Small Satellite Galaxies Research Articles

A Timeline of the M81 Group: Properties of the Extended Structures of M82 and NGC 3077

Mergers of and interactions between galaxies imprint a wide diversity of morphological, dynamical, and chemical characteristics in stellar halos and tidal streams. Measuring these characteristics elucidates aspects of the progenitors of the galaxies we observe today. The M81 group is the perfect galaxy group to understand the past, present, and future of a group of galaxies in the process of merging. Here, we measure the end of star formation (t 90) and metallicity ([M/H]) of the stellar halo of M82 and the eastern tidal stream of NGC 3077 to: (1) test the idea that M82 possesses a genuine stellar halo, formed before any interaction with M81; (2) determine if NGC 3077's tidal disruption is related to the star formation history in its tails; and (3) create a timeline of the assembly history of the central trio in the M81 group. We argue that M82 possesses a genuine, metal-poor ([M/H] ∼ −1.62 dex) stellar halo, formed from the merger of a small satellite galaxy roughly 6.6 Gyr ago. We also find that the stars present in NGC 3077's tails formed before tidal disruption with M81, and possess a roughly uniform metallicity as shown in S. Okamoto et al., implying that NGC 3077's progenitor had significant population gradients. Finally, we present a timeline of the central trio’s merger/interaction history.

Chronology of the chemical enrichment of the old Galactic stellar populations

Context. Over its history, the Milky Way has accreted several smaller satellite galaxies. These mergers added stars and gas to the Galaxy and affected the properties of the pre-existing stellar populations. Stellar chemical abundances and ages are needed to establish the chronological order of events that occur before, during, and after such mergers. Aims. We report the precise ages (∼6.5%) and chemical abundances for the TITANS, a sample of old metal-poor dwarfs and subgiants with accurate atmospheric parameters. We also obtain ages with an average precision of 10% for a selected sample of dwarf stars from the GALAH survey. We use these stars, located within ∼1 kiloparsec of the Sun, to analyse the chronology of the chemical evolution of in situ and accreted metal-poor stellar populations. Methods. We determined ages via isochrone fitting. For the TITANS, we determined Mg, Si, Ca, Ti, Ni, Ba, and Eu abundances using spectrum synthesis. The [Mg/Fe] abundances of the GALAH stars were re-scaled to be consistent with the abundances of the TITANS. We separated stellar populations by primarily employing chemical abundances and orbits. Results. We find that star formation in the so-called Gaia-Enceladus or Gaia-Sausage galaxy, the last major system to merge with the Milky Way, lasted at least 3 billion years and got truncated 9.6 ± 0.2 billion years ago. This marks with a very high level of precision the last stage of its merging process. We also identified stars of a heated metal-poor in-situ population with virtually null net rotation, probably disturbed by several of the early Milky Way mergers. We show that this population is more metal-rich than Gaia-Enceladus at any moment in time. Conclusions. The sequence of events uncovered in our analysis supports the hypothesis that Gaia-Enceladus truncated the formation of the high-α disc and caused the gas infall that forms the low-α disc.

Mock HI-galaxy catalogs and HI mass functions for future large-scale surveys

One of the interesting modern astrophysics topics is the discrepancy in HI-galaxy number density between the standard model ΛCDM prediction and observations, e.g., ALFALFA and HIPASS. Since then, the simulation over-predicted the abundance and HI mass (MHI) of small satellite and low-mass galaxies. Both problems still require sensitive instruments for wider and deeper HI surveys to increase the number of low-mass HI-galaxy samples. Current HI-galaxy surveys can only reach down to ∼ 107 Solar mass (M⊙). The sensitivity of upcoming radio surveys such as FAST or ASKAP are expected to reach lower MHI and higher redshift(z) HI-galaxy data. Alternatively, the HI source catalogs from cosmological simulations are used to determine the expected HIMF with the upcoming HI survey specification. This work aims to compare the HIMF in various HI conversion recipes. Various HIMF were determined by using the Multidark(Planck) catalog with the box (MB) and the light-cone (ML) types, and were parameterized by the Schechter function. In comparison to the Schechter function, we discovered that the HIMF from cold gas mass (Mcold) conversion method begins to diverge on MHI ≲ 108.5 M⊙. The HIMF from halo mass (Mhalo) recipe, which was constrained by the optical counterpart galaxy clustering, provided a satisfactory relevant shape of HIMF. The best-fit parameter α of MB and ML are equal to 0.00 ± 0.010 and −0.02 ± 0.0026, respectively. The flatter α values from simulations could indicate that the number density of low-MHI is underestimated.

The Fornax Cluster VLT Spectroscopic Survey

Context. Substructures in stellar haloes are a strong prediction of galaxy formation models in ΛCDM. Cold streams such as those from small satellite galaxies are extremely difficult to detect and kinematically characterize. The COld STream finder Algorithm (COSTA) is a novel algorithm able to find streams in the phase space of planetary nebulae (PNe) and globular cluster (GC) populations. COSTA isolates groups of (N) particles with small velocity dispersion (between 10 km s−1 and ∼120 km s−1) using an iterative (n) sigma-clipping over a defined number of (k) neighbor particles. Aims. We applied COSTA to a catalog of PNe and GCs from the Fornax Cluster VLT Spectroscopic Survey (FVSS) within ∼200 kpc from the cluster core in order to detect cold substructures and characterize their kinematics (mean velocity and velocity dispersion). Methods. We selected more than 2000 PNe and GCs from the FVSS catalogs and adopted a series of optimized setups of the COSTA parameters based on Montecarlo simulations of the PN and GC populations to search for realistic stream candidates. We find 13 cold substructures with velocity dispersion ranging from ∼20 to ∼100 km s−1, which are likely associated either to large galaxies or to ultra-compact dwarf (UCD) galaxies in the Fornax core. Results. The luminosities of these streams show a clear correlation with internal velocity dispersion, and their surface brightness correlates with their size and distance from the cluster center, which is compatible with the dissipative processes producing them. However, we cannot exclude that some of these substructures formed by violent relaxation of massive satellites that finally merged into the central galaxy. Among these substructures we have: (1) a stream connecting NGC 1387 to the central galaxy, NGC 1399, previously reported in the literature; (2) a new giant stream produced by the interaction of NGC 1382 with NGC 1380 and (possibly) NGC 1381; (3) a series of streams kinematically connected to nearby UCDs; and (4) clumps of tracers with no clear kinematical association to close cluster members. Conclusions. We show evidence for a variety of cold substructures predicted in simulations. Most of the streams are kinematically connected to UCDs, supporting the scenario that they can be remnants of disrupted dwarf systems. However, we also show the presence of long coherent substructures connecting cluster members and isolated clumps of tracers possibly left behind by their parent systems before these merged into the central galaxy. Unfortunately, the estimated low-surface brightness of these streams does not allow us to find their signatures in the current imaging data and deeper observations are needed to confirm them.

Dancing in the void: hydrodynamical N-body simulations of the extremely metal-poor galaxy DDO 68

ABSTRACT Using hydrodynamical N-body simulations, we show that the observed structure and kinematics of the extremely metal-poor dwarf irregular galaxy DDO 68 is compatible with a merger event with at least two smaller satellite galaxies. We were able to obtain a self-consistent model that simultaneously reproduces several of its observed features, including the very asymmetric and disturbed shape of the stellar component, the overall ${\rm H\, \small {\rm I}}$ distribution and its velocity field, the arc-like stellar structure to the west, and the low surface brightness stellar stream to the north. The model implies the interaction of the main progenitor of DDO 68 with two systems with dynamical masses $7\times 10^8\, \mathrm{ M}_{\odot }$ and almost $10^8\, \mathrm{ M}_{\odot }$ – 1/20 and 1/150 times the dynamical mass of DDO 68, respectively. We show that the merger between DDO 68 and the most massive of its satellites offers a route to explain the large offset of DDO 68 from the mass–metallicity relation. Assuming that the interacting galaxies have metallicities prior to the merger compatible with those of galaxies with similar stellar masses, we provide quantitative evidence that gas mixing alone does not suffice at diluting the gas of the two components; according to our simulations, the ${\rm H\, \small {\rm II}}$ regions observed along the cometary tail trace the low metallicity of the accreted satellite rather than that of DDO 68’s main body. In this case, the mass corresponding to the low metallicity is that of the secondary body and DDO 68 becomes consistent with the mass–metallicity relation.

Chronologically dating the early assembly of the Milky Way

The standard cosmological model ($\Lambda$-CDM) predicts that galaxies are built through hierarchical assembly on cosmological timescales$^{1,2}$. The Milky Way, like other disc galaxies, underwent violent mergers and accretion of small satellite galaxies in its early history. Thanks to Gaia-DR2$^3$ and spectroscopic surveys$^4$, the stellar remnants of such mergers have been identified$^{5-7}$. The chronological dating of such events is crucial to uncover the formation and evolution of the Galaxy at high redshift, but it has so far been challenging owing to difficulties in obtaining precise ages for these oldest stars. Here we combine asteroseismology -- the study of stellar oscillations -- with kinematics and chemical abundances, to estimate precise stellar ages ($\sim$ 11%) for a sample of stars observed by the $\mathit{Kepler}$ space mission$^8$. Crucially, this sample includes not only some of the oldest stars that were formed inside the Galaxy, but also stars formed externally and subsequently accreted onto the Milky Way. Leveraging this resolution in age, we provide compelling evidence in favour of models in which the Galaxy had already formed a substantial population of its stars (which now reside mainly in its thick disc) before the in-fall of the satellite galaxy Gaia-Enceladus/Sausage$^{5,6}$ around 10 billions years ago

Tracing the simulated high-redshift circum-galactic medium with Lyman α emission

Abstract With the Multi Unit Spectroscopic Explorer (MUSE), it is now possible to detect spatially extended Lyman α (Lyα) emission from individual faint (MUV ∼ −18) galaxies at redshifts, 3 < z < 6, tracing gas out to circum-galactic scales. To explore the implications of such observations, we present a cosmological radiation hydrodynamics simulation of a single galaxy, chosen to be typical of the Lyα-emitting galaxies detected by MUSE in deep fields. We use this simulation to study the origin and dynamics of the high-redshift circum-galactic medium (CGM). We find that the majority of the mass in the diffuse CGM is comprised of material infalling for the first time towards the halo center, but with the inner CGM also containing a comparable amount of mass that has moved past first-pericentric passage, and is in the process of settling into a rotationally supported configuration. Making the connection to Lyα emission, we find that the observed extended surface brightness profile is due to a combination of three components: scattering of galactic Lyα emission in the CGM, in-situ emission of CGM gas (mostly infalling), and Lyα emission from small satellite galaxies. The weight of these contributions vary with distance from the galaxy such that (1) scattering dominates the inner regions (r < 7 kpc), at surface brightness larger than a few 10−19 cgs, (2) all components contribute equally around r ∼ 10 kpc (or SB∼10−19), and (3) the contribution of small satellite galaxies takes over at large distances (or SB∼10−20).

Spectroscopic decomposition of the galaxy and halo of the cD galaxyNGC 3311

Information on the star-formation histories of cD galaxies and their extended stellar haloes lie in their spectra. Therefore, to determine whether these structures evolved together or through a two-phase formation, we need to spectroscopically separate the light from each component. We present a pilot study to use BUDDI to fit and extract the spectra of the cD galaxy NGC 3311 and its halo in an Integral Field Spectroscopy datacube, and carry out a simple stellar populations analysis to study their star-formation histories. Using MUSE data, we were able to isolate the light of the galaxy and its halo throughout the datacube, giving spectra representing purely the light from each of these structures. The stellar populations analysis of the two components indicates that, in this case, the bulk of the stars in both the halo and the central galaxy are very old, but the halo is more metal poor and less $\alpha$-enriched than the galaxy. This result is consistent with the halo forming through the accretion of much smaller satellite galaxies with more extended star formation. It is noteworthy that the apparent gradients in age and metallicity indicators across the galaxy are entirely consistent with the radially-varying contributions of galaxy and halo components, which individually display no gradients. The success of this study is promising for its application to a larger sample of cD galaxies that are currently being observed by IFU surveys.

The Origin of the Milky Way's Halo Age Distribution

Abstract We present an analysis of the radial age gradients for the stellar halos of five Milky Way (MW) mass-sized systems simulated as part of the Aquarius Project. The halos show a diversity of age trends, reflecting their different assembly histories. Four of the simulated halos possess clear negative age gradients, ranging from approximately −7 to −19 Myr kpc−1, shallower than those determined by recent observational studies of the Milky Way’s stellar halo. However, when restricting the analysis to the accreted component alone, all of the stellar halos exhibit a steeper negative age gradient with values ranging from −8 to −32 Myr kpc−1, closer to those observed in the Galaxy. Two of the accretion-dominated simulated halos show a large concentration of old stars in the center, in agreement with the Ancient Chronographic Sphere reported observationally. The stellar halo that best reproduces the current observed characteristics of the age distributions of the Galaxy is that formed principally by the accretion of small satellite galaxies. Our findings suggest that the hierarchical clustering scenario can reproduce the MW’s halo age distribution if the stellar halo was assembled from accretion and the disruption of satellite galaxies with dynamical masses less than ∼109.5 M ☉, and a minimal in situ contribution.

Not-so-missing galaxies were there all along

There ought to be more small satellite galaxies around the Milky Way than we have seen--and now, at last, they've been found. The main explanation is simple: they were there all along, we just missed them. Stacy Kim at Ohio State University in Columbus and her team pored through data from the Sloan Digital Sky Survey. They found that extrapolating from the number of galaxies in the survey area--about one third of the sky, thus far--we should be able to detect about 200 satellite dwarf galaxies

Models of the strongly lensed quasar DES J0408−5354

We present gravitational lens models of the multiply imaged quasar DES J0408-5354, recently discovered in the Dark Energy Survey (DES) footprint, with the aim of interpreting its remarkable quad-like configuration. We first model the DES single-epoch $grizY$ images as a superposition of a lens galaxy and four point-like objects, obtaining spectral energy distributions (SEDs) and relative positions for the objects. Three of the point sources (A,B,D) have SEDs compatible with the discovery quasar spectra, while the faintest point-like image (G2/C) shows significant reddening and a `grey' dimming of $\approx0.8$mag. In order to understand the lens configuration, we fit different models to the relative positions of A,B,D. Models with just a single deflector predict a fourth image at the location of G2/C but considerably brighter and bluer. The addition of a small satellite galaxy ($R_{\rm E}\approx0.2$") in the lens plane near the position of G2/C suppresses the flux of the fourth image and can explain both the reddening and grey dimming. All models predict a main deflector with Einstein radius between $1.7"$ and $2.0",$ velocity dispersion $267-280$km/s and enclosed mass $\approx 6\times10^{11}M_{\odot},$ even though higher resolution imaging data are needed to break residual degeneracies in model parameters. The longest time-delay (B-A) is estimated as $\approx 85$ (resp. $\approx125$) days by models with (resp. without) a perturber near G2/C. The configuration and predicted time-delays of J0408-5354 make it an excellent target for follow-up aimed at understanding the source quasar host galaxy and substructure in the lens, and measuring cosmological parameters. We also discuss some lessons learnt from J0408-5354 on lensed quasar finding strategies, due to its chromaticity and morphology.

The case of the missing satellites

In the late 1990s, computational technology had advanced sufficiently that astrophysicists were able to construct reasonably high resolution computer simulations of the Local Group of galaxies. These simulations indicated there should be around 250 small satellite galaxies orbiting the Milky Way and Andromeda. In the real Local Group, however, only around 40 satellites had been observed, and only twenty or so more have been discovered since then. Despite this discrepancy in numbers, claims have been made in recent years that the ‘missing satellites problem’ has been solved. Using the examples of the constructed luminosity curve, and hydrodynamic simulations, this paper explores how simulations are used in conjunction with observation to ‘solve’ the missing satellites problem. It is suggested that the simulated universes have sufficient complexity to be (temporarily) understood as worlds in their own right, ones that can be measured and observed. By demonstrating that these virtual worlds are sufficiently ‘realistic’ with respect to observations, astrophysicists are able to make a robust argument for the existence of ‘dark’, non-observable astrophysical objects. Observational and simulated data are combined to demonstrate the plausibility—a term that develops more ontologically meaningful connotations—of both the existence and the maintenance of dark satellites. It is thus through the conflation of the real and virtual worlds, the blending of simulation data with empirical data, that the missing satellites problem is ‘solved’.

Dissecting Halo Components in IFU Data

While most astronomers are now familiar with tools to decompose images into multiple components such as disks, bulges, and halos, the equivalent techniques for spectral data cubes are still in their infancy. This is unfortunate, as integral field unit (IFU) spectral surveys are now producing a mass of data in this format, which we are ill-prepared to analyze effectively. We have therefore been developing new tools to separate out components using this full spectral data. The results of such analyses will prove invaluable in determining not only whether such decompositions have an astrophysical significance, but, where they do, also in determining the relationship between the various elements of a galaxy. Application to a pilot study of IFU data from the cD galaxy NGC 3311 confirms that the technique can separate the stellar halo from the underlying galaxy in such systems, and indicates that, in this case, the halo is older and more metal poor than the galaxy, consistent with it forming from the cannibalism of smaller satellite galaxies. The success of the method bodes well for its application to studying the larger samples of cD galaxies that IFU surveys are currently producing.

An H i view of galaxy conformity: H i-rich environment around H i-excess galaxies

Using data taken as part of the Bluedisk project, we study the connection between neutral hydrogen (H i) in the environment of spiral galaxies and that in the galaxies themselves. We measure the total H i mass present in the environment in a statistical way by studying the distribution of noise peaks in the H i data cubes obtained for 40 galaxies observed with Westerbork Synthesis Radio Telescope. We find that galaxies whose H i mass fraction is high relative to standard scaling relations have an excess H i mass in the surrounding environment as well. Gas in the environment consists of gas clumps which are individually below the detection limit of our H i data. These clumps may be hosted by small satellite galaxies and/or be the high-density peaks of a more diffuse gas distribution in the intergalactic medium. We interpret this result as an indication for a picture in which the |${\rm H}\,{\small {I}}$|-rich central galaxies accrete gas from an extended gas reservoir present in their environment.

Dynamics and metallicity of far-infrared selected galaxies

We present near-infrared integral field spectroscopy of ten Herschel selected galaxies at z~1.5. From detailed mapping of the H$\alpha$ and [NII] emission lines we trace the dynamics, star formation rates, metallicities and also investigate gas fractions for these galaxies. For a few galaxies the distribution of star formation as traced by H$\alpha$ only traces a small fraction of the stellar disc, which could be tracing recent minor merging events. The rest-frame ultraviolet (UV) continuum emission often has a distribution completely different from H$\alpha$, which warns about the use of UV-SED based star formation tracers in these systems. Our analysis of galaxy dynamics shows that minor dynamical disruptions (e.g. minor merging) are generally not enough to cause a deviation from the established 'Main Sequence' relation. Most galaxies are found to follow the fundamental metallicity relation (FMR), although with large scatter. One galaxy, (a small satellite galaxy of a massive companion) is found to deviate strongly from the FMR. This deviation is in nice agreement with the correlation recently discovered in local galaxies between gas metallicity and environment, which has been ascribed to enriched inter-galactic medium (IGM) in dense environments, and therefore suggests that here the IGM was already significantly enriched by z~1.5.

Galaxy alignment on large and small scales

AbstractGalaxies are not randomly distributed across the universe but showing different kinds of alignment on different scales. On small scales satellite galaxies have a tendency to distribute along the major axis of the central galaxy, with dependence on galaxy properties that both red satellites and centrals have stronger alignment than their blue counterparts. On large scales, it is found that the major axes of Luminous Red Galaxies (LRGs) have correlation up to 30Mpc/h. Using hydro-dynamical simulation with star formation, we investigate the origin of galaxy alignment on different scales. It is found that most red satellite galaxies stay in the inner region of dark matter halo inside which the shape of central galaxy is well aligned with the dark matter distribution. Red centrals have stronger alignment than blue ones as they live in massive haloes and the central galaxy-halo alignment increases with halo mass. On large scales, the alignment of LRGs is also from the galaxy-halo shape correlation, but with some extent of mis-alignment. The massive haloes have stronger alignment than haloes in filament which connect massive haloes. This is contrary to the naive expectation that cosmic filament is the cause of halo alignment.

Satellite infall and mass deposition on the Galactic center

AbstractWe modeled numerically the infall of a small satellite galaxy on to the inner 200 parsec of our Galaxy, to test whether such an event perturbs gas orbiting in the central molecular zone (CMZ), as recently proposed by Lang et al. (2013). This process could have driven a large gas inflow around 10 Myr ago, explaining the past high accretion rate onto the supermassive black hole, and the presence of young stars in the inner parsecs of the Galaxy. Our simulations show a very small inflow of gas, not sufficient to produce the aforementioned effects.

A NOVEL APPROACH TO CONSTRAIN THE MASS RATIO OF MINOR MERGERS IN ELLIPTICAL GALAXIES: APPLICATION TO NGC 4889, THE BRIGHTEST CLUSTER GALAXY IN COMA

Minor mergers are thought to be important for the build-up and structural evolution of massive elliptical galaxies. In this work, we report the discovery of a system of four shell features in NGC 4889, one of the brightest members of the Coma cluster, using optical images taken with the Hubble Space Telescope and the Sloan Digital Sky Survey. The shells are well aligned with the major axis of the host and are likely to have been formed by the accretion of a small satellite galaxy. We have performed a detailed two-dimensional photometric decomposition of NGC 4889 and of the many overlapping nearby galaxies in its vicinity. This comprehensive model allows us not only to firmly detect the low-surface brightness shells, but, crucially, also to accurately measure their luminosities and colors. The shells are bluer than the underlying stars at the same radius in the main galaxy. We make use of the colors of the shells and the color-magnitude relation of the Coma cluster to infer the luminosity (or mass) of the progenitor galaxy. The shells in NGC 4889 appear to have been produced by the minor merger of a moderate-luminosity (M_I ~ -18.7 mag) disk (S0 or spiral) galaxy with a luminosity (mass) ratio of ~ 90:1 with respect to the primary galaxy. The novel methodology presented in this work can be exploited to decode the fossil record imprinted in the photometric substructure of other nearby early-type galaxies.

A Test to See in the Dark

Astrophysics![Figure][1] CREDIT: NASA AND EUROPEAN SPACE AGENCY According to current models of structure formation in the universe, large galaxies, like our Milky Way, formed from the merging of many smaller systems. The models predict that a large number of small satellite galaxies should

THE IMPACT OF GAS STRIPPING AND STELLAR MASS LOSS ON SATELLITE GALAXY EVOLUTION

Current semi-analytic models (SAMs) of galaxy formation over-predict the fraction of passive small late-type satellite galaxies in dense environments by a factor of two to three. We hypothesize that this is due to inaccurate prescriptions on cold gas evolution. In the hope of solving this problem we apply detailed prescriptions on the evolution of diffuse hot gases in satellites and on stellar mass loss, both of which are critical to model cold gas evolution. We replace the conventional shock-heating motivated instant stripping with a realistic gradual prescription based on ram pressure and tidal stripping. We also carefully consider stellar mass loss in our model. When both mechanisms are included, the fraction of passive late types matches the data much more closely. The satellite over-quenching problem is still present in small galaxies in massive haloes, however. In terms of the detectable residual star formation rates, gradual diffuse gas stripping appears to be much more important than stellar mass loss in our model. The implications of these results and other possibilities, such as redshift-dependent merging geometry and tidal disruption, are also discussed.