Host Virial Radius Research Articles

Symphony: Cosmological Zoom-in Simulation Suites over Four Decades of Host Halo Mass

We present Symphony, a compilation of 262 cosmological, cold-dark-matter-only zoom-in simulations spanning four decades of host halo mass, from 1011–1015 M ⊙. This compilation includes three existing simulation suites at the cluster and Milky Way–mass scales, and two new suites: 39 Large Magellanic Cloud-mass (1011 M ⊙) and 49 strong-lens-analog (1013 M ⊙) group-mass hosts. Across the entire host halo mass range, the highest-resolution regions in these simulations are resolved with a dark matter particle mass of ≈3 × 10−7 times the host virial mass and a Plummer-equivalent gravitational softening length of ≈9 × 10−4 times the host virial radius, on average. We measure correlations between subhalo abundance and host concentration, formation time, and maximum subhalo mass, all of which peak at the Milky Way host halo mass scale. Subhalo abundances are ≈50% higher in clusters than in lower-mass hosts at fixed sub-to-host halo mass ratios. Subhalo radial distributions are approximately self-similar as a function of host mass and are less concentrated than hosts’ underlying dark matter distributions. We compare our results to the semianalytic model Galacticus, which predicts subhalo mass functions with a higher normalization at the low-mass end and radial distributions that are slightly more concentrated than Symphony. We use UniverseMachine to model halo and subhalo star formation histories in Symphony, and we demonstrate that these predictions resolve the formation histories of the halos that host nearly all currently observable satellite galaxies in the universe. To promote open use of Symphony, data products are publicly available at http://web.stanford.edu/group/gfc/symphony.

Extracting galaxy merger time-scales II: a new fitting formula

ABSTRACT Predicting the merger time-scale (τmerge) of merging dark matter haloes, based on their orbital parameters and the structural properties of their hosts, is a fundamental problem in gravitational dynamics that has important consequences for our understanding of cosmological structure formation and galaxy formation. Previous models predicting τmerge have shown varying degrees of success when compared to the results of cosmological N-body simulations. We build on this previous work and propose a new model for τmerge that draws on insights derived from these simulations. We find that published predictions can provide reasonable estimates for τmerge based on orbital properties at infall, but tend to underpredict τmerge inside the host virial radius (R200) because tidal stripping is neglected, and overpredict it outside R200 because the host mass is underestimated. Furthermore, we find that models that account for orbital angular momentum via the circular radius Rcirc underpredict (overpredict) τmerge for bound (unbound) systems. By fitting for the dependence of τmerge on various orbital and host halo properties, we derive an improved model for τmerge that can be applied to a merging halo at any point in its orbit. Finally, we discuss briefly the implications of our new model for τmerge for semi-analytical galaxy formation modelling.

Do Halos that Form Early, Have High Concentration, Are Part of a Pair, or Contain a Central Galaxy Potential Host More Pronounced Planes of Satellite Galaxies?

Abstract The Milky Way, the Andromeda galaxy, and Centaurus A host flattened distributions of satellite galaxies that exhibit coherent velocity trends indicative of rotation. Comparably extreme satellite structures are very rare in cosmological ΛCDM simulations, giving rise to the “satellite plane problem.” As a possible explanation, it has been suggested that earlier-forming, higher-concentration host halos contain more flattened and kinematically coherent satellite planes. We have tested for such a proposed correlation between the satellite plane and host halo properties in the Exploring the Local Volume in Simulations suite of simulations. We find evidence for neither a correlation of plane flattening with halo concentration or formation time nor a correlation of kinematic coherence with concentration. The height of the thinnest subhalo planes does correlate with the host virial radius and the radial extent of the subhalo system. This can be understood as an effect of not accounting for differences in the radial distribution of subhalos and selecting them from different volumes than covered by the actual observations. Being part of a halo pair like the Local Group does not result in narrower or more correlated satellite planes either. Additionally, using the Phat ELVIS simulations, we show that the presence of a central galaxy potential does not favor narrower or more correlated satellite planes; rather, it leads to slightly wider planes. Such a central potential is a good approximation of the dominant effect baryonic physics in cosmological simulations has on a subhalo population. This suggests that, in contrast to other small-scale problems, the issue of planes of satellite galaxies is made worse by accounting for baryonic effects.

The Extent of Chemically Enriched Gas around Star-forming Dwarf Galaxies

Abstract Supernova driven winds are often invoked to remove chemically enriched gas from dwarf galaxies to match their low observed metallicities. In such shallow potential wells, outflows may produce massive amounts of enriched halo gas (circumgalactic medium, CGM) and pollute the intergalactic medium (IGM). Here, we present a survey of the CGM and IGM around 18 star-forming field dwarfs with stellar masses of at . Eight of these have CGM probed by quasar absorption spectra at projected distances, d, less than that of the host virial radius, . Ten are probed in the surrounding IGM at . The absorption measurements include neutral hydrogen, the dominant silicon ions for diffuse cool gas (T ∼ 104 K; Si ii, Si iii, and Si iv), moderately ionized carbon (C iv), and highly ionized oxygen (O vi). Metal absorption from the CGM of the dwarfs is less common and weaker compared to massive star-forming galaxies, though O vi absorption is still common. None of the dwarfs probed at have definitive metal-line detections. Combining the available silicon ions, we estimate that the cool CGM of the dwarfs accounts for only 2%–6% of the expected silicon budget from the yields of supernovae associated with past star formation. The highly ionized O vi accounts for ≈8% of the oxygen budget. As O vi traces an ion with expected equilibrium ion fractions of ≲0.2, the highly ionized CGM may represent a significant metal reservoir even for dwarfs not expected to maintain gravitationally shock heated hot halos.

The SAGA Survey. I. Satellite Galaxy Populations around Eight Milky Way Analogs

Abstract We present the survey strategy and early results of the “Satellites Around Galactic Analogs” (SAGA) Survey. The SAGA Survey’s goal is to measure the distribution of satellite galaxies around 100 systems analogous to the Milky Way down to the luminosity of the Leo I dwarf galaxy ( ). We define a Milky Way analog based on K-band luminosity and local environment. Here, we present satellite luminosity functions for eight Milky-Way-analog galaxies between 20 and 40 Mpc. These systems have nearly complete spectroscopic coverage of candidate satellites within the projected host virial radius down to using low-redshift gri color criteria. We have discovered a total of 25 new satellite galaxies: 14 new satellite galaxies meet our formal criteria around our complete host systems, plus 11 additional satellites in either incompletely surveyed hosts or below our formal magnitude limit. Combined with 13 previously known satellites, there are a total of 27 satellites around 8 complete Milky-Way-analog hosts. We find a wide distribution in the number of satellites per host, from 1 to 9, in the luminosity range for which there are 5 Milky Way satellites. Standard abundance matching extrapolated from higher luminosities predicts less scatter between hosts and a steeper luminosity function slope than observed. We find that the majority of satellites (26 of 27) are star-forming. These early results indicate that the Milky Way has a different satellite population than typical in our sample, potentially changing the physical interpretation of measurements based only on the Milky Way’s satellite galaxies.

Universal subhalo accretion in cold and warm dark matter cosmologies

The influence of the large scale structure on host halos may be studied by examining the angular infall pattern of subhalos. In particular, since warm and cold dark matter cosmologies predict different abundances and internal properties for halos at the low mass end of the mass function, it is interesting to examine if there are differences in how these low mass halos are accreted. The accretion events are defined as the moment a halo becomes a substructure, namely when it crosses its host's virial radius. We quantify the cosmic web at each point by the shear tensor and examine where, with respect to its eigenvectors, such accretion events occur in cold ($\Lambda$CDM) and warm (1keV sterile neutrino WDM) dark matter cosmological models. We find that the CDM and WDM subhalos are preferentially accreted along the principal axis of the shear tensor corresponding to the direction of weakest collapse. The beaming strength is modulated by the host and subhalo masses and by the redshift at which the accretion event occurs. Although strongest for the most massive hosts and subhalos at high redshift, the preferential infall is found to be always aligned with the axis of weakest collapse, thus we say that it has universal nature. We compare the strength of beaming in the WDM cosmology with the one found in the $\Lambda$CDM scenario. While the main findings remain the same, the accretion in the WDM model for the most massive host halos appears more beamed than in $\Lambda$CDM cosmology across all the redshifts.

SATELLITE DWARF GALAXIES IN A HIERARCHICAL UNIVERSE: THE PREVALENCE OF DWARF-DWARF MAJOR MERGERS

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 lower fraction for lower mass dwarf galaxies. Recent merger remnants are biased towards larger radial distance and more recent virial infall times, because most recent mergers occurred shortly before crossing within the virial radius of the host halo. Satellite-satellite mergers also occur within the host halo after virial infall, catalyzed by the large fraction of dwarf galaxies that fell in as part of a group. The merger fraction doubles for dwarf galaxies outside of the host virial radius, so the most distant dwarf galaxies in the Local Group are the most likely to have experienced a recent major merger. We discuss the implications of these results on observable dwarf merger remnants, their star formation histories, the gas content of mergers, and massive black holes in dwarf galaxies.

ORBITAL DEPENDENCE OF GALAXY PROPERTIES IN SATELLITE SYSTEMS OF GALAXIES

We study the dependence of satellite galaxy properties on the distance to the host galaxy and the orbital motion using the SDSS data. From SDSS DR7 we find 3515 isolated satellite systems of galaxies at z<0.03 that contain 8904 satellite galaxies. Using this sample we construct a catalog of 635 satellites associated with 215 host galaxies whose spin directions are determined by our inspection of the SDSS color images and/or by spectroscopic observations in the literature. We divide satellite galaxies into prograde and retrograde orbit subsamples depending on their orbital motion respect to the spin direction of the host. We find that the number of galaxies in prograde orbit is nearly equal to that of retrograde orbit galaxies: the fraction of satellites in prograde orbit is 50+/-2%. The velocity distribution of satellites with respect to their hosts is found almost symmetric: the median bulk rotation of satellites is -1+/- 8 km/s. It is found that the radial distribution of early-type satellites in prograde orbit is strongly concentrated toward the host while that of retrograde ones shows much less concentration. We also find the orbital speed of late-type satellites in prograde orbit increases as the projected distance to the host (R) decreases while the speed decreases for those in retrograde orbit. At R less than 0.1 times the host virial radius the orbital speed decreases in both prograde and retrograde orbit cases. Prograde satellites are on average fainter than retrograde satellites for both early and late morphological types. The u-r color becomes redder as R decreases for both prograde and retrograde orbit late-type satellites. The differences between prograde and retrograde orbit satellites may be attributed to their different origin or the different strength of physical processes that they have experienced through hydrodynamic interactions with their host galaxies.

The impact of baryonic physics on the shape and radial alignment of substructures in cosmological dark matter haloes

We use two simulations performed within the Constrained Local UniversE Simulation (CLUES) project to study both the shape and radial alignment of (the dark matter component of) subhaloes; one of the simulations is a dark matter only model while the other run includes all the relevant gas physics and star formation recipes. We find that the involvement of gas physics does not have a statistically significant effect on either property -- at least not for the most massive subhaloes considered in this study. However, we observe in both simulations including and excluding gasdynamics a (pronounced) evolution of the dark matter shapes of subhaloes as well as of the radial alignment signal since infall time. Further, this evolution is different when positioned in the central and outer regions of the host halo today; while subhaloes tend to become more aspherical in the central 50% of their host's virial radius, the radial alignment weakens in the central regime while strengthening in the outer parts. We confirm that this is due to tidal torquing and the fact that subhaloes at pericentre move too fast for the alignment signal to respond.

Anisotropy in the Distribution of Satellite Galaxy Orbits

Nearby clusters such as Virgo and Coma possess galaxy distributions that tend to be aligned with the principal axis of the cluster itself. This has also been confirmed by a recent statistical analysis of some 300 Abell clusters, where the effect has been linked to the dynamical state of the cluster. Moreover, the orbits of satellite galaxies in galactic systems like our own Milky Way also demonstrate a high degree of anisotropy—the so-called Holmberg effect, the origin of which has been the subject of debate for more than 30 years. This study presents the analysis of cosmological simulations focusing on the orbits of satellite galaxies within dark matter halos. The apocenters of the orbits of these satellites are preferentially found within a cone of opening angle ~40° around the major axis of the host halo, in accordance with the observed anisotropy found in galaxy clusters. We do, however, note that a link to the dynamical age of the cluster is not well established, as both of our oldest dark matter halos do show a clear anisotropy signal. Further analysis connects this distribution to the infall pattern of satellites along the filaments: the orbits are determined rather by the environment of the host halo than some "dynamical selection" during their life within the host's virial radius.