Clues to the formation and evolution of Nuclear Star Clusters (NSCs) lie in their stellar populations. However, these structures are often very faint compared to their host galaxy, and spectroscopic analysis of NSCs is hampered by contamination of li
ght from the rest of the system. With the introduction of wide-field IFU spectrographs, new techniques have been developed to model the light from different components within galaxies, making it possible to cleanly extract the spectra of the NSCs and study their properties with minimal contamination from the light of the rest of the galaxy. This work presents the analysis of the NSCs in a sample of 12 dwarf galaxies in the Fornax Cluster observed with MUSE. Analysis of the stellar populations and star-formation histories reveal that all the NSCs show evidence of multiple episodes of star formation, indicating that they have built up their mass further since their initial formation. The NSCs were found to have systematically lower metallicities than their host galaxies, which is consistent with a scenario for mass-assembly through mergers with infalling globular clusters, while the presence of younger stellar populations and gas emission in the core of two galaxies is indicative of in-situ star formation. We conclude that the NSCs in these dwarf galaxies likely originated as globular clusters that migrated to the core of the galaxy which have built up their mass mainly through mergers with other infalling clusters, with gas-inflow leading to in-situ star formation playing a secondary role.
Using the photometric data from the Next Generation Fornax Survey, we find a significant radial alignment signal among the Fornax dwarf galaxies. For the first time, we report that the radial alignment signal of nucleated dwarfs is stronger than that
of non-nucleated ones at 2.4$sigma$ confidence level, and the dwarfs located in the outer region ($R>R_{rm{vir}}/3$; $R_{rm{vir}}$ is the Fornax virial radius) show slightly stronger radial alignment signal than those in the inner region ($R<R_{rm{vir}}/3$) at $1.5sigma$ level. We also find that the significance of radial alignment signal is independent of the luminosities or sizes of the dwarfs.
With the published data of apparent axis ratios for 1109 ultra-diffuse galaxies (UDGs) located in 17 low-redshift (z~ 0.020 - 0.063) galaxy clusters and 84 UDGs in 2 intermediate-redshift (z~ 0.308 - 0.348) clusters, we take advantage of a Markov Cha
in Monte Carlo approach and assume a ubiquitous triaxial model to investigate the intrinsic morphologies of UDGs. In contrast to the conclusion of Burkert (2017), i.e., the underlying shapes of UDGs are purely prolate ($C=B<A$), we find that the data favor the oblate-triaxial models ($C<Blesssim A$) over the nearly prolate ones. We also find that the intrinsic morphologies of UDGs are relevant to their stellar masses/luminosities, environments, and redshifts. First, for the low-redshift UDGs in the same environment, the more-luminous ones are always thicker than the less-luminous counterparts, possibly due to the more voilent internal supernovae feedback or external tidal interactions for the progenitors of the more-luminous UDGs. The UDG thickness dependence on luminosity is distinct from that of the typical quiescent dwarf ellipticals (dEs) and dwarf spheroidals (dSphs) in the local clusters and groups, but resembles that of massive galaxies; in this sense, UDGs may not be simply treated as an extension of the dE/dSph class with similar evolutionary histories. Second, for the low-redshift UDGs within the same luminosity range, the ones with smaller cluster-centric distances are more puffed-up, probably attributed to tidal interactions. Finally, the intermediate-redshift cluster UDGs are more flattened, which plausibly suggests a `disky origin for high-redshift, initial UDGs.
We report the photometric properties of 16 dwarf galaxies, 15 of which are newly identified, in the Western halo of the nearby giant elliptical galaxy NGC5128. All candidates are found at projected distances $sim!100!-!225$kpc from their giant host,
with luminosities $-10.82!leq!M_V/{rm mag}!leq!-7.42$ and effective radii $4!leq!r_{rm eff}!leq!17$ (or $75!leq!r_{rm eff}/{rm pc}!leq!300$ at the distance of NGC5128). We compare to other low-mass dwarf galaxies in the local universe and find them to populate the faint/compact extension of the size-luminosity relation that was previously not well-sampled by dwarf galaxies in the Centaurus A system, with optical colors similar to compact stellar systems like globular clusters and ultra-compact dwarf galaxies despite having much more diffuse morphologies. From optical $ugriz$ photometry, stellar masses are estimated to be $5.17!leq!log{cal M}_star/M_odot!leq!6.48$, with colors that show them to fall redward of the dwarf galaxy mass-metallicity relation. These colors suggest star formation histories that require some mechanism that would give rise to extra metal enrichment such as primordial formation within the halos of their giant galaxy hosts, non-primordial star formation from previously enriched gas, or extended periods of star formation leading to self-enrichment. We also report the existence of at least two sub-groups of dwarf candidates, each subtending $10-20$ on the sky, corresponding to projected physical separations of $10!-!20$kpc. True physical associations of these groups, combined with their potentially extended star formation histories, would imply that they may represent dwarf galaxy groups in the early stage of interaction upon infall into a giant elliptical galaxy halo in the very nearby universe.
We present the first results from the GeMS/GSAOI Galactic Globular Cluster Survey (G4CS) of the Milky-Way globular clusters (GCs) NGC 3201 and NGC 2298. Using the Gemini South Adaptive Optics Imager (GSAOI), in tandem with the Gemini Multi-conjugate
adaptive optics System (GeMS) on the 8.1-meter Gemini-South telescope, we collected deep near-IR observations of both clusters, resolving their constituent stellar populations down to $K_ssimeq21$ Vega mag. Point spread function (PSF) photometry was performed on the data using spatially-variable PSFs to generate $JHK_{s}$ photometric catalogues for both clusters. These catalogues were combined with Hubble Space Telescope (HST) data to augment the photometric wavelength coverage, yielding catalogues that span the near-ultraviolet (UV) to near-infrared (near-IR). We then applied 0.14 mas/year accurate proper-motion cleaning, differential-reddening corrections and chose to anchor our isochrones using the lower main-sequence knee (MSK) and the main-sequence turn-off (MSTO) prior to age determination. As a result of the data quality, we found that the $K_{s}$ vs. F606W$-K_{s}$ and F336W vs. F336W$-K_{s}$ color-magnitude diagrams (CMDs) were the most diagnostically powerful. We used these two color combinations to derive the stellar-population ages, distances and reddening values for both clusters. Following isochrone-fitting using three different isochrone sets, we derived best-fit absolute ages of $12.2pm0.5$ Gyr and $13.2pm0.4$ Gyr for NGC 3201 and NGC 2298, respectively. This was done using a weighted average over the two aforementioned color combinations, following a pseudo-$chi^2$ determination of the best-fit isochrone set. Our derived parameters are in good agreement with recent age determinations of the two clusters, with our constraints on the ages being or ranking among the most statistically robust.
We derive stellar population parameters for a representative sample of ultracompact dwarfs (UCDs) and a large sample of massive globular clusters (GCs) with stellar masses $gtrsim$ 10$^{6}$ $M_{odot}$ in the central galaxy M87 of the Virgo galaxy clu
ster, based on model fitting to the Lick-index measurements from both the literature and new observations. After necessary spectral stacking of the relatively faint objects in our initial sample of 40 UCDs and 118 GCs, we obtain 30 sets of Lick-index measurements for UCDs and 80 for GCs. The M87 UCDs have ages $gtrsim$ 8 Gyr and [$alpha$/Fe] $simeq$ 0.4 dex, in agreement with previous studies based on smaller samples. The literature UCDs, located in lower-density environments than M87, extend to younger ages and smaller [$alpha$/Fe] (at given metallicities) than M87 UCDs, resembling the environmental dependence of the Virgo dE nuclei. The UCDs exhibit a positive mass-metallicity relation (MZR), which flattens and connects compact ellipticals at stellar masses $gtrsim$ 10$^{8}$ $M_{odot}$. The Virgo dE nuclei largely follow the average MZR of UCDs, whereas most of the M87 GCs are offset towards higher metallicities for given stellar masses. The difference between the mass-metallicity distributions of UCDs and GCs may be qualitatively understood as a result of their different physical sizes at birth in a self-enrichment scenario or of galactic nuclear cluster star formation efficiency being relatively low in a tidal stripping scenario for UCD formation. The existing observations provide the necessary but not sufficient evidence for tidally stripped dE nuclei being the dominant contributors to the M87 UCDs.
Substructure in globular cluster (GC) populations around large galaxies is expected in galaxy formation scenarios that involve accretion or merger events, and it has been searched for using direct associations between GCs and structure in the diffuse
galaxy light, or with GC kinematics. Here, we present a search for candidate substructures in the GC population around the Virgo cD galaxy M87 through the analysis of the spatial distribution of the GC colors.~The study is based on a sample of $sim!1800$ bright GCs with high-quality $u,g,r,i,z,K_s$ photometry, selected to ensure a low contamination by foreground stars or background galaxies.~The spectral energy distributions of the GCs are associated with formal estimates of age and metallicity, which are representative of its position in a 4-D color-space relative to standard single stellar population models.~Dividing the sample into broad bins based on the relative formal ages, we observe inhomogeneities which reveal signatures of GC substructures.~The most significant of these is a spatial overdensity of GCs with relatively young age labels, of diameter $sim!0.1$,deg ($sim!30,$kpc), located to the south of M87.~The significance of this detection is larger than about 5$sigma$ after accounting for estimates of random and systematic errors.~Surprisingly, no large Virgo galaxy is present in this area, that could potentially host these GCs.~But candidate substructures in the M87 halo with equally elusive hosts have been described based on kinematic studies in the past.~The number of GC spectra available around M87 is currently insufficient to clarify the nature of the new candidate substructure.
We present a photometric study of the dwarf galaxy population in the core region ($< r_{rm vir}/4$) of the Fornax galaxy cluster based on deep $ugi$ photometry from the Next Generation Fornax Cluster Survey. All imaging data were obtained with the Da
rk Energy Camera mounted on the 4-meter Blanco telescope at the Cerro-Tololo Interamerican Observatory. We identify 258 dwarf galaxy candidates with luminosities $-17 < M_{g} < -8$ mag, corresponding to typical stellar masses of $9.5gtrsim log{cal M}_{star}/M_odot gtrsim 5.5$, reaching $sim!3$ mag deeper in point-source luminosity and $sim!4$ mag deeper in surface-brightness sensitivity compared to the classic Fornax Cluster Catalog. Morphological analysis shows that surface-brightness profiles are well represented by single-component Sersic models with average Sersic indices of $langle nrangle_{u,g,i}=(0.78-0.83) pm 0.02$, and average effective radii of $langle r_erangle_{u,g,i}!=(0.67-0.70) pm 0.02$ kpc. Color-magnitude relations indicate a flattening of the galaxy red sequence at faint galaxy luminosities, similar to the one recently discovered in the Virgo cluster. A comparison with population synthesis models and the galaxy mass-metallicity relation reveals that the average faint dwarf galaxy is likely older than ~5 Gyr. We study galaxy scaling relations between stellar mass, effective radius, and stellar mass surface density over a stellar mass range covering six orders of magnitude. We find that over the sampled stellar mass range several distinct mechanisms of galaxy mass assembly can be identified: i) dwarf galaxies assemble mass inside the half-mass radius up to $log{cal M}_{star}$ ~8.0, ii) isometric mass assembly in the range $8.0 < log{cal M}_{star}/M_odot < 10.5$, and iii) massive galaxies assemble stellar mass predominantly in their halos at $log{cal M}_{star}$ ~10.5 and above.
Local Group (LG) galaxies have relatively accurate SFHs and metallicity evolution derived from resolved CMD modeling, and thus offer a unique opportunity to explore the efficacy of estimating stellar mass M$_{star}$ of real galaxies based on integrat
ed stellar luminosities. Building on the SFHs and metallicity evolution of 40 LG dwarf galaxies, we carried out a comprehensive study of the influence of SFHs, metallicity evolution, and dust extinction on the UV-to-NIR color-$M/L$ (color-log$Upsilon_{star}$($lambda$)) relations and M$_{star}$ estimation of local universe galaxies. We find that: The LG galaxies follow color-log$Upsilon_{star}$($lambda$) relations that fall in between the ones calibrated by previous studies; Optical color-log$Upsilon_{star}$($lambda$) relations at higher metallicities ([M/H]) are generally broader and steeper; The SFH concentration does not significantly affect the color-log$Upsilon_{star}$($lambda$) relations; Light-weighted ages and [M/H] together constrain log$Upsilon_{star}$($lambda$) with uncertainties ranging from $lesssim$ 0.1 dex for the NIR up to 0.2 dex for the optical passbands; Metallicity evolution induces significant uncertainties to the optical but not NIR $Upsilon_{star}$($lambda$) at given light-weighted ages and [M/H]; The $V$ band is the ideal luminance passband for estimating $Upsilon_{star}$($lambda$) from single colors, because the combinations of $Upsilon_{star}$($V$) and optical colors such as $B-V$ and $g-r$ exhibit the weakest systematic dependence on SFHs, [M/H] and dust extinction; Without any prior assumption on SFHs, M$_{star}$ is constrained with biases $lesssim$ 0.3 dex by the optical-to-NIR SED fitting. Optical passbands alone constrain M$_{star}$ with biases $lesssim$ 0.4 dex (or $lesssim$ 0.6 dex) when dust extinction is fixed (or variable) in SED fitting. [abridged]
Large samples of globular clusters (GC) with precise multi-wavelength photometry are becoming increasingly available and can be used to constrain the formation history of galaxies. We present the results of an analysis of Milky Way (MW) and Virgo cor
e GCs based on five optical-near-infrared colors and ten synthetic stellar population models. For the MW GCs, the models tend to agree on photometric ages and metallicities, with values similar to those obtained with previous studies. When used with Virgo core GCs, for which photometry is provided by the Next Generation Virgo cluster Survey (NGVS), the same models generically return younger ages. This is a consequence of the systematic differences observed between the locus occupied by Virgo core GCs and models in panchromatic color space. Only extreme fine-tuning of the adjustable parameters available to us can make the majority of the best-fit ages old. Although we cannot exclude that the formation history of the Virgo core may lead to more conspicuous populations of relatively young GCs than in other environments, we emphasize that the intrinsic properties of the Virgo GCs are likely to differ systematically from those assumed in the models. Thus, the large wavelength coverage and photometric quality of modern GC samples, such as used here, is not by itself sufficient to better constrain the GC formation histories. Models matching the environment-dependent characteristics of GCs in multi-dimensional color space are needed to improve the situation.
