Recent observations have revealed that the time-evolution of the S0 number fraction at intermediate and high redshifts (0.2 < z < 0.8) is more dramatic in groups of galaxies than in clusters. In ...order to understand the origin of S0s in groups, we investigate numerically the morphological transformation of spirals into S0s through group-related physical processes. Our chemodynamical simulations show that spirals in group environments can be strongly influenced by repetitive slow encounters with group member galaxies so that those with thin discs and prominent spiral arm structures can be transformed into S0s with thick discs and without any prominent spiral arm structure. Such tidal interactions can also trigger repetitive starbursts within the bulges of spirals and consequently increase significantly the masses of their bulges. Owing to the rapid consumption of gas initially in spirals during the bulge growth, the S0s can become gas-poor. The S0s transformed from spirals in this way have young and metal-rich stellar populations in the inner regions of their bulges. The simulated S0s have lower maximum rotational velocities and flatter radial line-of-sight velocity dispersion profiles in comparison to their progenitor spirals. The formation processes of S0s due to tidal interactions depend not only on the masses and orbits of the progenitor spirals, but also on the group mass. A significant fraction (10-30 per cent) of stars and gas can be stripped during this spiral to S0 morphological transformation so that intragroup stars and gas can be formed. Based on these results, we discuss structures, kinematics, chemical properties and the Tully-Fisher relation of S0s in groups.
We present the results of a study of stellar population properties at large galactocentric radii of 14 low-mass early-type galaxies in the Fornax and Virgo clusters. We derive radial profiles of age, ...total metallicity Z/H and α/Fe abundance ratios out to ∼1–3 effective radii by using nearly all of the Lick/IDS absorption-line indices in comparison to recent single stellar population models. We extend our study to higher galaxy mass via a novel literature compilation of 37 early-type galaxies, which provides stellar population properties out to one effective radius. We find that metallicity gradients correlate with galactic mass, and the relationship shows a sharp change in slope at a dynamical mass of ∼3.5 × 1010 M⊙. The central and mean values of the stellar population parameters (measured in r≤re/8 and at r=re, respectively) define positive mass trends. We suggest that the low metallicities, almost solar α/Fe ratios and the tight mass–metallicity gradient relation displayed by the low-mass galaxies are indicative of an early star-forming collapse with extended (i.e. ≥1 Gyr), low-efficiency star formation, and mass-dependent galactic outflows of metal-enriched gas. The flattening of metallicity gradients in high-mass galaxies and the broad scatter of the relationship are attributed to merger events. The high metallicities and supersolar abundances shown by these galaxies imply a rapid, high efficiency star formation. The observed Z/H–mass and α/Fe–mass relationships can be interpreted as a natural outcome of an early star-forming collapse. However, we find that hierarchical galaxy formation models implementing mass-dependent star formation efficiency, varying initial mass function, energy feedback via active galactic nucleus and the effects due to merger-induced starbursts can also reproduce both our observed relationships.
ABSTRACT
Using deep g, r, i imaging from the VST Early-type GAlaxy Survey (VEGAS), we have searched for ultradiffuse galaxies (UDGs) in the IC 1459 group. Assuming they are group members, we identify ...nine galaxies with physical sizes and surface brightnesses that match the UDG criteria within our measurement uncertainties. They have mean colours of g − i = 0.6 and stellar masses of ∼108 M⊙. Several galaxies appear to have associated systems of compact objects, e.g. globular clusters. Two UDGs contain a central bright nucleus, with a third UDG revealing a remarkable double nucleus. This appears to be the first reported detection of a double nucleus in a UDG – its origin is currently unclear.
ABSTRACT
The ultra diffuse galaxy in the NGC 5846 group (NGC 5846_UDG1) was shown to have a large number of globular cluster (GC) candidates from deep imaging as part of the VEGAS survey. Recently, ...Müller et al. published a velocity dispersion, based on a dozen of its GCs. Within their quoted uncertainties, the resulting dynamical mass allowed for either a dark matter free or a dark-matter-dominated galaxy. Here, we present spectra from KCWI that reconfirms membership of the NGC 5846 group and reveals a stellar velocity dispersion for UDG1 of σGC = 17 ± 2 km s−1. Our dynamical mass, with a reduced uncertainty, indicates a very high contribution of dark matter within the effective radius. We also derive an enclosed mass from the locations and motions of the GCs using the tracer mass estimator, finding a similar mass inferred from our stellar velocity dispersion. We find no evidence that the galaxy is rotating and is thus likely pressure supported. The number of confirmed GCs, and the total number inferred for the system (∼45), suggests a total halo mass of ∼2 × 1011 M⊙. A cored mass profile is favoured when compared to our dynamical mass. Given its stellar mass of 1.1 × 108 M⊙, NGC 5846_UDG1 appears to be an ultra diffuse galaxy with a dwarf-like stellar mass and an overly massive halo.
The two formation pathways of S0 galaxies Deeley, Simon; Drinkwater, Michael J; Sweet, Sarah M ...
Monthly Notices of the Royal Astronomical Society,
11/2021, Letnik:
508, Številka:
1
Journal Article
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ABSTRACT
Despite their ubiquity throughout the Universe, the formation of S0 galaxies remains uncertain. Recent observations have revealed that S0 galaxies make up a diverse population that is ...difficult to explain with a single formation pathway, suggesting that the picture of how these galaxies form is more complicated than originally envisioned. Here, we take advantage of the latest hydrodynamical cosmological simulations and follow up these studies with an investigation into the formation histories of S0s in IllustrisTNG. We first classify IllustrisTNG galaxies in a way that is fully consistent with the observations, and reproduce the observed photometric and environmental distributions seen for the S0 population. We then trace the formation histories of S0 galaxies back through time, identifying two main distinct pathways: those that experienced gas stripping via group infalls (37 per cent of S0s) or significant merger events (57 per cent). We find that those forming via mergers feature a transient star-forming ring, whose present-day occurrence rate matches observations. We find that these formation pathways together can reproduce the range in rotational support in observed S0s, concluding that there are two main formation pathways for S0 galaxies.
ABSTRACT
We examine ultra-diffuse galaxies (UDGs) and their relation to non-UDGs in mass–radius–luminosity space. We begin by publishing Keck/KCWI spectroscopy for the Coma cluster UDG Y358, for ...which we measure both a recessional velocity and velocity dispersion. Our recessional velocity confirms association with the Coma cluster and Y358’s status as a UDG. From our velocity dispersion (19 ± 3 km s−1), we calculate a dynamical mass within the half-light radius, which provides evidence for a core in Y358’s dark matter halo. We compare this dynamical mass, along with those for globular cluster (GC)-rich/-poor UDGs in the literature, to mass profiles for isolated, gas-rich UDGs, and UDGs in the NIHAO/FIRE simulations. We find GC-poor UDGs have dynamical masses similar to isolated, gas-rich UDGs, suggesting an evolutionary pathway may exist between the two. Conversely, GC-rich UDGs have dynamical masses too massive to be easily explained as the evolution of the isolated, gas-rich UDGs. The simulated UDGs match the dynamical masses of the GC-rich UDGs. However, once compared in stellar mass–halo mass space, the FIRE/NIHAO-simulated UDGs do not match the halo masses of either the isolated, gas-rich UDGs or the GC-rich UDGs at the same stellar mass. Finally, we supplement our data for Y358 with other UDGs that have measured velocity dispersions in the literature. We compare this sample to a wide range of non-UDGs in mass–radius–luminosity space, finding UDGs have a similar locus to non-UDGs of similar luminosity with the primary difference being their larger half-light radii.
Abstract
We present hydrodynamical N-body models which demonstrate that elliptical galaxies can transform into S0s by acquiring a disc. In particular, we show that the merger with a massive gas-rich ...satellite can lead to the formation of a baryonic disc around an elliptical. We model the elliptical as a massive, compact galaxy which could be observed as a ‘red nugget’ in the high-z universe. This scenario contrasts with existing S0 formation scenarios in the literature in two important ways. First, the progenitor is an elliptical galaxy whereas scenarios in the literature typically assume a spiral progenitor. Secondly, the physical conditions underlying our proposed scenario can exist in low-density environments such as the field, in contrast to scenarios in the literature which typically address dense environments like clusters and groups. As a consequence, S0s in the field may be the most likely candidates to have evolved from elliptical progenitors. Our scenario also naturally explains recent observations which indicate that field S0s may have older bulges than discs, contrary to cluster S0s which seem to have older discs than bulges.
ABSTRACT
We investigate the kinematic properties of nine nearby early-type galaxies with evidence of a disc-like component. Three of these galaxies are located in the field, five in the group, and ...only one in the cluster environment. By combining the kinematics of the stars with those of the globular clusters (GCs) and planetary nebulae (PNe), we probe the outer regions of our galaxies out to $\sim 4\!-\!6\, R_{\mathrm{e}}$. Six galaxies have PNe and red GCs that show good kinematic alignment with the stars, whose rotation occurs along the photometric major-axis of the galaxies, suggesting that both the PNe and red GCs are good tracers of the underlying stellar population beyond that traced by the stars. Additionally, the blue GCs also show rotation that is overall consistent with that of the red GCs in these six galaxies. The remaining three galaxies show kinematic twists and misalignment of the PNe and GCs with respect to the underlying stars, suggesting recent galaxy interactions. From the comparison with simulations, we propose that all six aligned galaxies that show similar dispersion-dominated kinematics at large radii ($\gt 2\!-\!3\, R_{\mathrm{e}}$) had similar late (z ≲ 1) assembly histories characterized by mini mergers (mass-ratio < 1:10). The different Vrot/σ profiles are then the result of an early (z > 1) minor merger (1:10 < mass-ratio < 1:4) for the four galaxies with peaked and decreasing Vrot/σ profiles and of a late minor merger for the two galaxies with flat Vrot/σ profiles. The three misaligned galaxies likely formed through multiple late minor mergers that enhanced their velocity dispersion at all radii, or a late major merger that spun-up both the GC subpopulations at large radii. Therefore, lenticular galaxies can have complex merger histories that shape their characteristic kinematic profile shapes.
ABSTRACT
Compact elliptical (cE) galaxies remain an elusively difficult galaxy class to study. Recent observations have suggested that isolated and host-associated cEs have different formation ...pathways, while simulation studies have also shown different pathways can lead to a cE galaxy. However, a solid link has not been established, and the relative contributions of each pathway in a cosmological context remains unknown. Here, we combine a spatially resolved observational sample of cEs taken from the Sydney-AAO Multi-object Integral field spectrograph Galaxy Survey with a matched sample of galaxies within the IllustrisTNG cosmological simulation to establish an overall picture of how these galaxies form. The observed cEs located near a host galaxy appear redder, smaller, and older than isolated cEs, supporting previous evidence for multiple formation pathways. Tracing the simulated cEs back through time, we find two main formation pathways; 32 ± 5 per cent formed via the stripping of a spiral galaxy by a larger host galaxy, while 68 ± 4 per cent formed through a gradual build-up of stellar mass in isolated environments. We confirm that cEs in different environments do indeed form via different pathways, with all isolated cEs in our sample having formed via in situ formation (i.e. none were ejected from a previous host), and 77 ± 6 per cent of host-associated cEs having formed via tidal stripping. Separating them by their formation pathway, we are able to reproduce the observed differences between isolated and host-associated cEs, showing that these differences can be fully explained by the different formation pathways dominating in each environment.
We use integral-field spectroscopy from the SAMI Galaxy Survey to identify galaxies that show evidence of recent quenching of star formation. The galaxies exhibit strong Balmer absorption in the ...absence of ongoing star formation in more than 10% of their spectra within the SAMI field of view. These -strong (HDS) galaxies (HDSGs) are rare, making up only ∼2% (25/1220) of galaxies with stellar mass > 10. The HDSGs make up a significant fraction of nonpassive cluster galaxies (15%; 17/115) and a smaller fraction (2.0%; 8/387) of the nonpassive population in low-density environments. The majority (9/17) of cluster HDSGs show evidence of star formation at their centers, with the HDS regions found in the outer parts of the galaxy. Conversely, the HDS signal is more evenly spread across the galaxy for the majority (6/8) of HDSGs in low-density environments and is often associated with emission lines that are not due to star formation. We investigate the location of the HDSGs in the clusters, finding that they are exclusively within 0.6R200 of the cluster center and have a significantly higher velocity dispersion relative to the cluster population. Comparing their distribution in projected phase space to those derived from cosmological simulations indicates that the cluster HDSGs are consistent with an infalling population that has entered the central 0.5r200,3D cluster region within the last ∼1 Gyr. In the eight of nine cluster HDSGs with central star formation, the extent of star formation is consistent with that expected of outside-in quenching by ram pressure stripping. Our results indicate that the cluster HDSGs are currently being quenched by ram pressure stripping on their first passage through the cluster.
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