Search

We present the High- z Evolution of Large and Luminous Objects (HELLO) project, a set of |$\sim \!30$| high-resolution cosmological simulations aimed to study Milky Way analogues (⁠|$M_\star \sim 10^{10-11}$|   |${\mathrm{M}}_{\odot }$|⁠) at high redshift (⁠|$z\sim [2-4]$|⁠). Based on the numerical investigation of a hundred astrophysical objects, HELLO features an updated scheme for chemical enrichment and the addition of local photoionization feedback. Independently of redshift and mass, our galaxies exhibit a smooth progression along the star formation main sequence until |$M_\star \sim \!10^{10.5}$|⁠ , around which our sample at |$z \sim 4$| remains mostly unperturbed while the most massive galaxies at |$z \sim 2$| reach their peak star formation rate (SFR) and its subsequent decline, due to a mix of gas consumption and stellar feedback. While active galactic nucleus feedback remains subdominant with respect to stellar feedback for energy deposition, its localized nature likely adds to the physical processes leading to declining SFRs. The phase in which a galaxy in our mass range can be found at a given redshift is set by its gas reservoir and assembly history. Finally, our galaxies are in excellent agreement with various scaling relations observed with the Hubble Space Telescope and the JWST , and hence can be used to provide the theoretical framework to interpret current and future observations from these facilities and shed light on the transition from star-forming to quiescent galaxies. [ABSTRACT FROM AUTHOR]

Cosmological galaxy formation simulations are powerful tools to understand the complex processes that govern the formation and evolution of galaxies. However, evaluating the realism of these simulations remains a challenge. The two common approaches for evaluating galaxy simulations is either through scaling relations based on a few key physical galaxy properties, or through a set of pre-defined morphological parameters based on galaxy images. This paper proposes a novel image-based method for evaluating the quality of galaxy simulations using unsupervised deep learning anomaly detection techniques. By comparing full galaxy images, our approach can identify and quantify discrepancies between simulated and observed galaxies. As a demonstration, we apply this method to SDSS imaging and NIHAO simulations with different physics models, parameters, and resolution. We further compare the metric of our method to scaling relations as well as morphological parameters. We show that anomaly detection is able to capture similarities and differences between real and simulated objects that scaling relations and morphological parameters are unable to cover, thus indeed providing a new point of view to validate and calibrate cosmological simulations against observed data. [ABSTRACT FROM AUTHOR]

We append two additional black hole (BH) accretion models, namely viscous disc and gravitational torque-driven accretion, into the Numerical Investigation of a Hundred Astrophysical Objects (NIHAO) project of galaxy simulations. We show that these accretion models, characterized by a weaker dependence on the BH mass compared to the commonly used Bondi-Hoyle accretion, naturally create a common evolutionary track (co-existence) between the mass of the BH and the stellar mass of the galaxy, even without any direct coupling via feedback (FB). While FB is indeed required to control the final BH and stellar mass of the galaxies, our results suggest that FB might not be the leading driver of the cosmic co-evolution between these two quantities; in these models, co-evolution is simply determined by the shared central gas supply. Conversely, simulations using Bondi-Hoyle accretion show a two-step evolution, with an early growth of stellar mass followed by exponential growth of the central supermassive black hole (SMBH). Our results show that the modelling of BH accretion (sometimes overlooked) is an extremely important part of BH evolution and can improve our understanding of how scaling relations emerge and evolve, and whether SMBH and stellar mass co-exist or co-evolve through cosmic time. [ABSTRACT FROM AUTHOR]

Recent observations of the Milky Way (MW) found an unexpected steepening of the star-forming gas metallicity gradient around the time of the Gaia–Sausage–Enceladus (GSE) merger event. Here, we investigate the influence of early (t merger ≲ 5 Gyr) massive (⁠|$M_{\rm {gas}}^{\rm {merger}}/M_{\rm {gas}}^{\rm {main}}(t_{\rm {merger}})\gtrsim 10~{{\ \rm per\ cent}}$|⁠) merger events such as the Gaia–Sausage–Enceladus merger in the MW on the evolution of the cold gas metallicity gradient. We use the NIHAO-UHD suite of cosmological hydrodynamical simulations of MW-mass galaxies to study the frequency of massive early mergers and their detailed impact on the morphology and chemistry of the gaseous discs. We find a strong steepening of the metallicity gradient at early times for all four galaxies in our sample which is caused by a sudden increase in the cold gas disc size (up to a factor of 2) in combination with the supply of unenriched gas (∼0.75 dex lower compared to the main galaxy) by the merging dwarf galaxies. The mergers mostly affect the galaxy outskirts and lead to an increase in cold gas surface density of up to 200 per cent outside of ∼8 kpc. The addition of unenriched gas breaks the self-similar enrichment of the inter-stellar-medium and causes a dilution of the cold gas in the outskirts of the galaxies. The accreted stars and the ones formed later out of the accreted gas inhabit distinct tracks offset to lower [α/Fe] and [Fe/H] values compared to the main galaxy's stars. We find that such mergers can contribute significantly to the formation of a second, low-α sequence as is observed in the MW. [ABSTRACT FROM AUTHOR]

We present a catalogue of dynamical properties for 2368 late-type galaxies from the MaNGA survey. The latter complements the catalogue of photometric properties for the same sample based on deep optical dark energy sky instrument legacy imaging survey (DESI) photometry processed with autoprof. Rotation curves (RCs), extracted by model-fitting H α velocity maps from the MaNGA Data Analysis Pipeline, extend out to 1.4 (1.9)  R e for the primary (secondary) MaNGA samples, respectively. The RCs and ancillary MaNGA Pipe3D data products were used to construct various fundamental galaxy scaling relations that are also compared uniformly with similar relations from numerical investigation of a hundred astrophysical object (NIHAO) zoom-in simulations. Simulated NIHAO galaxies were found to broadly reproduce the observed MaNGA galaxy population for |$\log (M_*/{\rm {\rm M}_{\odot })\, \gt\, 8.5}$|⁠. Some discrepancies remain, such as those pertaining to central stellar densities and the diversity of RCs due to strong feedback schemes. Also presented are spatially resolved scatters for the velocity-size-stellar mass (VRM*) structural relations using MaNGA and NIHAO samples. The scatter for these relations in the galaxian interiors is a consequence of the diversity of inner RC shapes, while scatter in the outskirts is dictated by the large range of stellar surface densities, which is itself driven by sporadic star formation. The detailed, spatially resolved scatter analysis highlights the complex interplay between local and global astrophysical processes and provides a strong constraint to numerical simulations. [ABSTRACT FROM AUTHOR]

The transition of high-mass galaxies from being blue and star-forming to being red and dead is a crucial step in galaxy evolution, yet not fully understood. In this work, we use the NIHAO (Numerical Investigation of a Hundred Astrophysical Objects) suite of galaxy simulations to investigate the relation between the transition time through the green valley and other galaxy properties. The typical green valley crossing time of our galaxies is approximately 400 Myr, somewhat shorter than observational estimates. The crossing of the green valley is triggered by the onset of active galactic nucleus (AGN) feedback and the subsequent shutdown of star formation. Interestingly, the time spent in the green valley is not related to any other galaxy properties, such as stellar age or metallicity, or the time at which the star formation quenching takes place. The crossing time is set by two main contributions: the ageing of the current stellar population and the residual star formation in the green valley. These effects are of comparable magnitude, while major and minor mergers have a negligible contribution. Most interestingly, we find the time that a galaxy spends to travel through the green valley is twice the e -folding time of the star formation quenching. This result is stable against galaxy properties and the exact numerical implementation of AGN feedback in the simulation. Assuming a typical crossing time of about 1 Gyr inferred from observations, our results imply that any mechanism or process aiming to quench star formation must do it on a typical time-scale of 500 Myr. [ABSTRACT FROM AUTHOR]

Although active galactic nuclei (AGN) feedback is required in simulations of galaxies to regulate star formation, further downstream effects on the dark matter (DM) distribution of the halo and stellar kinematics of the central galaxy can be expected. We combine simulations of galaxies with and without AGN physics from the Numerical Investigation of a Hundred Astrophysical Objects (NIHAO) to investigate the effect of AGN on the DM profile and central stellar rotation of the host galaxies. Specifically, we study how the concentration-halo mass (c – M) relation and the stellar spin parameter (λ R ) are affected by AGN feedback. We find that AGN physics is crucial to reduce the central density of simulated massive (⁠|$\gtrsim 10^{12}\, {\rm M}_{\odot }$|⁠) galaxies and bring their concentration to agreement with results from the Spitzer Photometry & Accurate Rotation Curves (SPARC) sample. Similarly, AGN feedback has a key role in reproducing the dichotomy between slow and fast rotators as observed by the ATLAS3D survey. Without star formation suppression due to AGN feedback, the number of fast rotators strongly exceeds the observational constraints. Our study shows that there are several collateral effects that support the importance of AGN feedback in galaxy formation, and these effects can be used to constrain its implementation in numerical simulations. [ABSTRACT FROM AUTHOR]

Recent observational and theoretical studies of the Local Group (LG) dwarf galaxies have highlighted their unique star-formation history, stellar metallicity, gas content, and kinematics. We investigate the commonality of these features by comparing constrained LG and field central dwarf halo simulations in the Numerical Investigation of a Hundred Astrophysical Objects (NIHAO) project. Our simulations, performed with NIHAO-like hydrodynamics which track the evolution of the Milky Way (MW) and M31 along with ∼100 dwarfs in the LG, reveal the total gas mass and stellar properties (velocity dispersion, evolution history, etc.) of present-day LG dwarfs to be similar to field systems. However, relative to field galaxies, LG dwarfs have more cold gas in their central parts and more metal-rich gas in the halo stemming from interactions with other dwarfs living in a high-density environment like the LG. Interestingly, the direct impact of massive MW/M31 analogues on the metallicity evolution of LG dwarfs is minimal; LG dwarfs accrete high-metallicity gas mostly from other dwarfs at late times. We have also tested for the impact of metal diffusion on the chemical evolution of LG dwarfs, and found that it does not affect the stellar or gaseous content of LG dwarfs. Our simulations suggest that the stellar components of LG dwarfs offer a unique and unbiased local laboratory for galaxy-formation tests and comparisons, especially against the overall dwarf population in the Universe. [ABSTRACT FROM AUTHOR]

Cosmological galaxy formation simulations are still limited by their spatial/mass resolution and cannot model from first principles some of the processes, like star formation, that are key in driving galaxy evolution. As a consequence they still rely on a set of 'effective parameters' that try to capture the scales and the physical processes that cannot be directly resolved in the simulation. In this study, we show that it is possible to use Machine Learning techniques applied to real and simulated images of galaxies to discriminate between different values of these parameters by making use of the full information content of an astronomical image instead of collapsing it into a limited set of values like size, or stellar/ gas masses. In this work, we apply our method to the NIHAO simulations and the THINGS and VLA-ANGST observations of H  i maps in nearby galaxies to test the ability of different values of the star formation density threshold n to reproduce observed H  i maps. We show that observations indicate the need for a high value of n ≳ 80 cm−3 (although the numerical value is model-dependent), which has important consequences for the dark matter distribution in galaxies. Our study shows that with innovative methods it is possible to take full advantage of the information content of galaxy images and compare simulations and observations in an interpretable, non-parametric, and quantitative manner. [ABSTRACT FROM AUTHOR]

With the advent of large spectroscopic surveys the amount of high quality chemodynamical data in the Milky Way (MW) increased tremendously. Accurately and correctly capturing and explaining the detailed features in the high-quality observational data is notoriously difficult for state-of-the-art numerical models. In order to keep up with the quantity and quality of observational data sets, improved prescriptions for galactic chemical evolution need to be incorporated into the simulations. Here we present a new, flexible, time-resolved chemical enrichment model for cosmological simulations. Our model allows us to easily change a number of stellar physics parameters such as the shape of the initial mass function (IMF), stellar lifetimes, chemical yields, or SN Ia delay times. We implement our model into the Gasoline2 code and perform a series of cosmological simulations varying a number of key parameters, foremost evaluating different stellar yield sets for massive stars from the literature. We find that total metallicity, total iron abundance, and gas phase oxygen abundance are robust predictions from different yield sets and in agreement with observational relations. On the other hand, individual element abundances, especially alpha-elements show significant differences across different yield sets and none of our models can simultaneously match constraints on the dwarf and MW mass scale. This offers a unique way of observationally constraining model parameters. For MW mass galaxies we find for most yield tables tested in this work a bimodality in the [α/Fe] versus [Fe/H] plane of rather low intrinsic scatter potentially in tension with the observed abundance scatter. [ABSTRACT FROM AUTHOR]

We use hydrodynamical cosmological simulations to show that it is possible to create, via tidal interactions, galaxies lacking dark matter (DM) in a DM-dominated universe. We select dwarf galaxies from the NIHAO project, obtained in the standard cold dark matter model and use them as initial conditions for simulations of satellite–central interactions. After just one pericentric passage on an orbit with a strong radial component, NIHAO dwarf galaxies can lose up to 80 per cent of their DM content, but, most interestingly, their central (≈8 kpc) DM-to-stellar mass ratio changes from a value of ∼25, as expected from numerical simulations and abundance matching techniques, to roughly unity as reported for NGC 1052-DF2 and NGC 1054-DF4. The stellar velocity dispersion drops from ∼30  |$\, \rm km\, s^{-1}$| before infall to values as low as 6 ± 2  |$\, \rm km\, s^{-1}$|⁠. These, and the half-light radius around 3 kpc, are in good agreement with observations from van Dokkum and collaborators. Our study shows that it is possible to create a galaxy without DM starting from typical dwarf galaxies formed in a DM-dominated universe, provided they live in a dense environment. [ABSTRACT FROM AUTHOR]

We investigate how the NIHAO galaxies match the observed star formation main sequence (SFMS) and what the origin of its scatter is. The NIHAO galaxies reproduce the SFMS and generally agree with observations, but the slope is about unity and thus significantly larger than observed values. This is because observed galaxies at large stellar masses, although still being part of the SFMS, are already influenced by quenching. This partial suppression of star formation by AGN feedback leads to lower star formation rates and therefore to lower observed slopes. We confirm that including the effects of AGN in our galaxies leads to slopes in agreement with observations. We find the deviation of a galaxy from the SFMS is correlated with its z  = 0 dark matter halo concentration and thus with its halo formation time. This means galaxies with a higher-than-average star formation rate (SFR) form later and vice versa. We explain this apparent correlation with the SFR by re-interpreting galaxies that lie above the SFMS (higher-than-average SFR) as lying to the left of the SFMS (lower-than-average stellar mass) and vice versa. Thus later forming haloes have a lower-than-average stellar mass, this is simply because they have had less-than-average time to form stars, and vice versa. It is thus the nature, i.e. how and when these galaxies form, that sets the path of a galaxy in the SFR versus stellar mass plane. [ABSTRACT FROM AUTHOR]

We use cosmological hydrodynamical galaxy formation simulations from the NIHAO project to investigate the response of cold dark matter (CDM) haloes to baryonic processes. Previous work has shown that the halo response is primarily a function of the ratio between galaxy stellar mass and total virial mass, and the density threshold above which gas is eligible to form stars, n [cm−3]. At low n all simulations in the literature agree that dwarf galaxy haloes are cuspy, but at high n ≳ 100 there is no consensus. We trace halo contraction in dwarf galaxies with n ≳ 100 reported in some previous simulations to insufficient spatial resolution. Provided the adopted star formation threshold is appropriate for the resolution of the simulation, we show that the halo response is remarkably stable for n ≳ 5, up to the highest star formation threshold that we test, n  = 500. This free parameter can be calibrated using the observed clustering of young stars. Simulations with low thresholds n ≤ 1 predict clustering that is too weak, while simulations with high star formation thresholds n ≳ 5, are consistent with the observed clustering. Finally, we test the CDM predictions against the circular velocities of nearby dwarf galaxies. Low thresholds predict velocities that are too high, while simulations with n ∼ 10 provide a good match to the observations. We thus conclude that the CDM model provides a good description of the structure of galaxies on kpc scales provided the effects of baryons are properly captured. [ABSTRACT FROM AUTHOR]

In recent years, a new window on galaxy evolution opened, thanks to the increasing discovery of galaxies with a low-surface brightness, such as Ultra Diffuse Galaxies (UDGs). The formation mechanism of these systems is still a much debated question and so are their kinematical properties. In this work, we address this topic by analysing the stellar kinematics of isolated UDGs formed in the hydrodynamical simulation suite Numerical Investigation of a Hundred Astrophysical Objects (NIHAO). We construct projected line-of-sight velocity and velocity dispersion maps to compute the projected specific angular momentum, λR, to characterize the kinematical support of the stars in these galaxies. We found that UDGs cover a broad distribution, ranging from dispersion to rotation-supported galaxies, with similar abundances in both regimes. The degree of rotation support of simulated UDGs correlates with several properties such as galaxy morphology, higher H  i fractions, and larger effective radii with respect to the dispersion-supported group, while the dark matter halo spin and mass accretion history are similar among the two populations. We demonstrate that the alignment of the infalling baryons into the protogalaxy at early z is the principal driver of the z  = 0 stellar kinematic state: pressure-supported isolated UDGs form via misaligned gas accretion while rotation-supported ones build up their baryons in an ordered manner. Accounting for random inclination effects, we predict that a comprehensive survey will find nearly half of field UDGs to have rotationally supported stellar discs, when selecting UDGs with effective radius larger than 1 kpc. [ABSTRACT FROM AUTHOR]

We present a comprehensive test of the relation between stellar and total masses in galaxies as predicted by popular models based on abundance matching (AM) techniques. We use the 'Spectroscopy and H -band Imaging of Virgo cluster galaxies' (SHIVir) survey with photometric and dynamical profiles for 190 Virgo cluster galaxies to establish a relation between the stellar and dynamical masses measured within the isophotal radius r 23.5. Various dark matter and galaxy scaling relations are combined with results from the NIHAO (Numerical Investigation of Hundred Astrophysical Objects) suite of hydrodynamical simulations to recast AM predictions in terms of these observed quantities. Our results are quite insensitive to the exact choice of dark matter profile and halo response to baryon collapse. We find that theoretical models reproduce the slope and normalization of the observed stellar-to-halo mass relation (SHMR) over more than three orders of magnitude in stellar mass (108 < M */M⊙ < 2 × 1011). However, the scatter of the observed SHMR exceeds that of AM predictions by a factor of ∼5. For systems with stellar masses exceeding 5 × 1010 M⊙, AM overpredicts the observed stellar masses for a given dynamical mass. The latter offset may support previous indications of a different stellar initial mass function in these massive galaxies. Overall, our results support the validity of AM predictions on a wide dynamical range. [ABSTRACT FROM AUTHOR]

We present a systematic analysis of the reaction of dark matter distribution to galaxy formation across more than eight orders of magnitude in stellar mass. We extend the previous work presented in the NIHAO-IV paper by adding 46 new high-resolution simulations of massive galaxies performed with the inclusion of black hole feedback. We show that outflows generated by the active galactic nucleus (AGN) are able to partially counteract the dark matter contraction due to the large central stellar component in massive haloes. The net effect is to relax the central dark matter distribution that moves to a less cuspy density profiles at halo mass larger than ≈3 × 1012 M⊙. The scatter around the mean value of the density profile slope (α) is fairly constant (Δα ≈ 0.3), with the exception of galaxies with halo masses around 1012 M⊙, at the transition from stellar to AGN feedback dominated systems, where the scatter increases by almost a factor of 3. We provide useful fitting formulae for the slope of the dark matter density profiles at few per cent of the virial radius for the whole stellar mass range: 105–1012 M⊙ (2 × 109 to 5 × 1013 M⊙ in halo mass). [ABSTRACT FROM AUTHOR]

Simulating thin and extended galactic discs has long been a challenge in computational astrophysics. We introduce the NIHAO-UHD suite of cosmological hydrodynamical simulations of Milky Way (MW) mass galaxies and study stellar disc properties such as stellar mass, size, and rotation velocity which agree well with observations of the MW and local galaxies. In particular, the simulations reproduce the age–velocity dispersion relation and a multicomponent stellar disc as observed for the MW. Half of our galaxies show a double exponential vertical profile, while the others are well described by a single exponential model which we link to the disc merger history. In all cases, mono-age populations follow a single exponential whose scale height varies monotonically with stellar age and radius. The scale length decreases with stellar age while the scale height increases. The general structure of the stellar discs is already set at time of birth as a result of the inside–out and upside–down formation. Subsequent evolution modifies this structure by increasing both the scale length and height of all mono-age populations. Thus, our results put tight constraints on how much dynamical memory stellar discs can retain over cosmological time-scales. Our simulations demonstrate that it is possible to form thin galactic discs in cosmological simulations provided there are no significant stellar mergers at low redshifts. Most of the stellar mass is formed in situ with only a few per cent (⁠|${\lesssim}5{{\ \rm per\ cent}}$|⁠) brought in by merging satellites at early times. Redshift zero snapshots and halo catalogues are publicly available. [ABSTRACT FROM AUTHOR]

We use 38 high-resolution simulations of galaxy formation between redshift 10 and 5 to study the impact of a 3 keV warm dark matter (WDM) candidate on the high-redshift Universe. We focus our attention on the stellar mass function and the global star formation rate and consider the consequences for reionization, namely the neutral hydrogen fraction evolution and the electron scattering optical depth. We find that three different effects contribute to differentiate warm and cold dark matter (CDM) predictions: WDM suppresses the number of haloes with mass less than few 109 M⊙; at a fixed halo mass, WDM produces fewer stars than CDM, and finally at halo masses below 109 M⊙, WDM has a larger fraction of dark haloes than CDM post-reionization. These three effects combine to produce a lower stellar mass function in WDM for galaxies with stellar masses at and below 107 M⊙. For z > 7, the global star formation density is lower by a factor of two in the WDM scenario, and for a fixed escape fraction, the fraction of neutral hydrogen is higher by 0.3 at z ∼ 6. This latter quantity can be partially reconciled with CDM and observations only by increasing the escape fraction from 23 per cent to 34 per cent. Overall, our study shows that galaxy formation simulations at high redshift are a key tool to differentiate between dark matter candidates given a model for baryonic physics. [ABSTRACT FROM AUTHOR]

The similarity between the distributions of spins for galaxies (λgal) and for dark-matter haloes (λhalo), indicated both by simulations and observations, is naively interpreted as a one-to-one correlation between the spins of a galaxy and its host halo. This is used to predict galaxy sizes in semi-analytic models via R e ≃ fj λhalo R vir, where R e is the half-mass radius of the galaxy, fj is the angular momentum retention factor, and R vir is the halo radius. Using two suites of zoom-in cosmological simulations, we find that λgal and the λhalo of its host halo are in fact barely correlated, especially at z ≥ 1, in line with previous indications. Since the spins of baryons and dark matter are correlated at accretion into R vir, the null correlation in the end reflects an anticorrelation between fj and λhalo, which can arise from mergers and a 'wet compaction' phase that many high-redshift galaxies undergo. It may also reflect that unrepresentative small fractions of baryons are tapped to the galaxies. The galaxy spin is better correlated with the spin of the inner halo, but this largely reflects the effect of the baryons on the halo. While λhalo is not a useful predictor for R e, our simulations reproduce a general relation of the form of R e = AR vir, in agreement with observational estimates. We find that the relation becomes tighter with A  = 0.02(c /10)−0.7, where c is the halo concentration, which in turn introduces a dependence on mass and redshift. [ABSTRACT FROM AUTHOR]

We introduce algorithms for black hole physics, i.e. black hole formation, accretion, and feedback, into the Numerical Investigation of a Hundred Astrophysical Objects (NIHAO) project of galaxy simulations. This enables us to study high mass, elliptical galaxies, where feedback from the central black hole is generally thought to have a significant effect on their evolution. We furthermore extend the NIHAO suite by 45 simulations that encompass z  = 0 halo masses from 1 × 1012 to |$4 \times 10^{13}\, \mathrm{M}_{\odot }$|⁠ , and resimulate five galaxies from the original NIHAO sample with black hole physics, which have z  = 0 halo masses from 8 × 1011 to |$3 \times 10^{12}\, \mathrm{M}_{\odot }$|⁠. Now NIHAO contains 144 different galaxies and thus has the largest sample of zoom-in simulations of galaxies, spanning z  = 0 halo masses from 9 × 108 to |$4 \times 10^{13}\, \mathrm{M}_{\odot }$|⁠. In this paper we focus on testing the algorithms and calibrating their free parameters against the stellar mass versus halo mass relation and the black hole mass versus stellar mass relation. We also investigate the scatter of these relations, which we find is a decreasing function with time and thus in agreement with observations. For our fiducial choice of parameters we successfully quench star formation in objects above a z  = 0 halo mass of |$10^{12}\, \mathrm{M}_{\odot }$|⁠ , thus transforming them into red and dead galaxies. [ABSTRACT FROM AUTHOR]

We study ultra-diffuse galaxies (UDGs) in zoom in cosmological simulations, seeking the origin of UDGs in the field versus galaxy groups. We find that while field UDGs arise from dwarfs in a characteristic mass range by multiple episodes of supernova feedback (Di Cintio et al.), group UDGs may also form by tidal puffing up and they become quiescent by ram-pressure stripping. The field and group UDGs share similar properties, independent of distance from the group centre. Their dark-matter haloes have ordinary spin parameters and centrally dominant dark-matter cores. Their stellar components tend to have a prolate shape with a Sérsic index n ∼ 1 but no significant rotation. Ram pressure removes the gas from the group UDGs when they are at pericentre, quenching star formation in them and making them redder. This generates a colour/star-formation-rate gradient with distance from the centre of the dense environment, as observed in clusters. We find that ∼20 per cent of the field UDGs that fall into a massive halo survive as satellite UDGs. In addition, normal field dwarfs on highly eccentric orbits can become UDGs near pericentre due to tidal puffing up, contributing about half of the group-UDG population. We interpret our findings using simple toy models, showing that gas stripping is mostly due to ram pressure rather than tides. We estimate that the energy deposited by tides in the bound component of a satellite over one orbit can cause significant puffing up provided that the orbit is sufficiently eccentric. We caution that while the simulations produce UDGs that match the observations, they under-produce the more compact dwarfs in the same mass range, possibly because of the high threshold for star formation or the strong feedback. [ABSTRACT FROM AUTHOR]

We use cosmological hydrodynamical galaxy formation simulations from the NIHAO project to investigate the impact of the threshold for star formation on the response of the dark matter (DM) halo to baryonic processes. The fiducial NIHAO threshold, |$n=10\, [{\rm cm}^{-3}]$|⁠, results in strong expansion of the DM halo in galaxies with stellar masses in the range |$10^{7.5} \lesssim {{M_{\rm star}}}\lesssim 10^{9.5} {{ {\rm M}_{\odot }}}$|⁠. We find that lower thresholds such as n  = 0.1 (as employed by the EAGLE/APOSTLE and Illustris/AURIGA projects) do not result in significant halo expansion at any mass scale. Halo expansion driven by supernova feedback requires significant fluctuations in the local gas fraction on sub-dynamical times (i.e. |$\lesssim $| 50 Myr at galaxy half-light radii), which are themselves caused by variability in the star formation rate. At one per cent of the virial radius, simulations with n  = 10 have gas fractions of ≃0.2 and variations of ≃0.1, while n  = 0.1 simulations have order of magnitude lower gas fractions and hence do not expand the halo. The observed DM circular velocities of nearby dwarf galaxies are inconsistent with CDM simulations with n  = 0.1 and n  = 1, but in a reasonable agreement with n  = 10. Star formation rates are more variable for higher n, lower galaxy masses, and when star formation is measured on shorter time-scales. For example, simulations with n  = 10 have up to 0.4 dex higher scatter in specific star formation rates than simulations with n  = 0.1. Thus observationally constraining the sub-grid model for star formation, and hence the nature of DM, should be possible in the near future. [ABSTRACT FROM AUTHOR]

State-of-the-art cosmological hydrodynamical simulations of galaxy formation have reached the point at which their outcomes result in galaxies with ever more realism. Still, the employed sub-grid models include several free parameters such as the density threshold, n, to localize the star-forming gas. In this work, we investigate the possibilities to utilize the observed clustered nature of star formation (SF) in order to refine SF prescriptions and constrain the density threshold parameter. To this end, we measure the clustering strength, correlation length, and power-law index of the two-point correlation function of young (τ < 50 Myr) stellar particles and compare our results to observations from the HST Legacy Extragalactic UV Survey (LEGUS). Our simulations reveal a clear trend of larger clustering signal and power-law index and lower correlation length as the SF threshold increases with only mild dependence on galaxy properties such as stellar mass or specific star formation rate. In conclusion, we find that the observed clustering of SF is inconsistent with a low threshold for SF (n < 1 cm−3) and strongly favours a high value for the density threshold of SF (n > 10 cm−3), as, for example employed in the NIHAO project. [ABSTRACT FROM AUTHOR]

We have used the NIHAO (Numerical Investigation of a Hundred Astrophysical Objects) simulations to explore how supernovae (SNe) affect star formation in galaxies. We find that SN feedback operates on all scales from the interstellar medium (ISM) to several virial radii. SNe regulate star formation by preventing condensation of Hi into h 2 and by moving cold neutral gas to the hot Hii phase. The first effect explains why the cold neutral gas in dwarf galaxies forms stars inefficiently. The second maintains the hot ISM of massive galaxies (Hii vents out at lower masses). At |$v_{\rm vir}\lower.5ex{\,\, \buildrel\gt \over \sim \,\,}67{\rm \, km\, s}^{-1}$|⁠, the outflow rate follows the relation: |$\skew4\dot{M}_{\rm out}=23\, (v_{\rm vir}/67{\rm \, km\, s}^{-1})^{-4.6}\, {\rm SFR}$|⁠. |$20{{\ \rm per\ cent}}$| – |$70{{\ \rm per\ cent}}$| of the gas expelled from galaxies escapes from the halo (ejective feedback) but outflows are dominated by cold swept-up gas, most of which falls back on to the galaxy on a |${\sim } 1\,$| Gyr time-scale. This 'fountain feedback' reduces the masses of galaxies by a factor of 2–4, since gas spends half to three quarter of its time in the fountain. Less than |$10{{\ \rm per\ cent}}$| of the ejected gas mixes with the hot circumgalactic medium and this gas is usually not reaccreted. On scales as large as 6 r vir, galactic winds divert the incoming gas from cosmic filaments and prevent it from accreting on to galaxies (pre-emptive feedback). This process is the main reason for the low baryon content of ultradwarves. [ABSTRACT FROM AUTHOR]

In this third paper of the series, we investigate the effects of warm dark matter (WDM) with a particle mass of |$m_\mathrm{WDM}=3\, \mathrm{keV}$| on the smallest galaxies in our Universe. We present a sample of 21 hydrodynamical cosmological simulations of dwarf galaxies and 20 simulations of satellite-host galaxy interaction that we performed both in a cold dark matter (CDM) and WDM scenario. In the WDM simulations, we observe a higher critical mass for the onset of star formation. Structure growth is delayed in WDM, as a result WDM haloes have a stellar population on average 2 Gyr younger than their CDM counterparts. Nevertheless, despite this delayed star formation, CDM and WDM galaxies are both able to reproduce the observed scaling relations for velocity dispersion, stellar mass, size, and metallicity at |$z$|  = 0. WDM satellite haloes in a Milky Way mass host are more susceptible to tidal stripping due to their lower concentrations, but their galaxies can even survive longer than the CDM counterparts if they live in a dark matter halo with a stee per central slope. In agreement with our previous CDM satellite study we observe a steepening of the WDM satellites' central dark matter density slope due to stripping. The difference in the average stellar age for satellite galaxies, between CDM and WDM, could be used in the future for disentangling these two models. [ABSTRACT FROM AUTHOR]

We used high resolution dark matter only cosmological simulations to investigate the structural properties of Lambda Cold Dark Matter (ΛCDM) haloes over cosmic time. The haloes in our study range in mass from ∼1010 to |${\sim } 10^{12} \, \rm M_{\odot}$|⁠, and are resolved with 105 to 107 particles. We fit the spherically averaged density profiles of DM haloes with the three parameter Einasto function. For our sample of haloes, the Einasto shape parameter, α, is uncorrelated with the concentration, c, at fixed halo mass, and at all redshifts. Previous reports of an anticorrelation are traced to fitting degeneracies, which our fits are less sensitive to due to our higher spatial resolution. However, for individual haloes the evolution in α and c is anticorrelated: at redshift |$z$|  = 7, α ≃ 0.4 and decreases with time, while c  ≃ 3 and increases with time. The evolution in structure is primarily due to accretion of mass at larger radii. We suggest that α traces the evolutionary state of the halo, with dynamically young haloes having high α (closer to a top-hat: α−1 = 0), and dynamically relaxed haloes having low α (closer to isothermal: α = 0). Such an evolutionary dependence reconciles the increase of α versus peak height, ν, with the dependence on the slope of the power spectrum of initial density fluctuations found by previous studies. [ABSTRACT FROM AUTHOR]

We use 85 pairs of high-resolution lambda cold dark matter (LCDM) cosmological simulations from the NIHAO project to investigate why in dwarf galaxies neutral hydrogen (H  i) linewidths measured at 50 per cent of the peak flux W 50/2 (from the hydrodynamical simulations) tend to underpredict the maximum circular velocity |${V_{\rm max}^{\rm DMO}}$| (from the corresponding dark matter only simulations). There are two main contributing processes. (1) Lower mass galaxies are less rotationally supported. This is confirmed observationally from the skewness of linewidths in bins of H  i mass in both ALFALFA and HIPASS observations. (2) The H  i distributions are less extended (relative to the dark matter halo) in dwarf galaxies. Coupled to the lower baryon-to-halo ratio results in rotation curves that are still rising at the last measured data point, in agreement with observations from SPARC. Combining these two effects, in both simulations and observations lower mass galaxies have, on average, lower W 50/ W 20. Additionally, mass-loss driven by supernovae and projection effects (dwarf galaxies are in general not thin discs) further reduce the linewidths. The implied H  i linewidth velocity function from NIHAO is in good agreement with observations in the nearby Universe of dwarf galaxies: |$10 \, {{ {\rm km\, s^{-1}} }} \lt W_{50}/2 \lt 80 \, {{ {\rm km\, s^{-1}} }}$|⁠. The dark matter only slope of ≈−2.9 is reduced to ≈−1.0 in the hydro simulations. Future radio observations of unbiased samples with higher spatial resolution will enable stricter tests of the simulations, and thus of the LCDM model. [ABSTRACT FROM AUTHOR]

We study the origin of bulge stars and their angular momentum (AM) evolution in 10 spiral galaxies with baryonic masses above |$10^{10}\rm\, M_{\odot }$| in the Numerical Investigation of a Hundred Astrophysical Objects galaxy formation simulations. The simulated galaxies are in good agreement with observations of the relation between the specific AM and mass of the baryonic component and the stellar bulge-to-total ratio (B / T). We divide the star particles at |$z$|  = 0 into disc and bulge components using a hybrid photometric/kinematic decomposition method that identifies all the central mass above an exponential disc profile as the 'bulge'. By tracking the bulge star particles back in time, we find that on average 95 per cent of the bulge stars formed in situ, 3 per cent formed ex situ in satellites of the same halo, and only 2 per cent formed ex situ in external galaxies. The evolution of the AM distribution of the bulge stars paints an interesting picture: The higher the final B / T, the more the specific AM remains preserved during the bulge formation. In all cases, bulge stars migrate significantly towards the central region, reducing their average galactocentric radius by roughly a factor 2, independently of the final B / T value. However, in the higher B / T (≳ 0.2) objects, the velocity of the bulge stars increases and the AM of the bulge is almost conserved, whereas at lower B / T values, the velocity of the bulge stars decreases and the AM of the bulge reduces. The correlation between the evolution of the AM and B / T suggests that bulge formation and disc formation are closely linked and cannot be treated as independent processes. [ABSTRACT FROM AUTHOR]

The satellites of the Milky Way and Andromeda represent the smallest galaxies we can observe in our Universe. In this series of papers, we aim to shed light on their formation and evolution using cosmological hydrodynamical simulations. In this first paper, we focus on the galaxy properties before accretion, by simulating 27 haloes with masses between 5 × 108 and 1010M☉. Out of this set 19 haloes successfully form stars, while 8 remain dark. The simulated galaxies match quite well present day observed scaling relations between stellar mass, size and metallicity, showing that such relations are in place before accretion. Our galaxies show a large variety of star formation histories, from extended star formation periods to single bursts. As in more massive galaxies, large star formation bursts are connected with major mergers events, which greatly contribute to the overall stellar mass build up. The intrinsic stochasticity of mergers induces a large scatter in the stellar mass-halo mass relation, up to two orders of magnitude. Despite the bursty star formation history, on these mass scales baryons are very ineffective in modifying the dark matter profiles, and galaxies with a stellar mass below ≈106M☉ retain their cuspy central dark matter distribution, very similar to results from pure N-body simulations. [ABSTRACT FROM AUTHOR]

In the first paper, we presented 27 hydrodynamical cosmological simulations of galaxies with total masses between 5 × 108 and 1010 M☉. In this second paper, we use a subset of these cosmological simulations as initial conditions (ICs) for more than 40 hydrodynamical simulations of satellite and host galaxy interaction. Our cosmological ICs seem to suggest that galaxies on these mass scales have very little rotational support and are velocity dispersion (σ) dominated. Accretion and environmental effects increase the scatter in the galaxy scaling relations (e.g. size-velocity dispersion) in very good agreement with observations. Star formation is substantially quenched after accretion. Mass removal due to tidal forces has several effects: it creates a very flat stellar velocity dispersion profile, and it reduces the dark matter content at all scales (even in the centre), which in turn lowers the stellar velocity on scales around 0.5 kpc even when the galaxy does not lose stellar mass. Satellites which start with a cored dark matter profile are more prone to either be destroyed or to end up in a very dark matter poor galaxy. Finally, we found that tidal effects always increase the 'cuspyness' of the dark matter profile, even for haloes that infall with a core. [ABSTRACT FROM AUTHOR]

We address the origin of ultra-diffuse galaxies (UDGs), which have stellar masses typical of dwarf galaxies but effective radii of Milky Way-sized objects. Their formation mechanism, and whether they are failed L⋆ galaxies or diffuse dwarfs, are challenging issues. Using zoom-in cosmological simulations from the Numerical Investigation of a Hundred Astrophysical Objects (NIHAO) project, we show that UDG analogues form naturally in dwarf-sized haloes due to episodes of gas outflows associated with star formation. The simulated UDGs live in isolated haloes of masses 1010–11 M⊙, have stellar masses of 107–8.5 M⊙, effective radii larger than 1 kpc and dark matter cores. They show a broad range of colours, an average Sérsic index of 0.83, a typical distribution of halo spin and concentration, and a non-negligible H i gas mass of 107 − 9 M⊙, which correlates with the extent of the galaxy. Gas availability is crucial to the internal processes which form UDGs: feedback-driven gas outflows, and subsequent dark matter and stellar expansion, are the key to reproduce faint, yet unusually extended, galaxies. This scenario implies that UDGs represent a dwarf population of low surface brightness galaxies and should exist in the field. The largest isolated UDGs should contain more H i gas than less extended dwarfs of similar M⋆. [ABSTRACT FROM AUTHOR]

We present the first numerical simulations in coupled dark energy cosmologies with high enough resolution to investigate the effects of the coupling on galactic and subgalactic scales. We choose two constant couplings and a time-varying coupling function and we run simulations of three Milky Way-sized haloes (~1012 M☉), a lower mass halo (6 × 1011 M☉) and a dwarf galaxy halo (5 × 109 M☉).We resolve each halo with several million dark matter particles. On all scales, the coupling causes lower halo concentrations and a reduced number of substructures with respect to Λ cold dark matter (ΛCDM). We show that the reduced concentrations are not due to different formation times. We ascribe them to the extra terms that appear in the equations describing the gravitational dynamics. On the scale of the Milky Way satellites, we show that the lower concentrations can help in reconciling observed and simulated rotation curves, but the coupling values necessary to have a significant difference from ΛCDM are outside the current observational constraints. On the other hand, if other modifications to the standard model allowing a higher coupling (e.g. massive neutrinos) are considered, coupled dark energy can become an interesting scenario to alleviate the small-scale issues of theΛCDM model. [ABSTRACT FROM AUTHOR]

We use ~100 cosmological galaxy formation 'zoom-in' simulations using the smoothed particle hydrodynamics code GASOLINE to study the effect of baryonic processes on the mass profiles of cold dark matter haloes. The haloes in our study range from dwarf (M200 ~ 1010M☉) to Milky Way (M200 ~ 1010M☉) masses. Our simulations exhibit a wide range of halo responses, primarily varying with mass, from expansion to contraction, with up to factor ~10 changes in the enclosed dark matter mass at 1 per cent of the virial radius. Confirming previous studies, the halo response is correlated with the integrated efficiency of star formation: ϵSF ≡ (Mstar/M200)/(Ωb/Ωm). In addition, we report a new correlation with the compactness of the stellar system: ϵR ≡ r1/2/R200. We provide an analytic formula depending on ϵSF and ϵR for the response of cold dark matter haloes to baryonic processes. An observationally testable prediction is that, at fixed mass, larger galaxies experience more halo expansion, while the smaller galaxies more halo contraction. This diversity of dark halo response is captured by a toy model consisting of cycles of adiabatic inflow (causing contraction) and impulsive gas outflow (causing expansion). For net outflow, or equal inflow and outflow fractions, f, the overall effect is expansion, with more expansion with larger f. For net inflow, contraction occurs for small f (large radii), while expansion occurs for large f (small radii), recovering the phenomenology seen in our simulations. These regularities in the galaxy formation process provide a step towards a fully predictive model for the structure of cold dark matter haloes. [ABSTRACT FROM AUTHOR]

We introduce project NIHAO (Numerical Investigation of a Hundred Astrophysical Objects), a set of 100 cosmological zoom-in hydrodynamical simulations performed using the GASOLINE code, with an improved implementation of the SPH algorithm. The haloes in our study range from dwarf (M200 ˜ 5 × 109M⊙) to Milky Way (M200 ˜ 2 × 1012M⊙) masses, and represent an unbiased sampling of merger histories, concentrations and spin parameters. The particle masses and force softenings are chosen to resolve the mass profile to below 1 per cent of the virial radius at all masses, ensuring that galaxy half-light radii are well resolved. Using the same treatment of star formation and stellar feedback for every object, the simulated galaxies reproduce the observed inefficiency of galaxy formation across cosmic time as expressed through the stellar mass versus halo mass relation, and the star formation rate versus stellar mass relation. We thus conclude that stellar feedback is the chief piece of physics required to limit the efficiency of star formation in galaxies less massive than the Milky Way. [ABSTRACT FROM AUTHOR]

We use cosmological hydrodynamical zoom-in simulations with the smoothed particle hydrodynamics code gasoline of four haloes of mass M200 ~ 1013 M⊙ to study the response of the dark matter to elliptical galaxy formation. Our simulations include metallicity-dependent gas cooling, star formation and feedback from massive stars and supernovae, but not active galactic nuclei (AGN). At z = 2 the progenitor galaxies have stellar-to-halo mass ratios consistent with halo abundance matching, assuming a Salpeter initial mass function. However, by z = 0 the standard runs suffer from the well-known overcooling problem, overpredicting the stellar masses by a factor of ≳ 4. To mimic a suppressive halo quenching scenario, in our forced quenching (FQ) simulations, cooling and star formation are switched off at z = 2. The resulting z = 0 galaxies have stellar masses, sizes and circular velocities close to what is observed. Relative to the control simulations, the dark matter haloes in the FQ simulations have contracted, with central dark matter density slopes d log ρ/d logr ~ -1.5, showing that dry merging alone is unable to fully reverse the contraction that occurs at z > 2. Simulations in the literature with AGN feedback, however, have found expansion or no net change in the dark matter halo. Thus, the response of the dark matter halo to galaxy formation may provide a new test to distinguish between ejective and suppressive quenching mechanisms. [ABSTRACT FROM AUTHOR]

In this first paper we discuss the linear theory and the background evolution of a new class of models we dub SCDEW: Strongly Coupled DE, plus WDM. In these models, WDM dominates today's matter density; like baryons, WDM is uncoupled. Dark energy is a scalar field Φ; its coupling to ancillary cold dark matter (CDM), whose today's density is <<1 per cent, is an essential model feature. Such coupling, in fact, allows the formation of cosmic structures, in spite of very low WDM particle masses (~100 eV). SCDEW models yield cosmic microwave background and linear large scale features substantially undistinguishable from ΛCDM, but thanks to the very low WDM masses they strongly alleviate ΛCDM issues on small scales, as confirmed via numerical simulations in the second associated paper. Moreover SCDEW cosmologies significantly ease the coincidence and fine tuning problems of ΛCDM and, by using a field theory approach, we also outline possible links with inflationary models. We also discuss a possible fading of the coupling at low redshifts which prevents non-linearities on the CDM component to cause computational problems. The (possible) low-z coupling suppression, its mechanism, and its consequences are however still open questions - not necessarily problems - for SCDEW models. The coupling intensity and the WDM particle mass, although being extra parameters in respect to ΛCDM, are found to be substantially constrained a priori so that, if SCDEW is the underlying cosmology, we expect most data to fit also ΛCDM predictions. [ABSTRACT FROM AUTHOR]

Ultra-compact dwarf galaxies (UCDs) have similar properties as massive globular clusters or the nuclei of nucleated galaxies. Recent observations suggesting a high dark matter content and a steep spatial distribution within groups and clusters provide new clues as to their origins. We perform high-resolution N-body/smoothed particle hydrodynamics simulations designed to elucidate two possible formation mechanisms for these systems: the merging of globular clusters in the centre of a dark matter halo, or the massively stripped remnant of a nucleated galaxy. Both models produce density profiles as well as the half-light radii that can fit the observational constraints. However, we show that the first scenario results to UCDs that are underluminous and contain no dark matter. This is because the sinking process ejects most of the dark matter particles from the halo centre. Stripped nuclei give a more promising explanation, especially if the nuclei form via the sinking of gas, funnelled down inner galactic bars, since this process enhances the central dark matter content. Even when the entire disc is tidally stripped away, the nucleus stays intact and can remain dark matter dominated even after severe stripping. Total galaxy disruption beyond the nuclei only occurs on certain orbits and depends on the amount of dissipation during nuclei formation. By comparing the total disruption of cold dark matter subhaloes in a cluster potential, we demonstrate that this model also leads to the observed spatial distribution of UCDs which can be tested in more detail with larger data sets. [ABSTRACT FROM AUTHOR]

We use the conditional luminosity function (CLF) and data from the 2-degree Field Galaxy Redshift Survey (2dFGRS) to constrain the average relation between light and mass in a Lambda cold dark matter (ΛCDM) cosmology with and (hereafter WMAP3 cosmology). Reproducing the observed luminosity dependence of the galaxy two-point correlation function results in average mass-to-light ratios that are ∼35 per cent lower than those in a ΛCDM cosmology with and (hereafter WMAP1 cosmology). This removes an important problem with previous halo occupation models which had a tendency to predict cluster mass-to-light ratios that were too high. For the WMAP3 cosmology, our model yields average mass-to-light ratios, central galaxy luminosities, halo occupation numbers, satellite fractions and luminosity-gap statistics, that are all in excellent agreement with those obtained from a 2dFGRS group catalogue and from other independent studies. We also use our CLF model to compute the probability distribution , that a central galaxy of luminosity Lcen resides in a halo of mass M. We find this distribution to be much broader than what is typically assumed in halo occupation distribution models, which has important implications for the interpretation of satellite kinematics and galaxy–galaxy lensing data. Finally, reproducing the luminosity dependence of the pairwise peculiar velocity dispersions in the 2dFGRS requires relatively low mass-to-light ratios for clusters and a satellite fraction that decreases strongly with increasing luminosity. This is only marginally consistent with the constraints obtained from the luminosity dependence of the galaxy two-point correlation function. We argue that a cosmology with parameters between those of the WMAP1 and WMAP3 cosmologies is likely to yield results with a higher level of consistency. [ABSTRACT FROM AUTHOR]