Accretion Of Galaxies Research Articles

Balmer Decrement Anomalies in Galaxies at z ∼ 6 Found by JWST Observations: Density-bounded Nebulae or Excited H i Clouds?

We investigate the physical origins of the Balmer decrement anomalies in GS-NDG-9422 and RXCJ2248-ID galaxies at z ∼ 6 whose Hα/Hβ values are significantly smaller than 2.7, the latter of which also shows anomalous Hγ/Hβ and Hδ/Hβ values beyond the errors. Because the anomalous Balmer decrements are not reproduced under the Case B recombination, we explore the nebulae with optical depths smaller and larger than the Case B recombination by physical modeling. We find two cases quantitatively explaining the anomalies: (1) density-bounded nebulae that are opaque only up to around Lyγ–Ly8 transitions and (2) ionization-bounded nebulae partly/fully surrounded by optically thick excited H i clouds. The case of (1) produces more Hβ photons via Lyγ absorption in the nebulae, requiring fine tuning in optical depth values, while this case helps ionizing photon escape for cosmic reionization. The case of (2) needs the optically thick excited Hi clouds with N 2 ≃ 1012−1013 cm−2, where N 2 is the column density of the hydrogen atom with the principal quantum number of n = 2. Interestingly, the high N 2 values qualitatively agree with the recent claims for GS-NDG-9422 with the strong nebular continuum requiring a number of 2s-state electrons and for RXCJ2248-ID with the dense ionized regions likely coexisting with the optically thick clouds. While the physical origin of the optically thick excited H i clouds is unclear, these results may suggest gas clouds with excessive collisional excitation caused by an amount of accretion and supernovae in the high-z galaxies.

Evolution of HOD and galaxy properties in filaments and nodes of the cosmic web

ABSTRACT We study the evolution of the halo occupation distribution (HOD) and galaxy properties of nodes and filamentary structures obtained by disperse from the Illustris TNG300-1 hydrodynamical simulation, in the redshift range $0 \le z \le 2$. We compute the HOD in filaments and nodes and fit the HOD parameters to study their evolution for both faint and bright galaxies. In nodes, the number of faint galaxies increases with decreasing redshift in the low-mass haloes, while no significant differences are seen in high-mass haloes. Limiting the HOD to bright galaxies shows that haloes increase in mass more than the number of bright galaxies they accrete. For filaments, no large differences in HOD are found for faint galaxies, although for brighter galaxies the behaviour is the same as in nodes. The HOD parametrization suggests that filaments have no effect on the mass required to host a galaxy (central or satellite), whereas nodes do. The results of the study indicate that with this parametrization, filaments do not seem to affect the stellar mass content of galaxies. In contrast, nodes appear to affect haloes with masses below approximately $10^{12.5} h^{-1} {\rm M}_{\odot }$ at local redshift. The analysis of the galaxy colour evolution shows a reddening towards lower redshift, although these processes seem to be more efficient in massive haloes, with a strong effect on bright galaxies. The general evolution suggests that the building of galaxy population within haloes is influenced by both the accretion of faint galaxies and the mass growth of the bright ones.

Hot gas accretion fuels star formation faster than cold accretion in high-redshift galaxies

ABSTRACT We use high-resolution ($\simeq$35pc) hydrodynamical simulations of galaxy formation to investigate the relation between gas accretion and star formation in galaxies hosted by dark matter haloes of mass $10^{12}$${{\mathrm{M}}_{\odot }}$ at $z = 2$. At high-redshift, cold-accreted gas is expected to be readily available for star formation, while gas accreted in a hot mode is expected to require a longer time to cool down before being able to form stars. Contrary to these expectations, we find that the majority of cold-accreted gas takes several hundred Myr longer to form stars than hot-accreted gas after it reaches the inner circumgalactic medium (CGM). Approximately 10 per cent of the cold-accreted gas flows rapidly through the inner CGM on to the galactic disc. The remaining 90 per cent is trapped in a turbulent accretion region that extends up to $\sim 50$ per cent of the virial radius, from which it takes several hundred Myr for the gas to be transported to the star-forming disc. In contrast, most hot shock-heated gas avoids this ‘slow track’, and accretes directly from the CGM on to the disc where stars can form. We find that shock-heating of cold gas after accretion in the inner CGM and supernova-driven outflows contribute to, but do not fully explain, the delay in star formation. These processes combined slow down the delivery of cold-accreted gas to the galactic disc and consequently limit the rate of star formation in Milky Way mass galaxies at $z \gt 2$.

Black holes regulate cool gas accretion in massive galaxies

The nucleus of almost all massive galaxies contains a supermassive black hole (BH)1. The feedback from the accretion of these BHs is often considered to have crucial roles in establishing the quiescence of massive galaxies2–14, although some recent studies show that even galaxies hosting the most active BHs do not exhibit a reduction in their molecular gas reservoirs or star formation rates15–17. Therefore, the influence of BHs on galaxy star formation remains highly debated and lacks direct evidence. Here, based on a large sample of nearby galaxies with measurements of masses of both BHs and atomic hydrogen (HI), the main component of the interstellar medium18, we show that the HI gas mass to stellar masses ratio (μHI = MHI/M⋆) is more strongly correlated with BH masses (MBH) than with any other galaxy parameters, including stellar mass, stellar mass surface density and bulge masses. Moreover, once the μHI–MBH correlation is considered, μHI loses dependence on other galactic parameters, demonstrating that MBH serves as the primary driver of μHI. These findings provide important evidence for how the accumulated energy from BH accretion regulates the cool gas content in galaxies, by ejecting interstellar medium gas and/or suppressing gas cooling from the circumgalactic medium.

AuriDESI: mock catalogues for the DESI Milky Way Survey

ABSTRACT The Dark Energy Spectroscopic Instrument Milky Way Survey (DESI MWS) will explore the assembly history of the Milky Way by characterizing remnants of ancient dwarf galaxy accretion events and improving constraints on the distribution of dark matter in the outer halo. We present mock catalogues that reproduce the selection criteria of MWS and the format of the final MWS data set. These catalogues can be used to test methods for quantifying the properties of stellar halo substructure and reconstructing the Milky Way’s accretion history with the MWS data, including the effects of halo-to-halo variance. The mock catalogues are based on a phase-space kernel expansion technique applied to star particles in the Auriga suite of six high-resolution lambda-cold dark matter magnetohydrodynamic zoom-in simulations. They include photometric properties (and associated errors) used in DESI target selection and the outputs of the MWS spectral analysis pipeline (radial velocity, metallicity, surface gravity, and temperature). They also include information from the underlying simulation, such as the total gravitational potential and information on the progenitors of accreted halo stars. We discuss how the subset of halo stars observable by MWS in these simulations corresponds to their true content and properties. These mock Milky Ways have rich accretion histories, resulting in a large number of substructures that span the whole stellar halo out to large distances and have substantial overlap in the space of orbital energy and angular momentum.

The progenitors of the intra-cluster light and intra-cluster globular clusters in galaxy groups and clusters

ABSTRACT We use the TNG50 from the IllustrisTNG suite of cosmological hydrodynamical simulation, complemented by a catalogue of tagged globular clusters, to investigate the properties and build up of two extended luminous components: the intra-cluster light (ICL) and the intra-cluster globular clusters (ICGCs). We select the 39 most massive groups and clusters in the box, spanning the range of virial masses $5 \times 10^{12} \lt \rm M_{200}/\rm {\rm M}_{\odot } \lt 2 \times 10^{14}$. We find good agreement between predictions from the simulations and current observational estimates of the fraction of mass in the ICL and its radial extension. The stellar mass of the ICL is only $\sim 10~{{\ \rm per\ cent}}$–20 per cent of the stellar mass in the central galaxy but encodes useful information on the assembly history of the group or cluster. About half the ICL in all our systems is brought in by galaxies in a narrow stellar mass range, M* = 1010–1011 M⊙. However, the contribution of low-mass galaxies (M* < 1010 M⊙) to the build up of the ICL varies broadly from system to system, $\sim 5~{{\ \rm per\ cent}}-45~{{\ \rm per\ cent}}$, a feature that might be recovered from the observable properties of the ICL at z = 0. At fixed virial mass, systems where the accretion of dwarf galaxies plays an important role have shallower metallicity profiles, less metal content, and a lower stellar mass in the ICL than systems where the main contributors are more massive galaxies. We show that intra-cluster GCs are also good tracers of this history, representing a valuable alternative when diffuse light is not detectable.

Accretion onto disc galaxies via hot and rotating CGM inflows

ABSTRACT Observed accretion rates onto the Milky Way and other local spirals fall short of that required to sustain star formation for cosmological timescales. A potential avenue for this unseen accretion is a rotating inflow in the volume-filling hot phase ($\sim 10^6\, {\rm K}$) of the circumgalactic medium (CGM), as suggested by some cosmological simulations. Using hydrodynamic simulations and a new analytic solution valid in the slow-rotation limit, we show that a hot inflow spins up as it approaches the galaxy, while remaining hot, subsonic, and quasi-spherical. Within the radius of angular momentum support ($\sim 15\, {\rm kpc}$ for the Milky Way) the hot flow flattens into a disc geometry and then cools from $\sim 10^6$ to $\sim 10^4\, {\rm K}$ at the disc–halo interface. Cooling affects all hot gas, rather than just a subset of individual gas clouds, implying that accretion via hot inflows does not rely on local thermal instability in contrast with ‘precipitation’ models for galaxy accretion. Prior to cooling and accretion the inflow completes ≈tcool/tff radians of rotation, where tcool/tff is the cooling time to free-fall time ratio in hot gas immediately outside the galaxy. The ratio tcool/tff may thus govern the development of turbulence and enhancement of magnetic fields in gas accreting onto low-redshift spirals. We show that if rotating hot inflows are common in Milky-Way-size disc galaxies, as predicted, then signatures of the expected hot gas rotation profile should be observable with X-ray telescopes and fast radio burst surveys.

The origin of lopsided satellite galaxy distribution around isolated systems in MillenniumTNG

ABSTRACT Dwarf satellites in galaxy groups are distributed in an anisotropic and asymmetric manner, which is called the ‘lopsided satellite distribution’. This lopsided signal has been observed not only in galaxy pairs but also in isolated systems. However, the physical origin of the lopsided signal in isolated systems is still unknown. In this work, we investigate this in the state-of-the-art hydrodynamical simulation of the MillenniumTNG Project by tracing each system back to high redshift. We find that the lopsided signal is dominated by satellites located in the outer regions of the halo and is also dominated by recently accreted satellites. The lopsided signal originates from the anisotropic accretion of galaxies from the surrounding large-scale structure and that, after accretion, the non-linear evolution of satellites inside the dark matter halo weakens the lopsidedness. The signal decreases as cosmic time passes because of a competition between anisotropic accretion and internal evolution within dark matter haloes. Our findings provide a useful perspective for the study of galaxy evolution, especially for the origin of the spatial satellite galaxy distributions.

On the accretion of a new group of galaxies onto Virgo – III. The stellar population radial gradients of dEs

ABSTRACT Using MUSE data, we investigate the radial gradients of stellar population properties (namely age, [M/H], and the abundance ratio of α elements [α/Fe]) for a sample of nine dwarf early-type (dE) galaxies with log(M⋆/M⊙) ∼ 9.0 and an infall time onto the Virgo cluster of 2–3 Gyr ago. We followed a similar approach as in Bidaran et al. to derive their stellar population properties and star formation histories (SFHs) through fitting observed spectral indices and full spectral fitting, respectively. We find that these nine dE galaxies have truncated [Mg/Fe] versus [Fe/H] profiles than equally massive Virgo dE galaxies with longer past infall times. Short profiles of three dE galaxies are the result of their intense star formation which has been quenched long before their accretion onto the Virgo cluster, possibly as a result of their group environment. In the remaining six dE galaxies, profiles mainly trace a recent episode of star burst within 0.4Re which results in higher light-weighted [α/Fe] values. The latter SFH peak can be due to ram pressure exerted by the Virgo cluster at the time of the accretion of the dE galaxies. Also, we show that younger, more metal-rich, and less α-enhanced stellar populations dominate their inner regions (i.e. < 0.4Re) resulting in mainly flat ∇age, negative ∇[M/H], and positive ∇[α/Fe]. We find that with increasing log(σRe) of dE galaxies, ∇age and ∇[α/Fe] flatten, and the latter correlation persists even after including early-type galaxies up to log(σRe ∼ 2.5), possibly due to the more extended star formation activity in the inner regions of dEs, as opposed to more massive early-type galaxies.

GaiaData Release 3

Context.The motion of stars has been used to reveal details of the complex history of the Milky Way, in constant interaction with its environment. Nevertheless, to reconstruct the Galactic history puzzle in its entirety, the chemo-physical characterisation of stars is essential. PreviousGaiadata releases were supported by a smaller, heterogeneous, and spatially biased mixture of chemical data from ground-based observations.Aims.GaiaData Release 3 opens a new era of all-sky spectral analysis of stellar populations thanks to the nearly 5.6 million stars observed by the Radial Velocity Spectrometer (RVS) and parametrised by the GSP-Spec module. In this work, we aim to demonstrate the scientific quality ofGaia’s Milky Way chemical cartography through a chemo-dynamical analysis of disc and halo populations.Methods.Stellar atmospheric parameters and chemical abundances provided byGaiaDR3 spectroscopy are combined with DR3 radial velocities and EDR3 astrometry to analyse the relationships between chemistry and Milky Way structure, stellar kinematics, and orbital parameters.Results.The all-skyGaiachemical cartography allows a powerful and precise chemo-dynamical view of the Milky Way with unprecedented spatial coverage and statistical robustness. First, it reveals the strong vertical symmetry of the Galaxy and the flared structure of the disc. Second, the observed kinematic disturbances of the disc – seen as phase space correlations – and kinematic or orbital substructures are associated with chemical patterns that favour stars with enhanced metallicities and lower [α/Fe] abundance ratios compared to the median values in the radial distributions. This is detected both for young objects that trace the spiral arms and older populations. Severalα, iron-peak elements and at least one heavy element trace the thin and thick disc properties in the solar cylinder. Third, young disc stars show a recent chemical impoverishment in several elements. Fourth, the largest chemo-dynamical sample of open clusters analysed so far shows a steepening of the radial metallicity gradient with age, which is also observed in the young field population. Finally, theGaiachemical data have the required coverage and precision to unveil galaxy accretion debris and heated disc stars on halo orbits through their [α/Fe] ratio, and to allow the study of the chemo-dynamical properties of globular clusters.Conclusions.GaiaDR3 chemo-dynamical diagnostics open new horizons before the era of ground-based wide-field spectroscopic surveys. They unveil a complex Milky Way that is the outcome of an eventful evolution, shaping it to the present day.

Hidden depths in the local Universe: The Stellar Stream Legacy Survey

Context. Mergers and tidal interactions between massive galaxies and their dwarf satellites are a fundamental prediction of the Lambda-cold dark matter cosmology. These events are thought to provide important observational diagnostics of non-linear structure formation. Stellar streams in the Milky Way and Andromeda are spectacular evidence for ongoing satellite disruption. However, constructing a statistically meaningful sample of tidal streams beyond the Local Group has proven a daunting observational challenge, and the full potential for deepening our understanding of galaxy assembly using stellar streams has yet to be realised. Aims. Here we introduce the Stellar Stream Legacy Survey, a systematic imaging survey of tidal features associated with dwarf galaxy accretion around a sample of ∼3100 nearby galaxies within z ∼ 0.02, including about 940 Milky Way analogues. Methods. Our survey exploits public deep imaging data from the DESI Legacy Imaging Surveys, which reach surface brightness as faint as ∼29 mag arcsec−2 in the r band. As a proof of concept of our survey, we report the detection and broad-band photometry of 24 new stellar streams in the local Universe. Results. We discuss how these observations can yield new constraints on galaxy formation theory through comparison to mock observations from cosmological galaxy simulations. These tests will probe the present-day mass assembly rate of galaxies, the stellar populations and orbits of satellites, the growth of stellar halos, and the resilience of stellar disks to satellite bombardment.

The Observable Properties of Galaxy Accretion Events in Milky Way–like Galaxies in the FIRE-2 Cosmological Simulations

In the Λ-Cold Dark Matter model of the universe, galaxies form in part through accreting satellite systems. Previous works have built an understanding of the signatures of these processes contained within galactic stellar halos. This work revisits that picture using seven Milky Way–like galaxies in the Latte suite of FIRE-2 cosmological simulations. The resolution of these simulations allows a comparison of contributions from satellites aboveM * ≳ 10 × 7 M ⊙, enabling the analysis of observable properties for disrupted satellites in a fully self-consistent and cosmological context. Our results show that the time of accretion and the stellar mass of an accreted satellite are fundamental parameters that in partnership dictate the resulting spatial distribution, orbital energy, and [α/Fe]-[Fe/H] compositions of the stellar debris of such mergers at present day. These parameters also govern the resulting dynamical state of an accreted galaxy at z = 0, leading to the expectation that the inner regions of the stellar halo (R GC ≲ 30 kpc) should contain fully phase-mixed debris from both lower- and higher-mass satellites. In addition, we find that a significant fraction of the lower-mass satellites accreted at early times deposit debris in the outer halo (R GC > 50 kpc) that are not fully phased-mixed, indicating that they could be identified in kinematic surveys. Our results suggest that, as future surveys become increasingly able to map the outer halo of our Galaxy, they may reveal the remnants of long-dead dwarf galaxies whose counterparts are too faint to be seen in situ in higher-redshift surveys.

HALOGAS: Strong constraints on the neutral gas reservoir and accretion rate in nearby spiral galaxies

Context. Galaxies in the local Universe are thought to require ongoing replenishment of their gas reservoir in order to maintain the observed star formation rates. Cosmological simulations predict that this accretion can occur in both a dynamically hot and cold mode, depending on the redshift, halo mass, and the details of the included feedback processes. However, until now, observational evidence of the accretion required to match the observed star formation histories is lacking. Aims. Within the framework of the Hydrogen Accretion in LOcal GalaxieS (HALOGAS) survey, this paper attempts to determine whether galaxies in the local Universe possess a significant reservoir of cold neutral gas and the possible accretion rates these reservoirs could provide. Additionally, with this moderately sized sample, we can start to investigate whether the observed accretion is connected to intrinsic properties of the galaxies such as Hubble type, star formation rate, or environment. Methods. We searched the vicinity of 22 nearby galaxies in a systematic and automated manner for isolated H I clouds or distinct streams that are not yet connected to the galaxy disk. The HALOGAS observations were carried out with the Westerbork Synthesis Radio Telescope and represent one of the most sensitive and detailed H I surveys to date. These observations typically reach column density sensitivities of ∼1019 cm−2 over a 20 km s−1 line width. Results. We find 14 secure H I cloud candidates without an observed optical counterpart in the entire HALOGAS sample. These cloud candidates appear to be analogs to the most massive clouds detected in the extensive cloud distributions around the Milky Way and M 31. However, their numbers seem significantly reduced on average compared to the numbers in these galaxies. Within the framework of cold accretion, we constrain upper limits for H I accretion in the local Universe. The average H I mass currently observed in a state suggestive of accretion onto the galaxies amounts to a rate of 0.05 M⊙ yr−1 with a stringent upper limit of 0.22 M⊙ yr−1, confirming previous estimates. This is much lower than the average star formation rate in this sample. Our best estimate, based on the Green Bank Telescope detection limits of several galaxies in the sample, suggests that another 0.04 M⊙ yr−1 of neutral hydrogen at most could be accreted from clouds and streams that remain undetected. Conclusions. These results show that in nearby galaxies, neutral hydrogen is not being accreted at the same rate as stars are currently being formed. Our study cannot exclude that other forms of gas accretion are at work, such as those provided by direct infall of ionized intergalactic gas or the condensation of coronal gas, triggered by galactic fountain activities. However, these observations do not reveal extended neutral gas reservoirs around most nearby spiral galaxies either.

Origin of highly r-process-enhanced stars in a cosmological zoom-in simulation of a Milky Way-like galaxy

ABSTRACT The r-process-enhanced (RPE) stars provide fossil records of the assembly history of the Milky Way (MW) and the nucleosynthesis of the heaviest elements. Observations by the R-Process Alliance (RPA) and others have confirmed that many RPE stars are associated with chemo-dynamically tagged groups, which likely came from accreted dwarf galaxies of the MW. However, we do not know how RPE stars are formed. Here, we present the result of a cosmological zoom-in simulation of an MW-like galaxy with r-process enrichment, performed with the highest resolution in both time and mass. Thanks to this advancement, unlike previous simulations, we find that most highly RPE (r-II; [Eu/Fe] > +0.7) stars are formed in low-mass dwarf galaxies that have been enriched in r-process elements for [Fe/H] $\lt -2.5$, while those with higher metallicity are formed in situ, in locally enhanced gas clumps that were not necessarily members of dwarf galaxies. This result suggests that low-mass accreted dwarf galaxies are the main formation site of r-II stars with [Fe/H] $\, \lt -2.5$. We also find that most low-metallicity r-II stars exhibit halo-like kinematics. Some r-II stars formed in the same halo show low dispersions in [Fe/H] and somewhat larger dispersions of [Eu/Fe], similar to the observations. The fraction of simulated r-II stars is commensurate with observations from the RPA, and the distribution of the predicted [Eu/Fe] for halo r-II stars matches that observed. These results demonstrate that RPE stars can be valuable probes of the accretion of dwarf galaxies in the early stages of their formation.

TOPoS

Context. The TOPoS project has the goal to find and analyse Turn-Off (TO) stars of extremely low metallicity. To select the targets for spectroscopic follow-up at high spectral resolution, we have relied on low-resolution spectra from the Sloan Digital Sky Survey. Aims. In this paper we use the metallicity estimates we have obtained from our analysis of the SDSS spectra to construct the metallicity distribution function (MDF) of the Milky Way, with special emphasis on its metal-weak tail. The goal is to provide the underlying distribution out of which the TOPoS sample was extracted. Methods. We make use of SDSS photometry, Gaia photometry and distance estimates derived from the Gaia parallaxes to derive a metallicity estimate for a large sample of over 24 million TO stars. This sample is used to derive the metallicity bias of the sample for which SDSS spectra are available. Results. We determined that the spectroscopic sample is strongly biased in favour of metal-poor stars, as intended. A comparison with the unbiased photometric sample allows to correct for the selection bias. We select a sub-sample of stars with reliable parallaxes for which we combine the SDSS radial velocities with Gaia proper motions and parallaxes to compute actions and orbital parameters in the Galactic potential. This allows us to characterize the stars dynamically, and in particular to select a sub-sample that belongs to the Gaia-Sausage-Enceladus (GSE) accretion event. We are thus able to provide also the MDF of GSE. Conclusions. The metal-weak tail derived in our study is very similar to that derived in the H3 survey and in the Hamburg/ESO Survey. This allows us to average the three MDFs and provide an error bar for each metallicity bin. Inasmuch the GSE structure is representative of the progenitor galaxy that collided with the Milky Way, that galaxy appears to be strongly deficient in metal-poor stars compared to the Milky Way, suggesting that the metal-weak tail of the latter has been largely formed by accretion of low mass galaxies rather than massive galaxies, such as the GSE progenitor.

Multiple gas acquisition events in galaxies with dual misaligned gas disks

Frequent accretion of external cold gas is thought to play an important role in galaxy assembly. However, almost all known kinematically misaligned galaxies identify only one gas disk that is misaligned with the stellar disk, implying a single gas acquisition event. Here we report a new configuration in two galaxies where both contain two gas disks misaligned with each other and also with the stellar disk. Such systems are not expected to be stable or long-lasting, challenging the traditional picture of gas accretion of galaxies and their angular momentum build-up. The differences in kinematic position angles are larger than 120{\deg} between the two gas disks, and 40{\deg} between each gas disk and the stellar component. The star formation activity is enhanced at the interface of the two gas disks compared with the other regions within the same galaxy. Such systems illustrate that low-redshift galaxies can still experience multiple gas acquisition events, and provide a new view into the origins of galactic gas.

Accretion of Galaxies around Supermassive Black Holes and a Theoretical Model of the Tully-Fisher and M-Sigma Relations

The observed Tully-Fisher and Faber-Jackson laws between the baryonic mass of galaxies and the velocity of motion of stars at the edge of galaxies are explained within the framework of the model of accretion of galaxies around supermassive black holes (SMBH). The accretion model can also explain the M-sigma relation between the mass of a supermassive black hole and the velocity of stars in the bulge. The difference in the mechanisms of origin of elliptical galaxies with low angular momentum and disk galaxies with high angular momentum can be associated with 3D and 2D accretion.

The Galaxy Replacement Technique (GRT): A New Approach to Study Tidal Stripping and Formation of Intracluster Light in a Cosmological Context

Abstract We introduce the Galaxy Replacement Technique (GRT) that allows us to model tidal stripping of galaxies with very high mass (m star = 5.4 × 104 M ⊙ h −1) and high spatial resolution (10 pc h −1), in a fully cosmological context, using an efficient and fast technique. The technique works by replacing multiple low-resolution dark-matter (DM) halos in the base cosmological simulation with high-resolution models, including a DM halo and stellar disk. We apply the method to follow the hierarchical buildup of a cluster since redshift ∼8 to now, through the hierarchical accretion of galaxies, individually or in substructures such as galaxy groups. We find we can successfully reproduce the observed total stellar masses of observed clusters since redshift ∼1. The high resolution allows us to accurately resolve the tidal stripping process and well describe the formation of ultralow surface brightness features in the cluster (μ V < 32 mag arcsec−2) such as the intracluster light (ICL), shells, and tidal streams. We measure the evolution of the fraction of light in the ICL and brightest cluster galaxy using several different methods. While their broad response to the cluster-mass growth history is similar, the methods show systematic differences, meaning we must be careful when comparing studies that use distinct methods. The GRT represents a powerful new tool for studying tidal effects on galaxies and exploring the formation channels of the ICL in a fully cosmological context and with large samples of simulated groups and clusters.

Phenomenological distribution of dark matter halos of galaxy comprising of stellar streams

Using the potential associated with NFW mass profile (Navarro et al. in Mon. Not. R. Astron. Soc. 402:21, 2010) enclosed at scale radius $r_{s}$ , we calculate the partition function of the clustering of dark halos comprising of stellar streams, which are the outcome of accretion of globular clusters or dwarf galaxies onto a massive host galaxy. The thermodynamics of the system of halos of stellar streams may provide crucial information on the properties of galaxy clustering. The distribution function is studied through statistical formalism. The study can give us an idea of the extended structure of galaxies and the contribution of halos to the thermodynamics and the mass content of the host galaxy. In an attempt to fit the theoretical orbital velocity to the galaxy rotation curve for the data available for galaxy $M33$ , we developed another potential that exactly leads to the best fit orbital velocity of the galaxy. Thus a phenomenological theory for the galaxy with a dark halo is established, with a new density profile.

Age-dating Red Giant Stars Associated with Galactic Disk and Halo Substructures

Abstract The vast majority of Milky Way stellar halo stars were likely accreted from a small number (≲3) of relatively large dwarf galaxy accretion events. However, the timing of these events is poorly constrained and predominantly relies on indirect dynamical mixing arguments or imprecise age measurements of stars associated with debris structures. Here, we aim to infer robust stellar ages for stars associated with galactic substructures to more directly constrain the merger history of the Galaxy. By combining kinematic, asteroseismic, and spectroscopic data where available, we infer stellar ages for a sample of 10 red giant stars that were kinematically selected to be within the stellar halo, a subset of which are associated with the Gaia–Enceladus–Sausage halo substructure, and compare their ages to 3 red giant stars in the Galactic disk. Despite systematic differences in both absolute and relative ages determined here, age rankings of stars in this sample are robust. Passing the same observable inputs to multiple stellar age determination packages, we measure a weighted average age for the Gaia–Enceladus–Sausage stars in our sample of 8 ± 3 (stat.) ± 1 (sys.) Gyr. We also determine hierarchical ages using isochrones for the populations of Gaia–Enceladus–Sausage, in situ halo and disk stars, finding a Gaia–Enceladus–Sausage population age of 8.0 − 2.3 + 3.2 Gyr. Although we cannot distinguish hierarchical population ages of halo or disk structures with our limited data and sample of stars, this framework should allow a distinct characterization of Galactic substructures using larger stellar samples and additional data available in the near future.