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The galaxy Malin 1 contains the largest stellar disk known but the formation mechanism of this structure has been elusive. In this paper, we report a Malin 1 analogue in the 100 Mpc IllustrisTNG simulation and describe its formation history. At redshift zero, this massive galaxy, having a maximum circular velocity $V_{\rm max}$ of 430 ${\rm km\ s^{-1}}$, contains a 100 kpc gas/stellar disk with morphology similar to Malin 1. The simulated galaxy reproduces well many observed features of Malin 1's vast disk, including its stellar ages, metallicities, and gas rotation curve. We trace the extended disk back in time and find that a large fraction of the cold gas at redshift zero originated from the cooling of hot halo gas, triggered by the merger of a pair of intruding galaxies. Our finding provides a novel way to form large galaxy disks as extreme as Malin 1 within the current galaxy formation framework., Comment: 5 page, 5 figures; resubmitted to MNRAS Letters after first referee report; Comments welcome. Animations of gas/stars can be found here at https://youtu.be/Ibl4Cyybw2Q and https://youtu.be/3aIpCu6e9dQ, or downloaded at http://www.tng-project.org/movies/tng/TNG_Malin_1_gas.mpeg and http://www.tng-project.org/movies/tng/TNG_Malin_1_star.mpeg

We present and analyse mock stellar catalogues that match the selection criteria and observables (including uncertainties) of the Gaia satellite data release 2 (DR2). The source are six cosmological high-resolution magneto-hydrodynamic $\Lambda$CDM zoom simulations of the formation of Milky Way analogues from the AURIGA project. Mock data are provided for stars with $V < 16$ mag, and $V < 20$ mag at $|b|>20$ degrees. The mock catalogues are made using two different methods: the public SNAPDRAGONS code, and a method based on that of Lowing et al. that preserves the phase-space distribution of the model stars. These publicly available catalogues contain 5-parameter astrometry, radial velocities, multi-band photometry, stellar parameters, dust extinction values, and uncertainties in all these quantities. In addition, we provide the gravitational potential and information on the origin of each star. By way of demonstration, we apply the mock catalogues to analyses of the young stellar disc and the stellar halo. We show that: i) the young outer stellar disc exhibits a flared distribution that is detectable in the height and vertical velocity distribution of A- and B-dwarf stars up to radii of ~15 kpc; and ii) the spin of the stellar halo out to 100 kpc can be accurately measured with Gaia DR2 RR Lyrae stars. These catalogues are well suited for comparisons with observations and should help to: i) develop and test analysis methods for the Gaia DR2 data; ii) gauge the limitations and biases of the data and iii) interpret the data in the light of theoretical predictions from realistic $ab$ $initio$ simulations of galaxy formation in the $\Lambda$CDM cosmological model., Comment: Accepted for publication in MNRAS (1st of September, 2018) after improvements to several figures and some discussion. The mock data are available for access and download at http://icc.dur.ac.uk/data and documentation and supplementary material is available at http://auriga.h-its.org/gaiamock.html

Strong gravitational lenses with measured time delay are a powerful tool to measure cosmological parameters, especially the Hubble constant ($H_0$). Recent studies show that by combining just three multiply-imaged AGN systems, one can determine $H_0$ to 2.4% precision. Furthermore, the number of time-delay lens systems is growing rapidly, enabling the determination of $H_0$ to 1% precision in the near future. However, as the precision increases it is important to ensure that systematic errors and biases remain subdominant. For this purpose, challenges with simulated datasets are a key component in this process. Following the experience of the past challenge on time delay, where it was shown that time delays can indeed be measured precisely and accurately at the sub-percent level, we now present the "Time Delay Lens Modeling Challenge" (TDLMC). The goal of this challenge is to assess the present capabilities of lens modeling codes and assumptions and test the level of accuracy of inferred cosmological parameters given realistic mock datasets. We invite scientists to model a set of simulated HST observations of 50 mock lens systems. The systems are organized in rungs, with the complexity and realism increasing going up the ladder. The goal of the challenge is to infer $H_0$ for each rung, given the HST images, the time delay, and stellar velocity dispersion of the deflector for a fixed background cosmology. The TDLMC challenge starts with the mock data release on 2018 January 8th. The deadline for blind submission is different for each rung. The deadline for Rung 0-1 is 2018 September 8; the deadline for Rung 2 is 2019 April 8 and the one for Rung 3 is 2019 September 8. This first paper gives an overview of the challenge including the data design, and a set of metrics to quantify the modeling performance and challenge details., Comment: 6 pages, 2 figures, submitted to MNRAS

Galaxy formation entails the hierarchical assembly of mass, along with the condensation of baryons and the ensuing, self-regulating star formation. The stars form a collisionless system whose orbit distribution retains dynamical memory that can constrain a galaxy's formation history. The ordered-rotation dominated orbits with near maximum circularity $\lambda_z \simeq1$ and the random-motion dominated orbits with low circularity $\lambda_z \simeq0$ are called kinematically cold and kinematically hot, respectively. The fraction of stars on `cold' orbits, compared to the fraction of stars on `hot' orbits, speaks directly to the quiescence or violence of the galaxies' formation histories. Here we present such orbit distributions, derived from stellar kinematic maps via orbit-based modelling for a well defined, large sample of 300 nearby galaxies. The sample, drawn from the CALIFA survey, includes the main morphological galaxy types and spans the total stellar mass range from $10^{8.7}$ to $10^{11.9}$ solar masses. Our analysis derives the orbit-circularity distribution as a function of galaxy mass, $p(\lambda_z~|~M_\star)$, and its volume-averaged total distribution, $p(\lambda_z)$. We find that across most of the considered mass range and across morphological types, there are more stars on `warm' orbits defined as $0.25\le \lambda_z \le 0.8$ than on either `cold' or `hot' orbits. This orbit-based "Hubble diagram" provides a benchmark for galaxy formation simulations in a cosmological context., Comment: Published in Nature Astronomy, 1 January 2018; doi:10.1038/s41550-017-0348-1; 22pages, 8 figures, 2 tables

We study galaxy shapes in the Illustris cosmological hydrodynamic simulation. We find that massive galaxies have a higher probability of being prolate. For galaxies with stellar mass larger than $10^{11}\rm M_{\odot}$, 35 out of total 839 galaxies are prolate. For 21 galaxies with stellar mass larger than $10^{12}\rm M_{\odot}$, 9 are prolate, 4 are triaxial while the others are close to being oblate. There are almost no prolate galaxies with stellar mass smaller than $3\times10^{11}\rm M_{\odot}$. We check the merger history of the prolate galaxies, and find that they are formed by major dry mergers. All the prolate galaxies have at least one such merger, with most having mass ratios between $1:1$ and $1:3$. The gas fraction (gas mass to total baryon mass) of the progenitors is 0-3 percent for nearly all these mergers, except for one whose second progenitor contains $\sim 15\%$ gas mass, while its main progenitor still contains less than $5\%$. For the 35 massive prolate galaxies that we find, 18 of them have minor axis rotation, and their angular momenta mostly come from the spin angular momenta of the progenitors (usually that of the main progenitor). We analyse the merger orbits of these prolate galaxies and find that most of them experienced a nearly radial merger orbit. Oblate galaxies with major dry mergers can have either radial or circular merger orbits. We further discuss various properties of these prolate galaxies, such as spin parameter $\lambda_{\rm R}$, spherical anisotropy parameter $\beta$, dark matter fraction, as well as inner density slopes for the stellar, dark matter and total mass distributions., Comment: Accepted for publication in MNRAS. 24 pages, 14 figures

The flux ratios in the multiple images of gravitationally lensed quasars can provide evidence for dark matter substructure in the halo of the lensing galaxy if the flux ratios differ from those predicted by a smooth model of the lensing galaxy mass distribution. However, it is also possible that baryonic structures in the lensing galaxy, such as edge-on discs, can produce flux-ratio anomalies. In this work, we present the first statistical analysis of flux-ratio anomalies due to baryons from a numerical simulation perspective. We select galaxies with various morphological types in the Illustris simulation and ray-trace through the simulated halos, which include baryons in the main lensing galaxies but exclude any substructures, in order to explore the pure baryonic effects. Our ray-tracing results show that the baryonic components can be a major contribution to the flux-ratio anomalies in lensed quasars and that edge-on disc lenses induce the strongest anomalies. We find that the baryonic components increase the probability of finding high flux-ratio anomalies in the early-type lenses by about 8% and by about 10 - 20% in the disc lenses. The baryonic effects also induce astrometric anomalies in 13% of the mock lenses. Our results indicate that the morphology of the lens galaxy becomes important in the analysis of flux-ratio anomalies when considering the effect of baryons, and that the presence of baryons may also partially explain the discrepancy between the observed (high) anomaly frequency and what is expected due to the presence of subhalos as predicted by the CDM simulations., Comment: 16 pages, 11 figures, accepted by MNRAS

Early-type galaxies provide unique tests for the predictions of the cold dark matter cosmology and the baryonic physics assumptions entering models for galaxy formation. In this work, we use the Illustris simulation to study correlations of three main properties of early-type galaxies, namely, the stellar orbital anisotropies, the central dark matter fractions and the central radial density slopes, as well as their redshift evolution since $z=1.0$. We find that lower-mass galaxies or galaxies at higher redshift tend to be bluer in rest-frame colour, have higher central gas fractions, and feature more tangentially anisotropic orbits and steeper central density slopes than their higher-mass or lower-redshift counterparts, respectively. The projected central dark matter fraction within the effective radius shows a very mild mass dependence but positively correlates with galaxy effective radii due to the aperture effect. The central density slopes obtained by combining strong lensing measurements with single aperture kinematics are found to differ from the true density slopes. We identify systematic biases in this measurement to be due to two common modelling assumptions, isotropic stellar orbital distributions and power-law density profiles. We also compare the properties of early-type galaxies in Illustris to those from existing galaxy and strong lensing surveys, we find in general broad agreement but also some tension, which poses a potential challenge to the stellar formation and feedback models adopted by the simulation., Comment: 27 pages, 19 figures, 7 tables, accepted for publication in MNRAS. Reported galaxy properties (regardless of galaxy types) are cataloged as the "Photometry, Morphology, Strong-lensing and Dynamics (PMSD)" supplementary catalog, publicly available from the Illustris website (http://www.illustris-project.org)