In the standard cosmological model, dark matter drives the structure formation and constructs potential wells within which galaxies may form. The baryon fraction in dark halos can reach the universal value (15.7%) in massive clusters and decreases rapidly as the mass of the system decreases. The formation of dwarf galaxies is sensitive both to baryonic processes and the properties of dark matter owing to the shallow potential wells in which they form. In dwarf galaxies in the Local Group, dark matter dominates the mass content even within their optical-light half-radii (r_e ~ 1 kpc). However, recently it has been argued that not all dwarf galaxies are dominated by dark matter. Here we report 19 dwarf galaxies that could consist mainly of baryons up to radii well beyond r_e, at which point they are expected to be dominated by dark matter. Of these, 14 are isolated dwarf galaxies, free from the influence of nearby bright galaxies and high dense environments. This result provides observational evidence that could challenge the formation theory of low-mass galaxies within the framework of standard cosmology. Further observations, in particular deep imaging and spatially-resolved kinematics, are needed to constrain the baryon fraction better in such galaxies.
In the standard Lambda-CDM paradigm, dwarf galaxies are expected to be dark-matter-rich, as baryonic feedback is thought to quickly drive gas out of their shallow potential wells and quench star formation at early epochs. Recent observations of local dwarfs with extremely low dark matter content appear to contradict this picture, potentially bringing the validity of the standard model into question. We use NewHorizon, a high-resolution cosmological simulation, to demonstrate that sustained stripping of dark matter, in tidal interactions between a massive galaxy and a dwarf satellite, naturally…
Recent observations have discovered a number of extremely gas-rich very faint dwarf galaxies possibly embedded in low-mass dark matter halos. We investigate star formation histories of these gas-rich dwarf ("almost dark") galaxies both for isolated and interacting/merging cases. We find that although star formation rates (SFRs) are very low (<10^-5 M_sun/yr) in the simulated dwarfs in isolation for the total halo masses (M_h) of 10^8-10^9 M_sun, they can be dramatically increased to be ~ 10^{-4} M_sun/yr when they interact or merge with other dwarfs. These interacting faint dwarfs with central…
Dark Matter (DM) is an ingredient essential to the current cosmological concordance model. It provides the gravitational pull needed for the baryons to form galaxies. Therefore, the existence of galaxies without DM is both disquieting and extremely interesting. Guo et al. recently presented "further evidence for a population of DM-deficient dwarf galaxies", however, their analysis bypasses the triaxiality of the dwarf galaxies. We carry out a Monte Carlo simulation showing how triaxiality must be considered to measure dynamical masses from projected axial ratios, calling into question the evid…
The recent discovery of diffuse dwarf galaxies that are deficient in dark matter appears to challenge the current paradigm of structure formation in our Universe. We describe the numerical experiments to determine if the so-called dark matter deficient galaxies (DMDGs) could be produced when two gas-rich, dwarf-sized galaxies collide with a high relative velocity of $\sim 300\,{\rm kms^{-1}}$. Using idealized high-resolution simulations with both mesh-based and particle-based gravito-hydrodynamics codes, we find that DMDGs can form as high-velocity galaxy collisions separate dark matter from t…
The nature of the several microlensing events observed by the MACHO team towards the Large Magellanic Cloud (LMC) is still a subject of debate. Low-mass substellar objects and stars with masses larger than ~M_{sun} have been ruled out as major components of a Massive Astrophysical Compact Halo Object (MACHO) Galactic halo, while stars of half a solar mass seem to be viable candidates. Main sequence stars have been already discarded, and there are tight restrictions on the role played by white dwarfs with hydrogen-dominated atmospheres. In this paper we evaluate the contribution to the dark mat…
It has been suggested that the dark matter-deficient galaxies NGC1052-DF2 and NGC1052-DF4 might not be members of the NGC1052 group but in the foreground at $\sim 13$ Mpc, and satellites of the bright spiral galaxy NGC1042. We previously showed that the CMDs of the galaxies are inconsistent with this hypothesis, and derived distances of 19-20 Mpc from their surface brightness fluctuation signals. Here we note that NGC1042 is almost certainly a member of the NGC1052 group as well, based on its radial velocity, the HI distribution in the NGC1052/NGC1042 system, and the Tully-Fisher relation.
It has been suggested that a trail of diffuse galaxies, including two dark matter deficient galaxies (DMDGs), in the vicinity of NGC1052 formed because of a high-speed collision between two gas-rich dwarf galaxies, one bound to NGC1052 and the other one on an unbound orbit. The collision compresses the gas reservoirs of the colliding galaxies, which in turn triggers a burst of star formation. In contrast, the dark matter and pre-existing stars in the progenitor galaxies pass through it. Since the high pressures in the compressed gas are conducive to the formation of massive globular clusters (…
Satellite galaxies undergo a variety of physical processes when they are accreted by groups and clusters, often resulting in the loss of baryonic and dark matter (DM) mass. In this work, we evaluate the predictions from the IllustrisTNG hydrodynamical simulation regarding the evolution of the matter content of satellites, focusing on a population that are accreted at $z>1$ and retain their identity as satellites down to $z=0$. At fixed host halo mass, the amount of DM and stellar mass stripped depends mostly on the pericentric distance, $d_{\rm peri}$, here normalised by host halo virial radiu…
Observations show that spiral galaxies in galaxy clusters tend to have on average less neutral hydrogen (HI) than galaxies of the same type and size in the field. There is accumulating evidence that such HI-deficient galaxies are also relatively frequent in galaxy groups. An important question is, which mechanisms are responsible for the gas deficiency in galaxy groups. To gain a better understanding of how environment affects the gas content of galaxies, we identified a sample of six HI-deficient galaxies from the HI Parkes All Sky Survey (HIPASS) using HI-optical scaling relations. One of th…
Imprints of tidal interactions on the stellar distribution of satellite galaxies: implications for dark matter deficient galaxies
Institute for Astronomy, School of Physics, Zhejiang University（浙江大学物理学院天体物理研究所）；Center for Cosmology and Computational Astrophysics, Zhejiang University（浙江大学宇宙学与计算天体物理中心）；Department of Astronomy, Yale University（耶鲁大学天文学系）
Low mass galaxies are expected to be dark matter dominated even within their centrals. Recently two observations reported two dwarf galaxies in group environment with very little dark matter in their centrals. We explore the population and origins of dark-matter-deficient galaxies (DMDGs) in two state-of-the-art hydrodynamical simulations, the EAGLE and Illustris projects. For all satellite galaxies with $10^9<M_*<10^{10}$ M$_{\odot}$ in groups with $M_{200}>10^{13}$ M$_{\odot}$, we find that about $2.6\%$ of them in the EAGLE, and $1.5\%$ in the Illustris are DMDGs with dark matter fractions …
How diffuse dwarf galaxies that are deficient in dark matter -- such as NGC1052-DF2 and NGC1052-DF4 -- formed remains a mystery. Along with their luminous member globular clusters (GCs), the so-called dark matter deficient galaxies (DMDGs) have challenged observers and theorists alike. Here we report a suite of galaxy collision simulations using the adaptive mesh refinement code ENZO with 1.25 pc resolution, which demonstrates that high-velocity galaxy collisions induce the formation of DMDGs and their star clusters (SCs) simultaneously. With a numerical resolution significantly better than ou…
Previous studies have shown that dark matter deficient galaxies (DMDG) such as NGC1052-DF2 (hereafter DF2) can result from tidal stripping. An important question, though, is whether such a stripping scenario can explain DF2's large specific frequency of globular clusters (GCs). After all, tidal stripping and shocking preferentially remove matter from the outskirts. We examine this using idealized, high-resolution simulations of a regular dark matter dominated galaxy that is accreted onto a massive halo. As long as the initial (pre-infall) dark matter halo of the satellite is cored, which is co…
Dark-Matter-Deficient Galaxies from Collisions: A New Probe of Bursty Feedback and Dark Matter Physics
Key Laboratory of Dark Matter and Space Astronomy, Purple Mountain Observatory, Chinese Academy of Sciences（中国科学院紫金山天文台暗物质与空间天文重点实验室）；School of Astronomy and Space Science, University of Science and Technology of China（中国科学技术大学天文与空间科学学院）
Surveying dark matter deficient galaxies (those with dark matter mass to stellar mass ratio $M_{\rm dm}/M_{\rm star}<1$) in the Illustris simulation of structure formation in the flat-$Λ$CDM cosmogony, we find $M_{\rm star} \approx 2 \times 10^8\, M_\sun$ galaxies that have properties similar to those ascribed by \citet{vanDokkumetal2018a} to the ultra-diffuse galaxy NGC1052-DF2. The Illustris simulation also contains more luminous dark matter deficient galaxies. Illustris galaxy subhalo 476171 is a particularly interesting outlier, a massive and very compact galaxy with $M_{\rm star} \approx …
We have studied the spectral time variations of candidate luminous blue variable stars (cLBV) in two low-metallicity blue compact dwarf galaxies, DDO 68 and PHL 293B. The LBV in DDO 68, located in HII region #3, shows an outburst, with an increase of more than 1000 times in Halpha luminosity during the period 2008-2010. The broad emission of the HI and HeI lines display a P Cygni profile, with a relatively constant terminal velocity of ~800 km/s, reaching a maximum luminosity L(Halpha) of ~2x10^38 erg/s, with a FWHM of ~1000-1200 km/s. On the other hand, since the discovery of a cLBV in 2001 i…
We have obtained optical spectroscopy of one of the most metal-poor dwarf star-forming galaxies (SFG) in the local Universe, J1234+3901, with the Large Binocular Telescope (LBT)/Multi-Object Dual Spectrograph (MODS). This blue compact dwarf (BCD) galaxy with a redshift z=0.133 was selected from the Data Release 14 (DR14) of the Sloan Digital Sky Survey (SDSS). Its properties are extreme in many ways. Its oxygen abundance 12 + log O/H = 7.035+/-0.026 is among the lowest ever observed for a SFG. Its absolute magnitude Mg = -17.35 mag makes it the brightest galaxy among the known BCDs with 12 + l…
Understanding the properties of dwarf galaxies is important not only to put them in their proper cosmological context, but also to understand the formation and evolution of the most common type of galaxies. Dwarf galaxies are divided into two main classes, dwarf irregulars (dIrrs) and dwarf spheroidals (dSphs), which differ from each other mainly because the former are gas-rich objects currently forming stars, while the latter are gas-deficient with no on-going star formation. Transition types (dT) are thought to represent dIs in the process of losing their gas, and can therefore shed light in…
The Rogue One: A Dark Matter-Dominated Dwarf in the Trail of Dark Matter-Deficient Galaxies
Swinburne University of Technology（斯威本科技大学）；Universidade do Porto（波尔图大学）；San José State University（圣何塞州立大学）；University of California Santa Cruz（加州大学圣克鲁兹分校）；University of Toronto（多伦多大学）
CROCODILE-SIDM: Tidal Formation of Dark Matter-Deficient Galaxies as a Test Case
The University of Osaka（大阪大学）；The University of Tokyo（东京大学）；University of Nevada, Las Vegas（内华达大学拉斯维加斯分校）；EPFL（洛桑联邦理工学院）；Excellence Cluster ORIGINS（ORIGINS卓越集群）；European Southern Observatory (ESO)（欧洲南方天文台）；Universitäts-Sternwarte, Fakultät für Physik（大学天文台，物理学院）
We perform a deep wide-field imaging survey of nearby galaxies using H$α$ and broadband filters to investigate the characteristics of star formation in galaxies. Motivated by the finding that star formation rates (SFRs) derived from H$α$ fluxes in dwarf galaxies are systematically lower than those inferred from far-ultraviolet (FUV) fluxes, we attempt to determine whether the same trend exists in the extended disks of two star-forming galaxies. We perform spatially resolved photometry using grid-shaped apertures to measure the FUV and H$α$ fluxes of star-forming regions. We also perform spectr…
It was recently proposed that the dark matter-deficient ultra-diffuse galaxies DF2 and DF4 in the NGC1052 group could be the products of a "bullet dwarf" collision between two gas-rich progenitor galaxies. In this model DF2 and DF4 formed at the same time in the immediate aftermath of the collision, and a strong prediction is that their globular clusters should have nearly identical stellar populations. Here we test this prediction by measuring accurate F606W-F814W colors from deep HST/ACS imaging. We find that the clusters are extremely homogeneous. The mean color difference between the globu…
The current Lambda CDM cosmological model predicts that galaxy evolution proceeds more slowly in lower density environments, suggesting that voids are a prime location to search for relatively pristine galaxies that are representative of the building blocks of early massive galaxies. To test the assumption that void galaxies are more pristine, we compare the evolutionary properties of a sample of dwarf galaxies selected specifically to lie in voids with a sample of similar isolated dwarf galaxies in average density environments. We measure gas-phase oxygen abundances and gas fractions for eigh…
Using fully GPU $N$-body simulations, we demonstrate for the first time that subhalos sink and transfer energy via dynamical friction into the centres of dwarf galaxies. This dynamical heating kicks any central massive black hole (MBH) out to tens of parsecs, especially at early epochs ($z$=1.5-3). This mechanism helps explain the observed off-center BHs in dwarf galaxies and also predicts that off-center BHs are more common in higher mass dwarf galaxies since dynamical friction becomes significantly weaker and BHs take more time to sink back towards the centres of their host galaxies. One con…
A recent survey of the Galaxy and M31 reveals that more than 90% of dwarf galaxies within 270 kpc of their host galaxy are deficient in HI gas. At such an extreme radius, the coronal halo gas is an order of magnitude too low to remove HI gas through ram-pressure stripping for any reasonable orbit distribution. However, all dwarfs are known to have an ancient stellar population (\geq 10 Gyr) from early epochs of vigorous star formation which, through heating of HI, could allow the hot halo to remove this gas. Our model looks at the evolution of these dwarf galaxies analytically as the host-gala…
We review basic properties of the population of dwarf galaxies in the Local Group focusing on dwarf spheroidal galaxies found in the immediate vicinity of the Milky Way. The evidence for dark matter in these objects is critically assessed. We describe the methods of dynamical modelling of such objects, using a few examples of the best-studied dwarfs and discuss the sources of uncertainties in mass estimates. We conclude with perspectives for dwarf galaxies as targets for dark matter detection experiments.
Radial velocities measured with the 6-meter telescope are given for 5 faint dwarf galaxies. All of these galaxies are confirmed as very nearby objects. Two of them, KK135 (dIr) and UGC 1703 (dSph/dTr), are local isolated dwarfs, and the three others, UGCA 127sat (dIr), NGC 2683dw1 (dIr), and NGC891dwA (dTr), belong to companions of nearby massive spirals.
In the context of the current $Λ$CDM cosmological model small dark matter haloes are abundant and satellites of dwarf galaxies are expected to be predominantly dark. Since low mass galaxies have smaller baryon fractions interactions with these satellites may leave particularly dramatic imprints. We uncover the influence of the most massive of these dark satellites on disky dwarf galaxies and the possible dynamical and morphological transformations that result from these interactions. We use a suite of carefully set-up, controlled simulations of isolated dwarf galaxies. The primary dwarf galaxi…
The neutral atomic hydrogen (HI) content of spiral galaxies has been observed to vary with environment, with more HI-deficient spirals residing in high density environments. This can be attributed to environmental effects such as ram pressure stripping and tidal interactions, which remove HI from the discs of galaxies. However, some spirals in low-density environments have also been observed to have relatively low HI mass fractions. The low densities of the Intergalactic Medium and lack of nearby galaxies in such environments make ram pressure stripping and tidal interactions unlikely candidat…
R Coronae Borealis (RCB) and dustless Hydrogen-deficient Carbon (dLHdC) stars are believed to be remnants of low mass white dwarf mergers. These supergiant stars have peculiar hydrogen-deficient carbon-rich chemistries and stark overabundances of $^{18}$O. RCB stars undergo dust formation episodes resulting in large-amplitude photometric variations that are not seen in dLHdC stars. Recently, the sample of known dLHdC stars in the Milky Way has more than quintupled with the discovery of 27 new dLHdC stars. We present medium resolution (R$\approx3000$) near-infrared spectra of 20 newly discovere…
Context. The fraction of field binaries on retrograde orbits about the Milky Way is significantly lower compared to its prograde counterpart. Chemical and dynamical evidence suggests that the retrograde stellar population originates from $ω$ Centauri, which is either the most massive globular cluster (GC) of the Milky Way or the putative core of a former dwarf galaxy. Aims. Star formation conditions required to produce the retrograde binary population are constrained assuming that the retrograde stellar population originates from $ω$ Centauri's progenitor. Methods. We match the observed low bi…
In addition to their low stellar densities, ultra-diffuse galaxies (UDGs) have a broad variety of dynamical mass-to-light ratios, ranging from dark matter (DM) dominated systems to objects nearly devoid of DM. To investigate the origin of this diversity, we develop a simple, semi-empirical model that predicts the structural evolution of galaxies, driven by feedback from massive star clusters, as a function of their departure from the mean SMHM relation. The model predicts that a galaxy located $\gtrsim 0.5$ dex above the mean relation at $M_{\rm halo}=10^{10}M_{\odot}$ will host a factor of $\…
Astronomers have been using the measured luminosity to estimate the {\em luminous mass} of stars, based on empirically established mass-to-light ratio which seems to be only applicable to a special class of stars---the main-sequence stars---with still considerable uncertainties. Another basic tool to determine the mass of a system of stars or galaxies comes from the study of their motion, as Newton demonstrated with his law of gravitation, which yields the {\em gravitational mass}. Because the luminous mass can at best only represent a portion of the gravitational mass, finding the luminous ma…
Globular clusters (GCs) are dense, gravitationally bound systems of thousands to millions of stars. They are preferentially associated with the oldest components of galaxies, and measurements of their composition can therefore provide insight into the build-up of the chemical elements in galaxies in the early Universe. We report a massive GC in the Andromeda Galaxy (M31) that is extremely depleted in heavy elements. Its iron abundance is about 800 times lower than that of the Sun, and about three times lower than in the most iron-poor GCs previously known. It is also strongly depleted in magne…
Like massive galaxies, dwarf galaxies are expected to undergo major mergers with other dwarfs. However, the end state of these mergers and the role that merging plays in regulating dwarf star formation is uncertain. Using imaging from the Hyper Suprime-Cam Subaru Strategic program, we construct a sample of dwarf-dwarf mergers and examine the star formation and host properties of the merging systems. These galaxies are selected via an automated detection algorithm from a sample of 6875 spectroscopically selected isolated dwarf galaxies at $z<0.12$ and $\log(M_\star/M_\odot)<9.6$ from the Galaxy…
A model is proposed for the formation of gas-rich dwarf irregular galaxies and gas-poor, rotating dwarf elliptical galaxies following the interaction between two giant galaxies as a function of space density. The formation of dwarf galaxies is considered to depend on a random variable, the tidal index theta, an environmental parameter defined by Karachentsev et al. (2004), such that for theta less than zero, the formation of dwarf irregular galaxy is assured whereas for theta greater than zero, the formation of dwarf ellipticals is preferred. It is found that for particular ranges of the inter…
Mergers are a common phenomenon in hierarchical structure formation, especially for massive galaxies and clusters, but their importance for dwarf galaxies in the Local Group remains poorly understood. We investigate the frequency of major mergers between dwarf galaxies in the Local Group using the ELVIS suite of cosmological zoom-in dissipationless simulations of Milky Way- and M31-like host halos. We find that ~10% of satellite dwarf galaxies with M_star > 10^6 M_sun that are within the host virial radius experienced a major merger of stellar mass ratio closer than 0.1 since z = 1, with a low…
Integral field unit (IFU) spectra of face-on star-forming galaxies from the MaNGA survey are stacked in radial bins so as to reach a S/N high enough to measure emission lines and Lick indices out to 2.5-3 R_e. Two thirds of galaxies have stellar populations in the outer disks that are older, more metal poor and less dusty than in the inner disks. Recent bursts of star formation have occurred more frequently in the outer disk, but extinction-corrected Halpha equivalent widths are significantly lower at fixed D_n(4000) in these regions. I examine the properties of a subset of galaxies with the t…
We present the results of the spectroscopic and photometric follow-up of two field galaxies that were selected as possible stellar counterparts of local high velocity clouds. Our analysis shows that the two systems are distant (D>20 Mpc) dwarf irregular galaxies unrelated to the local HI clouds. However, the newly derived distance and structural parameters reveal that the two galaxies have luminosities and effective radii very similar to the recently identified Ultra Diffuse Galaxies (UDGs). At odds with classical UDGs, they are remarkably isolated, having no known giant galaxy within ~2.0 Mpc…
We study how the void environment affects galactic chemical evolution by comparing the oxygen and nitrogen abundances of dwarf galaxies in voids with dwarf galaxies in denser regions. Using spectroscopic observations from SDSS DR7, we estimate oxygen, nitrogen, and neon abundances of 889 void dwarf galaxies and 672 dwarf galaxies in denser regions. A substitute for the [OII] 3727 doublet is developed, permitting oxygen abundance estimates of SDSS dwarf galaxies at all redshifts with the Direct Te method. We find that void dwarf galaxies have about the same oxygen abundance and Ne/O ratio, slig…
The tidal tails of globular clusters have been shown to be sensitive to the external tidal field. We investigate how Galactic globular clusters with observed tails are affected by satellite dwarf galaxies by simulating tails in galaxy models with and without dwarf galaxies. The simulations indicate that tidal tails can be subdivided into into three categories based on how they are affected by dwarf galaxies: 1) dwarf galaxies perturb the progenitor cluster's orbit (NGC 4590, Pal 1, Pal 5), 2) dwarf galaxies perturb the progenitor cluster's orbit and individual tail stars (NGC 362, NGC 1851, NG…
We have identified two channels for the formation of compact dwarf galaxies in the Illustris simulation by reconstructing mass and distance histories of candidates located in the vicinity of the simulation's most massive cluster galaxies. One channel is tidal stripping of Milky Way mass galaxies that form outside of clusters and eventually sink into them, spiraling in toward central massive objects. Second channel of formation is an in-situ formation (in reference to the parent cluster) of dwarf mass galaxies, with negligible evolution and limited change in stellar mass. We find 19 compact dwa…
I present a dynamical analysis of the measured redshifts and distances of 64 dwarf galaxies at distances between 50 kpc and 2.6 Mpc. These dwarfs are assumed to move as test particles in the gravitational field of 12 massive actors---galaxies and groups of galaxies---under the mixed boundary conditions imposed by cosmology. The model fits most of the measured dwarf distances and redshifts. But more work, perhaps on the gravitational interaction among dwarf galaxies, is required to account for the motions of six galaxies in the NGC 3109 association and two in the DDO 210 association. The sample…
Previous studies have shown that the large, diffuse galaxies NGC1052-DF2 and NGC1052-DF4 both have populations of unusually luminous globular clusters as well as a very low dark matter content. Here we present newly-obtained deep Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) imaging of one of these galaxies, NGC1052-DF4. We use these data to measure the distance of the galaxy from the location of the tip of the red giant branch (TRGB). We find a rapid increase in the number of detected stars fainter than $m_{F814W} \sim 27.3$, which we identify as the onset of the red giant br…
In search for a synthetic understanding, a scenario for the evolution of the star formation rate and the chemical abundances in galaxies is proposed, combining gas infall from galactic halos, outflow of gas by supernova explosions, and an oscillatory star formation process. The oscillatory star formation model is a consequence of the modelling of the fractional masses changes of the hot, warm and cold components of the interstellar medium. The observed periods of oscillation vary in the range $(0.1-3.0)\times10^{7}$\,yr depending on various parameters existing from giant to dwarf galaxies. The…
The nature of possible evolutionary pathways between various types of dwarf galaxies is still not fully understood. Blue compact dwarf galaxies (BCDs) provide a unique window into dwarf galaxy formation and evolution and are often thought of as an evolutionary stage between different classes of dwarf galaxies. In this study we use deep optical and near-infrared observations of the underlying hosts of BCDs in order to study the structural differences between different types of dwarf galaxies. When compared with dwarf irregular galaxies of similar luminosities, we find that the underlying hosts …
