Search

We report on a study of 9 nearby primitive galaxies observed by Hubble's COS far-UV spectrograph that can serve as templates of high-z galaxies to be observed by JWST. By "primitive galaxies," we mean galaxies having a low stellar mass, and low gas metallicity, , whether they are local or at high redshift. We find that far-UV spectra of these galaxies show evidence of hard radiation, including X-rays. Following Thuan et al. (2004), we identify these galaxies as massive X-ray binaries containing a massive accreting stellar black hole. We further find that the lower the metallicity, the higher the probability of extremely strong X-radiation. Following Heger et al. (2003), we suggest that the accreting black hold is produced by direct collapse of stars having initial masses greater than 50. The X-radiation produced by black hole disk directly affects the surrounding interstellar medium, and many of these effects are observable in far-UV spectra. [ABSTRACT FROM AUTHOR]

Nebular Heii emission implies the presence of energetic photons (E≽54 eV). Despite the great deal of effort dedicated to understanding Heii ionization, its origin has remained mysterious, particularly in metal-deficient star-forming galaxies. Unfolding Heii-emitting, metal-poor starbursts at z∼0 can yield insight into the powerful ionization processes occurring in the primordial universe. Here we present a study on the origin of the extended nebular Heii emission in SBS 0335-052E, one of the most metal-poor (Z ∼ 3% Z⊙ Heii-emitter starbursts known locally. Based on optical VLT/MUSE spectroscopic and Chandra X-ray observations, and current stellar models we found that the Heii-ionization budget of SBS 0335-052E can only be produced by peculiar, nearly metal-free ionizing stars (called here "PopIII-like" stars) with a top-heavy initial mass function. This result is in line with recent simulations for PopIII star formation down to z=0. [ABSTRACT FROM AUTHOR]

IC 10 as a starburst dwarf galaxy in the Local Group (LG) has a large population of newly formed stars that are massive and intrinsically very bright in comparison with other LG galaxies. Using the Isaac Newton Telescope (INT) with the Wide Field Camera (WFC) in the i-band and V-band, we performed an optical monitoring survey to identify the most evolved asymptotic giant branch stars (AGBs) and red supergiant stars (RSGs) in this star-forming galaxy, which can be used to determine the star formation history (SFH). The E(B - V) as an effective factor for obtaining the precise magnitude of stars is measured for each star using a 2D dust map (SFD98) to obtain a total extinction for each star in both the i-band and V-band. We obtained the photometric catalog for 53579 stars within the area of 0.07 deg2 (13.5 kpc2), of which 762 stars are classified as variable candidates after removing the foreground stars and saturated ones from our catalog. To reconstruct the SFH for IC 10, we first identified 424 long-period variable (LPV) candidates within the area of two half-light radii (2r h ) from the center of the galaxy. We estimated the recent star formation rate (SFR) at ∼ 0.32 yr-1 for a constant metallicity Z = 0.0008, showing the galaxy is currently undergoing high levels of star formation. Also, a total stellar mass of is obtained within 2r h for that metallicity. [ABSTRACT FROM AUTHOR]

Understanding the nucleosynthetic origin of nitrogen and the evolution of the N/O ratio in the interstellar medium is crucial for a comprehensive picture of galaxy chemical evolution at high-redshift because most observational metallicity (O/H) estimates are implicitly dependent on the N/O ratio. The observed N/O at high-redshift shows an overall constancy with O/H, albeit with a large scatter. We show that these heretofore unexplained features can be explained by the pre-supernova wind yields from rotating massive stars (M≳10M⊙,ν/νcrit≳0.4). Our models naturally produce the observed N/O plateau, as well as the scatter at low O/H. We find the scatter to arise from varying star formation efficiency. However, the models that have supernovae dominated yields produce a poor fit to the observed N/O at low O/H. This peculiar abundance pattern at low O/H suggests that dwarf galaxies are most likely to be devoid of SNe yields and are primarily enriched by pre-supernova wind abundances. [ABSTRACT FROM AUTHOR]

We report on a study of 9 nearby primitive galaxies observed by Hubble's COS far-UV spectrograph that can serve as templates of high-z galaxies to be observed by JWST. By "primitive galaxies," we mean galaxies having a low stellar mass, and low gas metallicity, , whether they are local or at high redshift. We find that far-UV spectra of these galaxies show evidence of hard radiation, including X-rays. Following Thuan et al. (2004), we identify these galaxies as massive X-ray binaries containing a massive accreting stellar black hole. We further find that the lower the metallicity, the higher the probability of extremely strong X-radiation. Following Heger et al. (2003), we suggest that the accreting black hold is produced by direct collapse of stars having initial masses greater than 50. The X-radiation produced by black hole disk directly affects the surrounding interstellar medium, and many of these effects are observable in far-UV spectra. [ABSTRACT FROM AUTHOR]

Nebular Heii emission implies the presence of energetic photons (E≽54 eV). Despite the great deal of effort dedicated to understanding Heii ionization, its origin has remained mysterious, particularly in metal-deficient star-forming galaxies. Unfolding Heii-emitting, metal-poor starbursts at z∼0 can yield insight into the powerful ionization processes occurring in the primordial universe. Here we present a study on the origin of the extended nebular Heii emission in SBS 0335-052E, one of the most metal-poor (Z ∼ 3% Z⊙ Heii-emitter starbursts known locally. Based on optical VLT/MUSE spectroscopic and Chandra X-ray observations, and current stellar models we found that the Heii-ionization budget of SBS 0335-052E can only be produced by peculiar, nearly metal-free ionizing stars (called here "PopIII-like" stars) with a top-heavy initial mass function. This result is in line with recent simulations for PopIII star formation down to z=0. [ABSTRACT FROM AUTHOR]

Using an ultra-deep, untargeted survey with the MUSE integral field spectrograph on the ESO Very Large Telescope, we obtain spectroscopic redshifts to a depth never explored before: galaxies with observed magnitudes m > 30–32. Specifically, we detect objects via Lyman-α emission at 2.9 < z < 6.7 without individual continuum counterparts in areas covered by the deepest optical/near-infrared imaging taken by the Hubble Space Telescope, the Hubble Ultra Deep Field. In total, we find more than 100 such objects in 9 square arcminutes at these redshifts, also including a number of sources that are visible only in the HST band that contains Lyman-α. Detailed HST and IRAC stacking analyses confirm the Lyman-α emission as well as the 1216 Å breaks, faint UV continua (MUV ∼ −15), and optical emission lines: these objects are the faintest spectroscopically-confirmed galaxies at high-z. The blue UV continuum slopes and measurements/limits on the equivalent widths of Lyman-α, which in some cases exceeds 300 Å, are consistent with ages < 10 Myr, metallicities < 5% solar, and stellar masses < 107–8 solar masses. The nature of these types of objects is intriguing as they could be the faint star-forming sources of Reionization and could represent the initial (strong) phase of stellar mass growth in galaxies. [ABSTRACT FROM AUTHOR]

Galaxies are observed to obey a strict set of dynamical scaling relations. We review these relations for rotationally supported disk galaxies spanning many decades in mass, surface brightness, and gas content. The behavior of these widely varied systems can be summarized with a handful of empirical laws connected by a common acceleration scale. [ABSTRACT FROM AUTHOR]

I review the current status of dynamical modelling of dwarf spheroidal galaxies focusing on estimates of their dark matter content. Starting with the simplest methods using the velocity dispersion profiles I discuss the inherent issues of mass-anisotropy degeneracy and contamination by unbound stars. I then move on to methods of increasing complexity, aiming to break the degeneracy, up to recent applications of the Schwarzschild orbit superposition method. The dynamical modelling is placed in the context of possible scenarios for the formation of dwarf spheroidal galaxies, including the tidal stirring model and mergers of dwarf galaxies. The two scenarios are illustrated with examples from simulations: a comparison between the tidal evolution of dwarfs with cuspy and cored dark matter profiles and the formation of a dwarf spheroidal with prolate rotation. [ABSTRACT FROM AUTHOR]

RR Lyrae variables are powerful tools to study their host stellar populations. Globular clusters and dwarf galaxies are old and usually host this type of variables. With a growing number of low luminosity objects discovered in the halo of the Milky Way, classifying stars clusters and galaxies has become more challenging. In this study, we examine the properties of RR Lyrae stars in globular clusters and dwarf galaxies in the Local Group. We construct a catalog of RR Lyrae variables in the Local Group globular clusters and dwarf galaxies from previously published data and compare the properties of RR Lyrae variables between those two types of stellar systems. Our goal is to search for a physical difference in the properties of RR Lyrae variables in those two classes of stellar systems. We also analyze the global trend of RRLs in these systems to understand more about their formation and evolution history. [ABSTRACT FROM AUTHOR]

We present results of a multiwavelength study of the isolated dual AGN system Was 49. Observations show that the dominant component in this interacting system, Was 49a, is a spiral galaxy, while Was 49b is hosted in a dwarf galaxy located at 8 kpc from the nucleus of Was 49a, at the edge of its disk. The intriguing fact about this system is the luminosity of their corresponding AGNs. While Was 49a hosts a low luminosity Seyfert 2 with Lbol˜1043erg s–1, Was 49b has a Seyfert 2 with Lbol ˜ 1045erg s–1, in the luminosity range of Quasars. Furthermore, estimates of the black hole and host galaxy masses of Was 49b indicate a black hole significantly more massive than one would expect from scaling relations. This result is in contrast with findings that the most luminous merger-triggered AGNs are found in major mergers and that minor mergers predominantly enhance AGN activity in the primary galaxy. [ABSTRACT FROM AUTHOR]

Detecting the seed black holes from which quasars formed is extremely challenging; however, those seeds that did not grow into supermassive should be found as intermediate-mass black holes (IMBHs) of 100 – 105 M⊙ in local dwarf galaxies. The use of deep multiwavelength surveys has revealed that a population of actively accreting IMBHs (low-mass AGN) exists in dwarf galaxies at least out to z ˜3. The black hole occupation fraction of these galaxies suggests that the early Universe seed black holes formed from direct collapse of gas, which is reinforced by the possible flattening of the black hole-galaxy scaling relations at the low-mass end. This scenario is however challenged by the finding that AGN feedback can have a strong impact on dwarf galaxies, which implies that low-mass AGN in dwarf galaxies might not be the untouched relics of the early seed black holes. This has important implications for seed black hole formation models. [ABSTRACT FROM AUTHOR]

Angular momentum (AM) is a key parameter to understand galaxy formation and evolution. AM originates in tidal torques between proto-structures at turn around, and from this the specific AM is expected to scale as a power-law of slope 2/3 with mass. However, subsequent evolution re-shuffles this through matter accretion from filaments, mergers, star formation and feedback, secular evolution and AM exchange between baryons and dark matter. Outer parts of galaxies are essential to study since they retain most of the AM and the diagnostics of the evolution. Galaxy IFU surveys have recently provided a wealth of kinematical information in the local universe. In the future, we can expect more statistics in the outer parts, and evolution at high z, including atomic gas with SKA. [ABSTRACT FROM AUTHOR]

Certain types of large amplitude AGB variable are proving to be powerful distance indicators that will rival Cepheids in the James Webb Space Telescope era of high precision infrared photometry. These are predominantly found in old populations and have low mass progenitors. At the other end of the AGB mass-scale, large amplitude variables, particularly those undergoing hot bottom burning, are the most luminous representatives of their population. These stars are < 1 Gyr old, are often losing mass copiously and are vital to our understanding of the integrated light of distant galaxies as well as to chemical enrichment. However, the evolution of such very luminous AGB variables is rapid and remains poorly understood. Here I discuss recent infrared observations of both low- and intermediate-mass Mira variables in the Local Group and beyond. [ABSTRACT FROM AUTHOR]

The evolution of galaxies is driven by the birth and death of stars. AGB stars are at the end points of their evolution and therefore their luminosities directly reflect their birth mass; this enables us to reconstruct the star formation history. These cool stars also produce dust grains that play an important role in the temperature regulation of the interstellar medium (ISM), chemistry, and the formation of planets. These stars can be resolved in all of the nearby galaxies. Therefore, the Local Group of galaxies offers us a superb near-field cosmology site. Here we can reconstruct the formation histories, and probe the structure and dynamics, of spiral galaxies, of the many dwarf satellite galaxies surrounding the Milky Way and Andromeda, and of isolated dwarf galaxies. It also offers a variety of environments in which to study the detailed processes of galaxy evolution through studying the mass-loss mechanism and dust production by cool evolved stars. In this paper, I will first review our recent efforts to identify mass-losing Asymptotic Giant Branch (AGB) stars and red supergiants (RSGs) in Local Group galaxies and to correlate spatial distributions of the AGB stars of different mass with galactic structures. Then, I will outline our methodology to reconstruct the star formation histories using variable pulsating AGB stars and RSGs and present the results for rates of mass–loss and dust production by pulsating AGB stars and their analysis in terms of stellar evolution and galaxy evolution. [ABSTRACT FROM AUTHOR]

Results of mass imaging nearby dwarf galaxies in emission H α line and red continuum with the 6-meter BTA telescope are available via the address: http://www.sao.ru/lv/lvgdb. The sample of dwarfs limited by a distance of 11 Mpc contains about 500 objects. Their H α - fluxes are used to derive integrated and specific star formation rates of the galaxies. We evaluate the consistency between star formation rates obtained from our H α -survey and GALEX far-ultraviolet survey. We fix a systematic rise of the ratio SFR(FUV)/SFR(H α) with the decreasing stellar mass of dwarf galaxies. In the sample there were included only galaxies of late types: T = 10 (Ir), 9 (Im, BCD), 8 (Sdm), 7 (Sd), 6 (Scd), since elliptical and lenticular galaxies, and also spiral with massive bulges, have a significantly different history of star formation. [ABSTRACT FROM AUTHOR]

We have searched for a sign of the past dynamical disturbance events on NGC 1068, an archetypical Type-2 Seyfert galaxy, using deep and wide optical imaging data by the Subaru telescope. The data taken by Hyper Suprime-Cam (HSC) as well as the archived data by Suprime-Cam reveal several faint outer structures of the galaxy, most of which were never reported before. We discover three large (r e = 3 -5.5 kpc), extremely diffuse objects (UDOs) within 45 kpc from the center of NGC 1068. We suggest that two of these UDOs are actually a part of a large loop-like structure surrounding NGC 1068. Such an extremely faint loop or stream is the direct evidence for a past minor merger event. The third UDO has a distorted morphology, suggesting that it is under the influence of strong tidal field. Furthermore, we have identified another ultra-diffuse but compact (μ 0, r > 25 mag arcsec-2, re ~ 0.8kpc) dwarf galaxy within ~140 kpc from NGC 1068. We speculate that this ultra-diffuse dwarf could be the object related to the ancient tidal disruption event (tidal dwarf) during the early mass assembly period of NGC 1068. We also detect an asymmetric outer one-arm structure emanated from the western edge of the outermost disk of NGC 1068 together with a ripple-like structure at the opposite side. These structures are also expected to arise in a late phase (up to several billion years ago) of a minor merger, according to numerical simulations. Our findings are consistent with the idea that the AGN activity in NGC 1068 is caused by a past minor merger. [ABSTRACT FROM AUTHOR]

Mass-loss via stellar-feedback driven outflows is predicted to play a critical role in the baryon cycle of low-mass galaxies. However, observational constraints on warm winds are limited as outflows are transient, intrinsically low-surface brightness events and, thus, difficult to detect. Here, we search for outflows in a sample of eleven nearby starburst dwarf galaxies which are strong candidates for outflows. Despite deep H? imaging on galaxies, only a fraction of the sample show evidence of winds. The spatial extent of all detected ionized gas is limited and would still be considered part of the ISM by simulations. These new observations indicate that the physical extent of warm phase outflows is modest and most of the mass will be recycled to the galaxy. The sample is part of the panchromatic STARBurst IRegular Dwarf Survey (STARBIRDS) designed to characterize the starburst phenomenon and its impact on the evolution of low-mass galaxies. [ABSTRACT FROM AUTHOR]

We study the distribution of mid-infrared light in stellar structures in a large sample (∽ 400) of low-mass (M stellar <109 M Sun ) galaxies. Our sample is selected from the Spitzer Survey of Stellar Structures in Galaxies (S4G), which entails deep imaging of nearby galaxies with the IRAC instrument at 3.6/4.5 μ m. Based on the 2D decomposition of the 3.6 μ m images, we find that the majority (∽65%) of galaxies in our sample is well-fit by a single disk profile. The rest of the sample is more adequately fit by a disk and an additional component (e.g., bar, nucleus, bulge, second disk component). Bars are present in ∽11% of the sample, marking a sharp drop in the bar fraction compared to that found for more massive galaxies. The typical contribution of bars to the 3.6 μ m light in dwarfs is ∽1-2%, lower than that found in more massive galaxies. These results bring a number of issues into question: why do low-mass galaxies have such low bar fraction? does the bar instability act differently in low-mass galaxies such that a smaller proportion of stellar mass is typically involved in the bar structure? Is the fact that dwarfs are more dark matter dominated playing a role? [ABSTRACT FROM AUTHOR]

Dwarf galaxies make ideal laboratories to test galaxy evolution paradigms and cosmological models. Detailed studies of dwarfs across the spectrum allow us to gauge the efficacy of astrophysical processes at play in the lowest mass halos such as gas accretion, feedback, turbulence and chemical enrichment. Future observational studies will deliver unprecedented insights on the orbits of dwarf companions around the Milky Way, on their star formation histories and on the 3-D internal motions of their stars. Over large volumes, we will assess the impact of local environment on baryon cycling and star formation laws, leading to a full picture of the evolution of dwarfs across cosmic time. In combination, future discoveries promise to trace the history of assembly within the Local Group and beyond, probe how stars form under pristine conditions, and test models of structure formation on small scales. [ABSTRACT FROM AUTHOR]

Ultra-diffuse galaxies (UDGs) have sizes comparable to the Milky Way and stellar masses of about 1/1000 of it. They attracted a lot of attention as possible "dark galaxies" heavily dominated by dark matter, however, no reliable dynamical mass estimates were done because of their extremely low surface brightness. We have recently found 13 gas free diffuse young (300–500 Myr) post-starburst galaxies (PSGs) without ongoing star formation in Coma and Abell 2147, which, should they continue to evolve passively, will become UDGs in 5 Gyr. We obtained deep spectroscopic observations for 11 diffuse PSGs and derived their internal kinematics and stellar population properties. All of them possess disk-like kinematics (substantial rotation, low stellar velocity dispersion) and likely experienced starburst episodes prior to the star formation quenching by ram pressure stripping. Our results suggest that at least some UDGs were "normal" intermediate to large-sized disk galaxies in the past, which were later quenched by dense environment. [ABSTRACT FROM AUTHOR]

Dwarf galaxies represent the dominant population at high redshift and they most likely contributed in great part to star formation history of the Universe and cosmic reionization. The importance of dwarf galaxies at high redshift has been mostly recognized in the last decade due to large progress in observing facilities allowing deep galaxy surveys to identify low-mass galaxies. This population appear to have extreme emission lines and ionizing properties that challenge stellar population models. Star formation follows a stochastic process in these galaxies, which has important implication on the ionizing photon production and its escape fraction whose measurements are challenging for both simulations and observations. Outstanding questions include: what are the physical properties at the origin of such extreme properties? What are the smallest dark matter halos that host star formation? Are dwarf galaxies responsible for cosmic reionization? [ABSTRACT FROM AUTHOR]

The Local Group contains a great number of dwarf irregulars and spheroidals, for which the spectroscopy of individual stars can be obtained. Thus, the chemical evolution of these galaxies can be traced, with the only need of finding populations spanning a large age range and such that we can accurately derive the composition. Planetary nebulae (PNe) are old- and intermediate-age star remnants and their chemical abundances can be obtained up to 3-4 Mpc. H ii regions, which are brighter and much easily detected, represent galaxies young content. PNe and H ii regions share similar spectroscopic features and are analysed in the same way. Both are among the best tracers of the chemical evolution allowing to draw the chemical time line of nearby galaxies. The focus in this review are the PN and H ii region populations as constraints to the chemical evolution models and the mass-metallicity relation of the local universe. [ABSTRACT FROM AUTHOR]

We present a summary of the predictions from numerical simulations to our understanding of dwarf galaxies. It centers the discussion around the Λ Cold Dark Matter scenario (ΛCDM) but discusses also implications for alternative dark matter models. Four key predictions are identified: the abundance of dwarf galaxies, their dark matter content, their relation with environment and the existence of dwarf satellites orbiting dwarf field galaxies. We discuss tensions with observations and identify the most exciting predictions expected from simulations in the future, including i) the existence of "dark galaxies" (dark matter halos without stars), ii) the ability to resolve the structure (size, morphology, dark matter distribution) in dwarfs and iii) the number of ultra-faint satellites around dwarf galaxies. All of these predictions shall inform future observations, not only the faintest galaxies to be discovered within the Local Volume but also distant dwarfs driving galaxy formation in the early universe. [ABSTRACT FROM AUTHOR]

This review describes where we are today in light of the dust and gas properties and their relation to star formation, in low metallicity galaxies of the local universe following recent surveys from sensitive infrared space telescopes, mainly Spitzer and Herschel space observatories as well as ground-based observations of the molecular gas reservoir. Models to interpret the ISM properties are gaining sophistication in order to account for the wide range of valuable observational diagnostics that we have today to trace the different gas phases, the broad range of photometry we have, from mid-infrared to submillimetre dust emission and the various galactic size scales that we can sample today. This review summarizes the rich multi-phase observations we can exploit today, and the multi-phase modeling approach to interpret the observations. [ABSTRACT FROM AUTHOR]

The ubiquity of star-forming dwarf galaxies (SFDG) in the local Universe allows us to trace their evolution in all type of environments, from voids to rich clusters. SFDGs in low-density regions are still assembling their mass, they often show peculiar gas morphology and kinematics, likely associated to external gas accretion or galaxy interactions, and they can experience strong bursts of star formation. The most metal-poor SFDGs are found in the field and they are unique laboratories to investigate the star formation process in the low-metallicity regime, at conditions similar to their high-redshift analogues. On the other hand, SFDGs in intermediate- and high-density environments provide a key to understand the processes that remove their interstellar medium (ISM) and suppress star formation, leading to the different types of gas-poor early-type dwarfs. We review the most recent results on the properties of SFDGs at low and high galaxy densities focusing in particular on the impact of a cluster environment on their ISM components (dust, molecular, atomic and ionised gas). We analyse the population of SFDGs in the nearest rich clusters: Virgo, which is still in the process of assembly, and Fornax, which is more dynamically evolved, more compact and denser. We discuss how the different evolutionary stage of the two structures affects the properties of SFDGs. [ABSTRACT FROM AUTHOR]

The combination of sensitivity and large sky coverage of the ALFALFA HI survey has enabled the detection of difficult to observe low mass galaxies in large numbers, including dwarf galaxies overlooked in optical surveys. Three different, but connected, studies of dwarf galaxies from the ALFALFA survey are of particular interest: SHIELD (Survey of HI in Extremely Low-mass Dwarfs), candidate gas-rich ultra-faint dwarf galaxies, and the (Almost) Dark population. SHIELD is a systematic multiwavelength study of all dwarf galaxies from ALFALFA with M HI < 107.2 M ⊙ and clear optical counterparts. Candidate gas-rich ultra-faint dwarf galaxies extend the dwarf galaxy population to even lower masses. These galaxies are identified as isolated HI clouds with no discernible optical counterpart but subsequent observations reveal that some are extremely faint, gas-dominated galaxies. Leo P, discovered first as an HI detection, and then found to be an actively star-forming galaxy, bridges the gap between these candidate galaxies and the SHIELD sample. The (Almost) Dark sample consists of galaxies whose optical counterparts are overlooked in current optical surveys but which are clear detections in ALFALFA. This sample includes field gas-rich ultra-diffuse galaxies. Coma P, with a peak surface brightness of only ∼26.4 mag arcsec−2 in g', demonstrates the sort of extreme low surface brightness galaxy that can be discovered in an HI survey. [ABSTRACT FROM AUTHOR]

We use a matched filter to detect compact groups of old, metal-poor stars that we term FOSSILs (Fragments of Old Stellar Systems in Limbo). With size scales on the order of 10 arcminutes, distances ranging from 2 to 200 kpc, and memberships ranging from a handful to several dozen stars, these FOSSILs stand out from the surrounding field and are presumably signatures of, or debris from, ancient star clusters and dwarf galaxies. They may be localized concentrations of stars within more extensive tidal streams, and in some cases may be the signatures of extant but heretofore undetected ultrafaint galaxies. Using magnitudes and colors from the Pan-STARRs survey, we detect ∼ 70 such FOSSILs at 5 σ or greater in a 2200 square degree region in the vicinity of the north Galactic pole. A subsample of more populous FOSSILs that could be candidate ultrafaint dwarf galaxies suggests a total population of 200 such objects within 200 kpc of the Galactic center. Spectroscopic and astrometric follow-up of these FOSSILs will be required to determine the nature of these structures, deepen our understanding of the make-up and accretion history of the Galactic halo, and perhaps alleviate the missing satellites problem. [ABSTRACT FROM AUTHOR]

We have studied young stellar populations and star clusters in the dwarf irregular galaxy Leo A using multicolor (B, V, R, I, Hα) photometry data obtained with the Subaru Suprime-Cam and two-color photometry results measured on archival HST/ACS F475W & F814W frames. The analysis of the main sequence (MS) and blue supergiant (BSG – "blue loop") stars enabled us to study the star formation history in the Leo A galaxy during the last ~200 Myr. Also, we have discovered 5 low-mass (≲ 400 M⊙) star clusters within the ACS field. This finding, taking into account a low metallicity environment and a yet-undetected molecular gas in Leo A, constrains star formation efficiency estimates and scenarios. Inside the well known "hole" in the H i column density map (Hunter et al. 2012) we found a shock front (prominent in Hα), implying an unseen progenitor and reminding the "hole" problems widely discussed by Warren et al. (2011). [ABSTRACT FROM AUTHOR]

We report an investigation of the properties of dwarf galaxies (Mr < -15) inside 26 clusters at z = 0.15 – 0.25, using the X-ray data from the Chandra archive, and optical images taken with Subaru Suprime-Cam. Our results include: 1. Investigation of the dwarf galaxy density distribution is sensitive to the background galaxies and the choice of colour selection of galaxies. 2. Cluster-centric dwarf-to-giant ratio is highly sensitive to the level of subtracted background galaxies. 3. A certain fraction of faint galaxies always remain undetected by the detection algorithm near the center of clusters, even after carefully treating the halo or extra diffuse light created by bright galaxies. The number of 'undetected' faint galaxies varies significantly from cluster to cluster, and even from pointing to pointing. 4. Dwarf galaxies extend up to 2 Mpc from the center in most clusters. Meanwhile, the distribution of blue dwarf galaxies extends more to the outside. 5. For a given colour, the spatial distributions of dwarf galaxies and giant galaxies become similar. Namely, the most of the radial distribution comes from the colour, rather than the size, of galaxies. 6. Relative to the NFW profile, all of the galaxy populations are showing a deficit near the cluster core (r < 0.3 Mpc). 7. The dwarf-to-giant ratio shows no variation against cluster measures such as the richness and X-ray luminosity, as well as various cluster X-ray characteristics related to possible dynamical status of clusters. [ABSTRACT FROM AUTHOR]

We study how the void environment affects the chemical evolution of galaxies by comparing the metallicity of dwarf galaxies in voids with dwarf galaxies in denser regions. Using spectroscopic observations from SDSS DR7, we estimate oxygen and nitrogen abundances of 889 void dwarf galaxies and 672 dwarf galaxies in denser regions. A substitute for the [OII] λ 3727 doublet is developed, permitting oxygen abundance estimates of SDSS dwarf galaxies at all redshifts with the direct method. We find that void dwarf galaxies have about the same oxygen abundances and slightly lower N/O ratios than dwarf galaxies in denser environments. The lower N/O ratios seen in void dwarf galaxies may indicate both delayed star formation and a dependence of cosmic downsizing on the large-scale environment. Similar oxygen abundances in the two dwarf galaxy populations might be evidence of larger ratios of dark matter halo mass to stellar mass in voids. [ABSTRACT FROM AUTHOR]

We present the status of an ongoing project to map the detailed chemical abundances of stars across the main body of the Sagittarius dwarf Spheroidal galaxy (Sgr dSph). The Sgr dSph is the closest known dwarf galaxy, and is being tidally destroyed by its interaction with the Milky Way (MW), leaving behind a massive stellar stream. Sgr dSph is a chemically outstanding object, with peculiar abundance ratios, clear center-outskirts abundance gradients, and spanning more than 3 orders of magnitude in metallicity. We present here detailed abundances from UVES@VLT spectra for more than 50 giants across 8 fields along the major and minor axes of Sgr dSph, and 5 more outside the galaxy main body, but possibly associated to its stellar stream. [ABSTRACT FROM AUTHOR]

We present results from a non-cosmological, three-dimensional hydrodynamic simulation of an outflow from an intermediate-mass black hole in Dwarf Spheroidal Galaxies. Assuming an initial baryonic-to-dark-matter ratio derived from the CMB radiation and a cored, static dark matter potential, we evolved the galactic gas distribution over 3 Gyr, taking into account the outflow of a black hole. Our results indicate that in a homogeneous medium the outflow propagates freely in both directions with the same velocity and its capable of removing a fraction of the gas from the galaxy (it depends on the initial conditions of the outflow). When the SNe are taken into account, the effect of the outflow is substantially reduced. It is necessary an initial velocity around 1000 km/s and a density larger than 0.003 particles.cm−3 for the outflow to propagate. In these conditions, the removal of gas from the galaxy is almost negligible at the end of the 3 Gyr of the simulation. [ABSTRACT FROM AUTHOR]

Using spatially resolved spectroscopy from the SDSS-IV Mapping Nearby Galaxies at APO (MaNGA) survey, we identify 69 dwarf elliptical (dE) galaxies in the nearby Universe fainter than M r = −19 (M B = −18), selected independently of morphology and environment. The majority exhibit coherent rotation in their stellar kinematics, consistent with an origin as morphologically transformed disk galaxies. Six galaxies in this dE sample appear to host Active Galactic Nuclei (AGN) that are likely preventing current star formation through maintenance mode feedback. The ionised gas component of these dEs is typically kinematically offset from the stellar component, suggesting the gas is either recently accreted or outflowing. We therefore demonstrate the potential of IFU spectroscopy for understanding the physical properties of dwarf galaxies in detail. [ABSTRACT FROM AUTHOR]

Properties of dwarf galaxies formed and evolved in the lowest density environment remain largely unexplored and poorly understood. Especially this concerns the low-mass end (M bar < 109 M ⊙). We overview the results of systematic study of a hundred void dwarfs from the nearby Lynx-Cancer void. We describe the ongoing project aiming to form Nearby Void galaxy sample (R < 25 Mpc) over the whole sky. 1354 objects with distances less than 25 Mpc fall within 25 voids delineated by 460 luminous galaxies/groups. The void major sizes range from 13 to 37 Mpc. 1088 of 1354 void galaxies reside deeply in voids, having distances to the nearest luminous neighbour of 2–11 Mpc. 195 nearest void galaxies reside in the Local Volume. We summarize the main statistical properties of the new sample and outline the prospects of study of both, the void dwarf properties and the fine structure of voids. [ABSTRACT FROM AUTHOR]

Based on the data from the Next Generation Virgo cluster Survey (NGVS), we statistically study the photometric properties of globular clusters (GCs), ultra-compact dwarfs (UCDs) and dwarf nuclei in the Virgo core (M87) region. We found an obvious negative color (g - z) gradient in GC system associate with M87, i.e. GCs in the outer regions are bluer. However, such color gradient does not exist in UCD system, neither in dwarf nuclei system around M87. In addition, we found that many UCDs are surrounded by extended, low surface brightness envelopes. The dwarf nuclei and UCDs show different spatial distributions from GCs, with dwarf nuclei and UCDs (especially for the UCDs with visible envelopes) lying at larger distances to the Virgo center. These results support the view that UCDs (at least for a fraction of UCDs) are more tied to dwarf nuclei than to GCs. [ABSTRACT FROM AUTHOR]

Dwarf spheroidal galaxies of the Local Group share a similar characteristic nowadays: a low amount of gas in their interiors. In this work, we present results from a three-dimensional hydrodynamical simulation of the gas inside an object with similar characteristics of the Ursa Minor galaxy. We evolved the initial gas distribution over 3 Gyr, considering the effects of the types Ia and II supernovae. The instantaneous supernovae rates were derived from a chemical evolution model applied to spectroscopic data of the Ursa Minor galaxy. Our simulation shows that the amount of gas that is lost varies with time and galactocentric radius. The highest gas-loss rates occurred during the first 600 Myr of evolution. Our results also indicate that types Ia and II supernovae must be essential drivers of the gas loss in Ursa Minor galaxy (and probably in other similar dwarf galaxies). [ABSTRACT FROM AUTHOR]

We have searched optical u′g′r′i′z′ imaging of 22 deg2 centred on the nearby giant elliptical galaxy NGC 5128 for new dwarf galaxies in the Centaurus A group. We report 45 promising new candidates, which are broadly consistent with the properties of nearby dwarf spheroidal galaxies and extend the size-luminosity relation toward fainter total luminosities and smaller sizes for known dwarf galaxies outside the Local Group (LG). Altogether, these new results show NGC 5128 to be the host of a large reservoir of low-mass dwarf galaxies that is at least as rich as that of the LG and is ripe for detailed follow-up observations. [ABSTRACT FROM AUTHOR]

Absorption line spectroscopy of foreground gas in the spectra of background quasars has revealed some clear cases where neutral gas is present and associated with dwarf galaxies. Spectroscopy of Ly α and low-ionization metal lines can be combined to derive neutral gas phase metallicities. The damped Ly α absorbers (DLAs) in quasar spectra are the clearest cases of absorption by predominantly neutral gas regions. Here we present some results on neutral gas phase metallicities for cases where the DLA is clearly associated with a dwarf galaxy. We find that the neutral gas phase metallicities in these systems are similar to those in other DLAs. We argue that there may be many unrecognized cases where a DLA is actually associated with a dwarf galaxy even though there is a luminous galaxy within 100 kpc of the quasar sightline. [ABSTRACT FROM AUTHOR]

The overwhelming majority of galaxies in the Universe are dwarf galaxies. But although they are important components in understanding galaxy evolution, these systems are typically too faint to be observed at high redshifts. However, we are able to obtain an unobscured view of early star formation and chemical enrichment in these galaxies at low redshift and low-redshift analogs at high redshift. In this talk, I will review the mass-metallicity relation, the mass-star formation rate relation of galaxies, the classifications of dwarf galaxies, and the importance of dwarf galaxies for both astronomy and physics. Then I will introduce some work in our group on connections among between different types of dwarf galaxies,the mass-metallicity relations and the main sequence relations of dwarf galaxies, using the deep optical and near infrared images and spectra of large dwarf galaxy sample. At the end, I will talk about some projects of dwarf galaxies we are working on, including the spectroscopic survey for compact dwarf galaxies using the LAMOST. [ABSTRACT FROM AUTHOR]

Red Supergiant Stars (RSGs) are important probes of stellar and chemical evolution in star-forming environments. They represent the brightest near-IR stellar components of external galaxies and probe the most recent stellar population to provide robust, independent abundance estimates. The Local Group dwarf irregular galaxy, NGC6822, is a reasonably isolated galaxy with an interesting structure and turbulent history. Using RSGs as chemical abundance probes, we estimate metallicities in the central region of NGC6822, finding a suggestion of a metallicity gradient (in broad agreement with nebular tracers), however, this requires further study for confirmation. With intermediate resolution Multi-object spectroscopy (from e.g. KMOS, EMIR, MOSFIRE) combined with state-of-the-art stellar model atmospheres, we demonstrate how RSGs can be used to estimate stellar abundances in external galaxies. In this context, we compare stellar and nebular abundance tracers in NGC 6822 and by combining stellar and nebular tracers we estimate an abundance gradient of −0.18 ± 0.05 dex/kpc. [ABSTRACT FROM AUTHOR]

The hosts of long Gamma-ray bursts (GRBs) are places of intense star-formation, which, at low redshift, are primarily low-mass dwarf starburst galaxies. Spatially resolved studies of these galaxies are still sparse, even more so at high spectral resolution where we can probe gas kinematics, in- and outflows and differences in abundance between different components. Here we present the first high resolution IFU sample of six low redshift GRB hosts, all dwarf starbursts. All galaxies in our sample show evidence for excess emission or broad emission components, with velocities of 100-200 km s−1. For GRB 030329, outflowing gas had also been observed in absorption in spectra of the GRB afterglow. The high velocity emission is usually blue shifted, connected to the brightest star-forming regions and more metal rich than the narrow component associated with the emission of the general host ISM. This gives strong indications that the excess emission/broad component is indeed associated to a starburst wind as observed in many field star-burst galaxies and a sign for the intense ongoing star-formation in those galaxies. [ABSTRACT FROM AUTHOR]

The Milky Way's (MW) satellite population is a powerful probe of warm dark matter (WDM) models as the abundance of small substructures is very sensitive to the properties of the WDM particle. However, only a partial census of the MW's complement of satellite galaxies exists because surveys of the MW's close environs are incomplete both in depth and in sky coverage. We present a new Bayesian analysis that combines the sample of satellites recently discovered by the Dark Energy Survey (DES) with those found in the Sloan Digital Sky Survey (SDSS) to estimate the total satellite galaxy luminosity function down to M v = 0. We find that there should be at least $124_{ - 27}^{ + 40}$ (68% CL, statistical error) satellites as bright or brighter than M v = 0 within 300 kpc of the Sun, with only a weak dependence on MW halo mass. When it comes online the Large Synoptic Survey Telescope should detect approximately half of this population. We also show that WDM models infer the same number of satellites as in ΛCDM, which will allow us to rule out those models that produce insufficient substructure to be viable. [ABSTRACT FROM AUTHOR]

We analysed a population of bright-red (BR) stars in the dwarf irregular galaxy Leo A by using multicolour photometry data obtained with the Subaru/Suprime-Cam (B , V , R , I , Hα) and HST/ACS (F 475 W & F 814 W) instruments. In order to separate the Milky Way (MW) and Leo A populations of red stars, we developed a photometric method, which enabled us to study the spatial distribution of BR stars within the Leo A galaxy. We found a significant difference in the scale-length (S-L) of radial distributions of the "young" and "old" red giant branch (RGB) stars – 0′.82 ± 0′.04 and 1′53 ± 0′.03, respectively. Also, we determined the S-L of BR stars of 0′.85 ± 0′.05, which closely matches that of the "young" RGB stars. Additionally, we found a sequence of peculiar RGB stars and 8 dust-enshrouded stars in the Leo A galaxy. [ABSTRACT FROM AUTHOR]

To investigate molecular composition of low-metallicity environments, we conducted spectral line survey observations in the 3 mm band toward three dwarf galaxies, the Large Magellanic Cloud, IC 10, and NGC 6822 with the Mopra 22 m, the Nobeyama 45 m and the IRAM 30 m, respectively. The rotational transitions of CCH, HCN, HCO+, HNC, CS, SO, 13CO, and 12CO were detected in all three galaxies. We found that the spectral intensity patterns are similar to one another regardless of star formation activities. Compared with Solar-metallicity environments, the molecular compositions of dwarf galaxies are characterized by (1) deficient nitrogen-bearing molecules and (2) enhanced CCH and suppressed CH3OH. These are interpreted (1) as a direct consequence of the lower elemental abundance of nitrogen, and (2) as a consequence of extended photon dominated regions in cloud peripheries due to the lower abundance of dust grains, respectively. [ABSTRACT FROM AUTHOR]

Determining the star formation history (SFH) is key to understand the formation and evolution of dwarf galaxies. Recovering the SFH in resolved galaxies is mostly based on deep colour–magnitude diagrams (CMDs), which trace the signatures of multiple evolutionary stages of their stellar populations. In distant and unresolved galaxies, the integrated light of the galaxy can be decomposed, albeit made difficult by an age–metallicity degeneracy. Another solution to determine the SFH of resolved galaxies is based on evolved stars; these luminous stars are the most accessible tracers of the underlying stellar populations and can trace the entire SFH. Here we present a novel method based on long period variable (LPV) evolved asymptotic giant branch (AGB) stars and red supergiants (RSGs). We applied this method to reconstruct the SFH for IC1613, an irregular dwarf galaxy at a distance of 750 kpc. Our results provide an independent confirmation that no major episode of star formation occurred in IC1613 over the past 5 Gyr. [ABSTRACT FROM AUTHOR]

Based on radio and X-ray observations, it has been suggested that a black hole of mass ∼106 Mʘ resides in the dwarf starburst galaxy Henize 2-10. This unusual finding has important implications for the formation of massive black holes in the early universe since Henize 2-10 can be viewed as a low redshift analog to the first high- z galaxies. We present long-slit HST STIS spectra that include the central radio/X-ray source. While recent VLT-MUSE spectroscopic observations with 0″.7 seeing show no change in ionization near the central source, our higher spatial resolution STIS observations identify a distinct compact region at the location of the radio/X-ray source. Initial analysis reveals broader (FWHM ∼ 380 km s-1) blue-shifted lines of low ionization. Our analysis focuses on testing two scenarios: a LINER-like AGN and a young (few decades) SNR. [ABSTRACT FROM AUTHOR]

RR Lyrae stars are powerful tools to study their host populations. Information such as distance, metallicity, reddening, and age can be obtained from their pulsation properties. Dwarf spheroidal (dSph) galaxies are the most common type of galaxy in the Local Group. They are found around massive hosts such as the Milky Way (MW) and M31 and are suggested to be the present-day counterparts to systems from which spheroids and stellar halos of larger galaxies were assembled. By comparing RR Lyraes in these dSphs with their host galaxies, we hope to understand more about the formation of these galaxies. In order to achieve this goal, we have analyzed six fields in M31 using archival imaging from the Hubble Space Telescope. Published data for M31, M33, and several M31 dSphs are also included. The results are then compared with those in the MW to better constrain the early history of these systems. [ABSTRACT FROM AUTHOR]

We present dark-matter minihalo models for the Ultra-Compact High Velocity HI Clouds (UCHVCs) recently discovered in the 21 cm ALFALFA survey. We assume gravitational confinement of ~104 K HI gas by flat-cored dark-matter subhalos within the Local Group. For the UCHVCs we calculate the photoionization-limited hydrostatic gas profiles for any distance-dependent total observed HI mass and predict the associated (projected) HI half-mass radii. The observed 21 cm fluxes and half-mass angular radii then constrain the source distances or DM halo parameters. As a consistency check we model the gas-rich dwarf galaxy Leo T, for which the distance is known (420 kpc) and there is a well-resolved HI column density profile. We derive an upper limit for the pressure of any enveloping hot IGM gas at the distance of Leo T. Our analysis supports the scenario that some of the UCHVCs may constitute a population of 21-cm-selected but optically-faint dwarf galaxies in the Local Volume. [ABSTRACT FROM AUTHOR]

Half-dozen of extreme representatives of void dwarf galaxy population were found in our study of evolutionary status of a hundred galaxies in the nearby Lynx-Cancer void. They are very gas-rich, extremely low-metallicity [7.0 < 12 + log(O/H) < ∼ 7.3] objects, with blue colours of outer parts. The colours indicate the ages of the oldest visible stellar population of one to a few Gyr. They all are intrinsically faint, mostly Low Surface Brightness dwarfs, with M B range of –9.5 m to –14 m . Thus, their finding is a subject of the severe observational selection. The recent advancement in search for such objects in other nearby voids resulted in doubled their total number. We summarize all available data on this group of unusual void dwarf galaxies and discuss them in the general context of very low metallicity galaxies and their possible formation and evolutionary scenarios. [ABSTRACT FROM AUTHOR]