Host Halo Research Articles

When and How Ram Pressure Stripping in Low-mass Satellite Galaxies Enhances Star Formation

We investigate how a satellite's star formation rate (SFR) and surviving gas respond to ram pressure stripping (RPS) in various environments. Using a suite of high-resolution wind tunnel simulations with radiative cooling, star formation, and supernovae feedback, we model the first infall orbit of a low-mass disk galaxy (M * = 109.7 M ⊙) in different host halos, ranging from Milky Way–like to cluster hosts. When the ram pressure is moderate, we find that the stripping satellite shows an enhanced SFR relative to the isolated control case, despite gas loss due to stripping. The SFR enhancement is caused, not directly by compression, but by ram-pressure-driven mass flows, which can increase the dense gas fraction in the central disk regions. The spatially resolved star formation main sequence and Kennicutt–Schmidt relations in our simulations are consistent with recent findings of the VERTICO and GASP surveys. Our results predict the environmental signals of RPS in future multiwavelength, high-angular resolution observations: the star formation and gas surface densities will be centralized, and symmetrically enhanced within the stripping radius.

Not all subhaloes are created equal: modelling the diversity of subhalo density profiles in TNG50

ABSTRACT In this work, we analyse the density profiles of subhaloes with masses Msh ≥ 1.4 × 108 M⊙ in the TNG50 simulation, with the aim of including baryonic effects. We evaluate the performance of frequently used models, such as the standard Navarro–Frenk–White (NFW), the Einasto, and a smoothly truncated version of the NFW profile. We find that these models do not perform well for the majority of subhaloes, with the NFW profile giving the worst fit in most cases. This is primarily due to mismatches in the inner and outer logarithmic slopes, which are significantly steeper for a large number of subhaloes in the presence of baryons. To address this issue, we propose new three-parameter models and show that they significantly improve the goodness of fit independently of the subhalo’s specific properties. Our best-performing model is a modified version of the NFW profile with an inner log-slope of −2 and a variable truncation that is sharper and steeper than the slope transition in the standard NFW profile. Additionally, we investigate how both the parameter values of the best density profile model and the average density profiles vary with subhalo mass, Vmax, distance from the host halo centre, baryon content, and infall time, and we also present explicit scaling relations for the mean parameters of the individual profiles. The newly proposed fit and the scaling relations are useful to predict the properties of realistic subhaloes in the mass range 108 M⊙ ≤Msh ≤ 1013 M⊙ that can be influenced by the presence of baryons.

The Connection between the Intracluster Light and its Host Halo: Formation Time and Contribution from Different Channels

We extend the analysis presented by Contini et al. to higher redshifts, up to z = 2, by focusing on the relation between the intracluster light (ICL) fraction and the halo mass, its dependence on redshift, the role played by the halo concentration, and the formation time, in a large sample of simulated galaxy groups/clusters with . Moreover, a key focus is to isolate the relative contributions provided by the main channels for the ICL formation to the total amount. The ICL fraction at higher redshift is weakly dependent on halo mass and comparable with that at the present time, in agreement with recent observations. Stellar stripping, mergers, and preprocessing are the major channels responsible for the ICL formation, with stellar stripping accounting for ∼90% of the total ICL, regardless of halo mass and redshift. Preprocessing is an important process for clusters to accrete already formed ICL. The diffuse component forms very early, at z ∼ 0.6, and its formation depends on both concentration and formation time of the halo, with more concentrated haloes that formed early assembling their ICL earlier than those that formed later. The efficiency of this process is independent of halo mass but increases with decreasing redshift, which implies that stellar stripping becomes more important with time as the concentration increases. This highlights the link between the ICL and the dynamical state of a halo: groups/clusters that have a higher fraction of diffuse light are more concentrated, relaxed, and in an advanced stage of growth.

The Impact of Patchy Reionization on Ultrafaint Dwarf Galaxies

We investigate how patchy reionization affects the star formation history (SFH) and stellar metallicity of ultrafaint dwarf galaxies (UFDs). Patchy reionization refers to varying ultraviolet background strengths depending on a galaxy’s environment. Recent observations highlight the significance of this effect on UFDs, as UFDs can have different SFHs depending on their relative position with respect to their host halo during the period of reionization. However, most cosmological hydrodynamic simulations do not consider environmental factors such as patchy reionization, and the effect of reionization is typically applied homogeneously. Using a novel approach to implement patchy reionization, we show how SFHs of simulated UFDs can change. Our cosmological hydrodynamic zoom-in simulations focus on UFD analogs with M vir ∼ 109 M ⊙, M * ≲ 105 M ⊙ at z = 0. We find that patchy reionization can weaken the effect of reionization by 2 orders of magnitude up to z = 3, enabling late star formation in half of the simulated UFDs, with quenching times ∼460 Myr later than those with homogeneous reionization. We also show that halo merger and mass assembly can affect the SFHs of simulated UFDs, in addition to patchy reionization. The average stellar iron-to-hydrogen ratio, [Fe/H], of the simulated UFDs with patchy reionization increases by 0.22–0.42 dex. Finally, our findings suggest that patchy reionization could be responsible for the extended SFHs of Magellanic UFDs compared to non-Magellanic UFDs.

A Self-interacting Dark Matter Solution to the Extreme Diversity of Low-mass Halo Properties

The properties of low-mass dark matter (DM) halos appear to be remarkably diverse relative to cold, collisionless DM predictions, even in the presence of baryons. We show that self-interacting DM (SIDM) can simultaneously explain observations of halo diversity at two opposite extremes—the inner density profile of the dense substructure perturbing the strong lens galaxy SDSSJ0946+1006 and the rotation curves of isolated, gas-rich ultradiffuse galaxies (UDGs). To achieve this, we present the first cosmological zoom-in simulation featuring strong DM self-interactions in a galaxy group environment centered on a 1013 M ⊙ host halo. In our SIDM simulation, most surviving subhalos of the group-mass host are deeply core-collapsed, yielding excellent candidates for the observed dense strong-lens perturber. Self-interactions simultaneously create kiloparsec-scale cores in low-concentration isolated halos, which could host the observed UDGs. Our scenario can be further tested with observations of DM structure and galaxies over a wide mass range.

Modelling the orbital histories of satellites of Milky Way-mass galaxies: testing static host potentials against cosmological simulations

ABSTRACT Understanding the evolution of satellite galaxies of the Milky Way (MW) and M31 requires modelling their orbital histories across cosmic time. Many works that model satellite orbits incorrectly assume or approximate that the host halo gravitational potential is fixed in time and is spherically symmetric or axisymmetric. We rigorously benchmark the accuracy of such models against the FIRE-2 cosmological baryonic simulations of MW/M31-mass haloes. When a typical surviving satellite fell in ($3.4\!-\!9.7\, \rm {Gyr}$ ago), the host halo mass and radius were typically 26–86 per cent of their values today, respectively. Most of this mass growth of the host occurred at small distances, $r\lesssim 50\, \rm {kpc}$, opposite to dark matter only simulations, which experience almost no growth at small radii. We fit a near-exact axisymmetric gravitational potential to each host at z = 0 and backward integrate the orbits of satellites in this static potential, comparing against the true orbit histories in the simulations. Orbital energy and angular momentum are not well conserved throughout an orbital history, varying by 25 per cent from their current values already $1.6\!-\!4.7\, \rm {Gyr}$ ago. Most orbital properties are minimally biased, ≲10 per cent, when averaged across the satellite population as a whole. However, for a single satellite, the uncertainties are large: recent orbital properties, like the most recent pericentre distance, typically are ≈20 per cent uncertain, while earlier events, like the minimum pericentre or the infall time, are ≈40–80 per cent uncertain. Furthermore, these biases and uncertainties are lower limits, given that we use near-exact host mass profiles at z = 0.

The DESI One-Percent Survey: exploring a generalized SHAM for multiple tracers with the UNIT simulation

ABSTRACT We perform SubHalo Abundance Matching (SHAM) studies on UNIT simulations with {σ, Vceil, vsmear}-SHAM and {σ, Vceil, fsat}-SHAM. They are designed to reproduce the clustering on 5–30 $\, {\, h^{-1}\, {\rm Mpc}}$ of luminous red galaxies (LRGs), emission-line galaxies (ELGs), and quasi-stellar objects (QSOs) at 0.4 < z < 3.5 from DESI (Dark Energy Spectroscopic Instrument) One Percent Survey. Vceil is the incompleteness of the massive host (sub)haloes and is the key to the generalized SHAM. vsmear models the clustering effect of redshift uncertainties, providing measurements consistent with those from repeat observations. A free satellite fraction fsat is necessary to reproduce the clustering of ELGs. We find ELGs present a more complex galaxy–halo mass relation than LRGs reflected in their weak constraints on σ. LRGs, QSOs, and ELGs show increasing Vceil values, corresponding to the massive galaxy incompleteness of LRGs, the quenched star formation of ELGs and the quenched black hole accretion of QSOs. For LRGs, a Gaussian vsmear presents a better profile for subsamples at redshift bins than a Lorentzian profile used for other tracers. The impact of the statistical redshift uncertainty on ELG clustering is negligible. The best-fitting satellite fraction for DESI ELGs is around 4 per cent, lower than previous estimations for ELGs. The mean halo mass log10(〈Mvir〉) in ${{\, h^{-1}\, \mbox{M}_\odot }}{}$ for LRGs, ELGs, and QSOs are 13.16 ± 0.01, 11.90 ± 0.06, and 12.66 ± 0.45, respectively. Our generalized SHAM algorithms facilitate the production of multitracer galaxy mocks for cosmological tests.

Are the fates of supermassive black holes and galaxies determined by individual mergers, or by the properties of their host haloes?

ABSTRACT The fates of massive galaxies are tied to the evolution of their central supermassive black holes (BHs), due to the influence of AGN feedback. Correlations within simulated galaxy populations suggest that the masses of BHs are governed by properties of their host dark matter haloes, such as the binding energy and assembly time, at a given halo mass. However, the full picture must be more complex, as galaxy mergers have also been shown to influence the growth of BHs and the impact of AGN. In this study, we investigate this problem through a controlled experiment, using the genetic modification technique to adjust the assembly history of a Milky Way-like galaxy simulated with the EAGLE model. We change the halo assembly time (and hence the binding energy) in the absence of any disruptive merger events, and find little change in the integrated growth of the BH. We attribute this to the angular momentum support provided by a galaxy disc, which reduces the inflow of gas towards the BH and effectively decouples the BH’s growth from the halo’s properties. Introducing major mergers into the assembly history disrupts the disc, causing the BH to grow ≈4 × more massive and inject feedback that reduces the halo baryon fraction by a factor of ≈2 and quenches star formation. Merger events appear essential to the diversity in BH masses in EAGLE, and we also show that they increase the halo binding energy; correlations between these quantities may therefore be the result of merger events.

Impact of tidal environment on galaxy clustering in GAMA

ABSTRACT We constrain models of the galaxy distribution in the cosmic web using data from the Galaxy and Mass Assembly (GAMA) survey. We model the redshift-space behaviour of the 2-point correlation function (2pcf) and the recently proposed Voronoi volume function (VVF) – which includes information beyond two-point statistics. We extend the standard halo model using extra satellite degrees of freedom and two assembly bias parameters: αcen and αsat, which correlate the occupation numbers of central and satellite galaxies with their host halo’s tidal environment, respectively. We measure $\alpha _{\rm sat}=1.44^{+0.25}_{-0.43}$ and $\alpha _{\rm cen}=-0.79^{+0.29}_{-0.11}$ using a combination of 2pcf and VVF measurements, representing a detection of assembly bias at the 3.3σ (2.4σ) significance level for satellite (central) galaxies. This result remains robust to possible anisotropies in the halocentric distribution of satellites as well as technicalities of estimating the data covariance. We show that the growth rate (fσ8) deduced using models with assembly bias is about 7 per cent (i.e. 1.5σ) lower than if assembly bias is ignored. When projected on to the Ωm–σ8 plane, the model constraints without assembly bias overlap with Planck expectations, while allowing assembly bias introduces significant tension with Planck, preferring either a lower Ωm or a lower σ8. Finally, we find that the all-galaxy weak-lensing signal is unaffected by assembly bias, but the central and satellite sub-populations individually show significantly different signals in the presence of assembly bias. Our results illustrate the importance of accurately modelling galaxy formation for cosmological inference from future surveys.

Tracking the evolution of satellite galaxies: mass stripping and dark-matter deficient galaxies

ABSTRACT 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, dperi, here normalized by host halo virial radius. The closest encounters result in significant loss of DM, with subhaloes retaining between 20 and a few per cent of their z = 1 mass. At fixed dperi, DM mass stripping seems more severe in lower mass haloes. Conversely, the average satellite in higher mass haloes has its stellar mass growth halted earlier, having lost a higher fraction of stellar mass by z = 0. We also show that mass stripping has a strong impact on the quenched fractions. The IllustrisTNG boxes are qualitatively consistent in these predictions, with quantitative differences mostly originating from the distinct subhalo mass ranges covered by the boxes. Finally, we have identified DM-deficient systems in all TNG boxes. These objects are preferentially found in massive clusters (Mhost ≳ 1013 M⊙), had very close encounters with their central galaxies ($d_{\rm peri}\simeq 0.05\, R_{\rm vir}$), and were accreted at high redshift (zinfall ≳ 1.4), reinforcing the notion that tidal stripping is responsible for their remarkable lack of DM.

The Galaxy Number Density Profile of Halos

More precise measurements of galaxy clustering will be provided by the next generation of galaxy surveys, such as DESI, WALLABY, and the Square Kilometre Array. To utilize this information to improve our understanding of the Universe, we need to accurately model the distribution of galaxies in their host dark matter halos. In this work, we present a new galaxy number density profile of halos, which makes predictions for the positions of galaxies in the host halo, different to the widely adopted Navarro–Frenk–White (NFW) profile, since galaxies tend to be found more in the outskirts of halos (nearer the virial radius) than an NFW profile. The parameterized galaxy number density profile model of halos is fit and tested using the Dark Sage semi-analytic model of galaxy formation. We find that our galaxy number density profile model of halos can accurately reproduce the halo occupation distribution and galaxy two-point correlation function of the Dark Sage simulation. We also derive the analytic expressions for the circular velocity and gravitational potential energy for this profile model. We use the SDSS Data Release 10 galaxy group catalog to validate this galaxy number density profile model of halos. Compared to the NFW profile, we find that our model more accurately predicts the positions of galaxies in their host halo and the galaxy two-point correlation function.

Transferability exploration of weight based on ImageNet for galaxy classification

The accurate classification of galaxies is essential for understanding the structure of the cosmos and establishing connections with host haloes. Manual classification methods are time-consuming, subjective, and prone to errors. This study investigates the efficiency of automated galaxy categorization utilising transfer learning with pre-trained models from ImageNet by utilising deep learning techniques. The Galaxy10 DECals Dataset is used in the study, and the DenseNet-121 network architecture is used for transfer learning. Experiments are used to assess the effects of various ImageNet weight configurations on the performance of the model. The findings reveal that the model trained with 20% ImageNet data achieves the highest classification accuracy of 79% for galaxies. Grad-CAM visualisation further highlights the impact of weight initialization on categorisation by displaying the different focal points of models trained with various ImageNet weights. The results indicate that using higher weights from ImageNet transfers advanced features, whereas using fewer weights transfers basic features, potentially producing inaccurate results. Prior to transfer learning, the study emphasises the significance of choosing the best weights from ImageNet. The knowledge acquired aids in the classification of galaxies and offers direction for further study.

The influence of subhaloes on host halo properties

ABSTRACT Within the ΛCDM cosmology, dark matter haloes are composed of both a smooth component and a population of smaller gravitationally bound subhaloes. These components are often treated as a single halo when properties, such as density profiles, are extracted from simulations. Recent work has shown that density profiles change substantially when subhalo mass is excluded. In this paper, we expand on this result by analysing three specific host halo properties – concentration (cNFW), spin (λB), and shape (c/a) – when calculated only from the smooth component of the halo. This analysis is performed on both Milky Way-mass haloes and cluster-mass haloes in high-resolution zoom-in N-body simulations. We find that when subhaloes are excluded, the median value of (1) cNFW is enhanced by $\approx 30\pm 11$ and $\approx 77\pm 8.1~{{\ \rm per\ cent}}$ for Milky Way-mass ($10^{12.1}\, \text{M}_\odot$) and cluster-mass ($10^{14.8}\, \text{M}_\odot$) haloes, respectively, (2) λB is reduced for Milky Way-mass by $\approx 11\pm 4.9~{{\ \rm per\ cent}}$ and cluster-mass haloes by $\approx 27\pm 3.5~{{\ \rm per\ cent}}$. Additionally, with the removal of subhaloes, cluster-mass haloes tend to become more spherical as the ratio of minor-to-major axis, c/a, increases by $\approx 11\pm 3.6~{{\ \rm per\ cent}}$, whereas Milky Way-mass haloes remain approximately the same shape with c/a changed by $\approx 1.0\pm 5.8~{{\ \rm per\ cent}}$. Fractional changes of each of these properties depend primarily on the amount of mass in subhaloes and, to a lesser extent, mass accretion history. Our findings demonstrate that the properties of the smooth components of dark matter haloes are biased relative to the total halo mass.

Resolving the physics of quasar Ly α nebulae (RePhyNe): I. Constraining quasar host halo masses through circumgalactic medium kinematics

ABSTRACT Ly α nebulae ubiquitously found around z > 2 quasars can supply unique constraints on the properties of the circumgalactic medium, such as its density distribution, provided the quasar halo mass is known. We present a new method to constrain quasar halo masses based on the line-of-sight velocity dispersion maps of Ly α nebulae. By using MUSE-like mock observations obtained from cosmological hydrodynamic simulations under the assumption of maximal quasar fluorescence, we show that the velocity dispersion radial profiles of Ly α emitting gas are strongly determined by gravity and that they are thus self-similar with respect to halo mass when rescaled by the virial radius. Through simple analytical arguments and by exploiting the kinematics of He ii1640 Å emission for a set of observed nebulae, we show that Ly α radiative transfer effects plausibly do not change the shape of the velocity dispersion profiles but only their normalization without breaking their self-similarity. Taking advantage of these results, we define the variable $\eta ^{140-200}_{40-100}$ as the ratio of the median velocity dispersion in two specifically selected annuli and derive an analytical relation between $\eta ^{140-200}_{40-100}$ and the halo mass which can be directly applied to observations. We apply our method to 37 observed quasar Ly α nebulae at 3 < z < 4.7 and find that their associated quasars are typically hosted by ∼1012.16 ± 0.14M⊙ haloes independent of redshift within the explored range. This measurement, which is completely independent of clustering methods, is consistent with the lowest mass estimates based on quasar autocorrelation clustering at z∼3 and with quasar-galaxies cross-correlation results.

Formation of first star clusters under the supersonic gas flow – I. Morphology of the massive metal-free gas cloud

ABSTRACT We performed 42 simulations of first star formation with initial supersonic gas flows relative to the dark matter at the cosmic recombination era. Increasing the initial streaming velocities led to delayed halo formation and increased halo mass, enhancing the mass of the gravitationally shrinking gas cloud. For more massive gas clouds, the rate of temperature drop during contraction, in other words, the structure asymmetry, becomes more significant. When the maximum and minimum gas temperature ratios before and after contraction exceed ∼10, the asymmetric structure of the gas cloud prevails, inducing fragmentation into multiple dense gas clouds. We continued our simulations until 105 yr after the first dense core formation to examine the final fate of the massive star-forming gas cloud. Among the 42 models studied, we find the simultaneous formation of up to four dense gas clouds, with a total mass of about $2254\, \mathrm{ M}_\odot$. While the gas mass in the host halo increases with increasing the initial streaming velocity, the mass of the dense cores does not change significantly. The star formation efficiency decreases by more than one order of magnitude from ϵIII ∼ 10−2 to 10−4 when the initial streaming velocity, normalized by the root mean square value, increases from 0 to 3.

Beyond Ultra-diffuse Galaxies. I. Mass–Size Outliers among the Satellites of Milky Way Analogs

Large diffuse galaxies are hard to find, but understanding the environments where they live, their numbers, and ultimately their origins, is of intense interest and importance for galaxy formation and evolution. Using Subaru’s Hyper Suprime-Cam Strategic Survey Program, we perform a systematic search for low surface brightness galaxies and present novel and effective methods for detecting and modeling them. As a case study, we surveyed 922 Milky Way analogs in the nearby Universe (0.01 < z < 0.04) and built a large sample of satellite galaxies that are outliers in the mass–size relation. These “ultra-puffy” galaxies (UPGs), defined to be 1.5σ above the average mass–size relation, represent the tail of the satellite size distribution. We find that each MW analog hosts N UPG = 0.31 ± 0.05 UPGs on average, which is consistent with but slightly lower than the observed abundance at this halo mass in the Local Volume. We also construct a sample of ultra-diffuse galaxies (UDGs) in MW analogs and find an abundance of N UDG = 0.44 ± 0.05 per host. With literature results, we confirm that the UDG abundance scales with the host halo mass following a sublinear power law. We argue that our definition of UPGs, which is based on the mass–size relation, is more physically motivated than the common definition of UDGs, which depends on the surface brightness and size cuts and thus yields different surface mass density cuts for quenched and star-forming galaxies.

A jolt to the system: ram pressure on low-mass galaxies in simulations of the Local Group

ABSTRACT Low-mass galaxies are highly susceptible to environmental effects that can efficiently quench star formation. We explore the role of ram pressure in quenching low-mass galaxies ($M_{*}\sim 10^{5}{-}10^{9}\, \rm {M}_{\odot }$) within 2 Mpc of Milky Way (MW) hosts using the FIRE-2 simulations. Ram pressure is highly variable across different environments, within individual MW haloes, and for individual low-mass galaxies over time. The impulsiveness of ram pressure – the maximum ram pressure scaled to the integrated ram pressure prior to quenching – correlates with whether a galaxy is quiescent or star forming. The time-scale between maximum ram pressure and quenching is anticorrelated with impulsiveness, such that high impulsiveness corresponds to quenching time-scales &amp;lt;1 Gyr. Galaxies in low-mass groups ($M_\mathrm{*,host}10^{7}{-}10^{9}\, \rm {M}_{\odot }$) outside of MW haloes experience typical ram pressure only slightly lower than ram pressure on MW satellites, helping to explain effective quenching via group preprocessing. Ram pressure on MW satellites rises sharply with decreasing distance to the host, and, at a fixed physical distance, more recent pericentre passages are typically associated with higher ram pressure because of greater gas density in the inner host halo at late times. Furthermore, the ram pressure and gas density in the inner regions of Local Group-like paired host haloes are higher at small angles off the host galaxy disc compared to isolated hosts. The quiescent fraction of satellites within these low-latitude regions is also elevated in the simulations and observations, signaling possible anisotropic quenching via ram pressure around MW-mass hosts.

Jellyfish galaxies with the IllustrisTNG simulations - No enhanced population-wide star formation according to TNG50

ABSTRACT Due to ram-pressure stripping (RPS), jellyfish galaxies are thought to lose large amounts, if not all, of their interstellar medium. Nevertheless, some, but not all, observations suggest that jellyfish galaxies exhibit enhanced star formation compared to control samples, even in their ram pressure-stripped tails. We use the TNG50 cosmological gravity + magnetohydrodynamical simulation, with an average spatial resolution of 50–200 pc in the star-forming regions of galaxies, to quantify the star formation activity and star formation rates (SFRs) of more than 700 jellyfish galaxies at z = 0–1 with stellar masses $10^{8.3-10.8} \ {\rm M}_\odot$ in hosts with mass $10^{10.5-14.3} \ {\rm M}_\odot$. We extract their global SFRs, the SFRs within their main stellar body versus within the tails, and we follow the evolution of the star formation along their individual evolutionary tracks. We compare the findings for jellyfish galaxies to those of diversely constructed control samples, including against satellite and field galaxies with matched redshift, stellar mass, gas fraction, and host halo mass. According to TNG50, star formation and RPS can indeed occur simultaneously within any given galaxy, and frequently do so. Moreover, star formation can also take place within the ram pressure-stripped tails, even though the latter is typically subdominant. However, TNG50 does not predict elevated population-wide SFRs in jellyfish compared to analogue satellite galaxies with the same stellar mass or gas fraction. Simulated jellyfish galaxies do undergo bursts of elevated star formation along their history but, at least according to TNG50, these do not translate into a population-wide enhancement at any given epoch.

Mass Dependence of Galaxy–Halo Alignment in LOWZ and CMASS

We measure the galaxy-ellipticity (GI) correlations for the Sloan Digital Sky Survey Data Release 12 LOWZ and CMASS samples with the shape measurements from the DESI Legacy Imaging Surveys. We model the GI correlations in an N-body simulation with our recent accurate stellar–halo mass relation from the Photometric object Around Cosmic webs (PAC) method. The large data set and our accurate modeling turns out an accurate measurement of the alignment angle between central galaxies and their host halos. We find that the alignment of central elliptical galaxies with their host halos increases monotonically with galaxy stellar mass or host halo mass, which can be well described by a power law for the massive galaxies. We also find that central elliptical galaxies are more aligned with their host halos in LOWZ than in CMASS, which might indicate an evolution of galaxy–halo alignment, though future studies are needed to verify this is not induced by the sample selections. In contrast, central disk galaxies are aligned with their host halos about 10 times more weakly in the GI correlation. These results have important implications for intrinsic alignment (IA) correction in weak lensing studies, IA cosmology, and theory of massive galaxy formation.

Frequency of the dark matter subhalo collisions and bifurcation sequence arising formation of dwarf galaxies

ABSTRACT The cold dark matter model predicts galaxies have 100 times more dark matter mass than stars. Nevertheless, recent observations report the existence of dark-matter-deficient galaxies with less dark matter than expected. To solve this problem, we investigate the physical processes of galaxy formation in head-on collisions between gas-containing dark matter subhaloes (DMSHs). Analytical estimation of the collision frequency between DMSHs associated with a massive host halo indicates that collisions frequently occur within one-tenth of the virial radius of the host halo, with a collision time-scale of about $10\, \mathrm{Myr}$, and the most frequent relative velocity increases with increasing radius. Using analytical models and numerical simulations, we show the bifurcation channel of the formation of dark-matter-dominated and dark-matter-deficient galaxies. In the case of low-velocity collisions, a dark-matter-dominated galaxy is formed by the merging of two DMSHs. In the case of moderate-velocity collisions, the two DMSHs penetrate each other. However, the gas medium collides, and star formation begins as the gas density increases, forming a dwarf galaxy without dark matter at the collision surface. In the case of high-velocity collisions, shock-breakout occurs due to the shock waves generated at the collision surface reaching the gas surface, and no galaxy forms. For example, the simulation demonstrates that a pair of DMSHs with a mass of $10^9\, \mathrm{M_\odot }$ containing gas of 0.1 solar metallicity forms a dark-matter-deficient galaxy with a stellar mass of $10^7\, \mathrm{M_\odot }$ for a relative velocity of $200\, \mathrm{km\, s^{-1}}$.