Dwarf galaxies in the MATLAS Survey: Hubble Space Telescope observations of nuclear star clusters

Abstract

In dwarf galaxies, nuclear star clusters (NSCs) are believed to primarily form from the migration and merger of globular clusters (GCs), with a possible contribution from in-situ star-forming activity triggered by gas infall. We present the study of NSCs in 41 MATLAS survey dwarf galaxies including ultra-diffuse galaxies (UDGs), as part of a large follow-up imaging program with the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) using the F606W and F814W filters. The sample is biased towards low-surface brightness and large dwarfs, i.e., UDG-like galaxies, and includes two galaxies with a double nucleus, 13 newly identified nucleated dwarfs thanks to HST's high spatial resolution, and five candidate ultra-compact dwarf progenitors. We modeled the NSCs with a Sérsic profile and derived their structural properties and photometry. We find the NSC Sérsic index to increase with the luminosity and stellar mass, while no obvious trend is seen on the effective radius and ellipticity. The faint NSCs tend to have a constant color profile, whereas the bright ones have a bluer center, suggesting that the most massive NSCs in our sample might have experienced a mixed formation scenario, including in-situ star formation. A significant portion of our NSCs tend to be more massive than for other galaxy samples of similar stellar mass, which could be due to some dwarfs ongoing tidal disruption or an initial formation of massive NSCs from multiple GC mergers and in-situ star forming activity. More observations of resolved NSC are needed to be able to infer their formation scenario from the structural properties and photometry in dwarfs.

Figures (10)

• Figure 1: Comparison between the CFHT $g$-band (left cutout) and the HST F606W filter (right cutout) observations of the central 10$\times$10 area of three nucleated dwarfs from the MATLAS sample (left two columns) and three newly identified (right two columns). We draw an 1 radius circle at the position of the nucleus in each image. North is up and east is to the left.
• Figure 2: HST F606W+F814W color cutouts of the 41 nucleated dwarf galaxies. The names of the 13 new nucleated galaxies are in boldfaced, and those of the two double-nucleated dwarfs are in cyan. We used a different stretch for MATLAS-1332 and MATLAS-1938 to enhance the visibility of the NSC. Each image is 1$\times$1. North up and east is to the left.
• Figure 3: Scaling relations between the structural properties of the MATLAS NSCs studied in this work (black open circles) compared to NSCs from the LV (blue dots), Virgo (orange triangles), and Fornax (red crosses) clusters, and located in LTGs (light blue crosses), using the structural parameter-based sample described in Sect. \ref{['section:NSCcatalogs']}. Top: Effective radius as a function of the absolute magnitude in the $V$-band (left) and the stellar mass (right). We show the relation log R$_{\rm e,\rm NSC}$=$-2.0-0.25$M$_{\rm V,\rm NSC}$ fitted by Georgiev2014 (black line), R$_{\rm e} \propto$ M$_{\ast}^{0.38}$ (black dashed line) found by Bekki2004, and R$_{\rm e} \propto$ M$_{\ast}$ (black dotted line) suggested by Turner2012. Bottom left: Ellipticity vs. the absolute magnitude in the $V$-band. Bottom right: Relation between the Sérsic index and the stellar mass. The dotted line indicates $n=6$.
• Figure 4: NSC stellar mass to galaxy stellar mass relation (top) and NSC-to-galaxy mass ratio vs. galaxy stellar mass (bottom) for the MATLAS NSCs from this work (black open circles), LV (blue dots), Virgo (orange triangles), Fornax (red crosses), and LTG (light blue crosses) photometry-based sample from Sect. \ref{['section:NSCcatalogs']}. We computed the running average of log M$_{\ast}^{\rm NSC}$ in bins with a width of 1 dex and a moving step size of 0.2 dex in log M$_{\ast}^{\rm gal}$ considering all the NSCs (black solid line). In the bottom plot, we compare the running average to Eqs. 1 (purple dashed line) and 2 (purple line) of Neumayer2020, as well as the relation found for ETGs in Scott2013 (purple dotted line) and dEs in Graham2003 (gray line; with dashed lines for the errors). The error bars (lower right corner) represent the median uncertainties.
• Figure 5: Color profiles observed within 3R$_{\rm e,\rm NSC}$ for the 15 modeled NSCs with R$_{\rm e,\rm NSC}$>1 pix. The NSCs are order by increasing M$_{\ast}^{\rm NSC}$ and split into two subplots: M$_{\ast}^{\rm NSC}<10^{6.5}$ M$_{\odot}$ (top) mostly showing constant profiles, and M$_{\ast}^{\rm NSC}>10^{6.5}$ M$_{\odot}$ (center) with varying profiles from redder outskirts to a bluer center. The peculiar profile, from bluer outskirts to an inner redder color, of MATLAS-49 is highlighted with a dotted line.