VEGAS-SSS: Tracing Globular Cluster Populations in the Interacting NGC3640 Galaxy Group

Description

Table of contents (English)

Figures show the cutout from the VST r − band image centered on the dwarf galaxies (left panel) identified around NGC3640, the surface brightness profile in the r-band (middle panel) and the color profile for each of the galaxies (right panel). The green circle and blue dashed line represent the Re in the r − band.  The error bars are estimated by propagating the error on the isophotal flux, background value, zeropoint and extinction. As uncertainty on the background we assumed the 30% of the fluctuation estimated by Autoprof19. Judging from the images and radial profiles, Dwarf 12 and 13 might be nucleated.

Technical info (English)

To analyze the properties of the galaxies identified in this work, we developed a code that integrates the AutoProf pipeline (Stone et al. 2021). In brief, AutoProf automatically fits elliptical isophotes to galaxy images and extracts accurate flux measurements along them. It also estimates the local background value for each object and masks out any sources (e.g., stars, background galaxies, GCs) that could affect surface photometry measurements. In our code, we enhanced the masking procedure by incorporating additional sources to mask –particularly focusing on the inner region of the galaxy– and added a module to derive the Sersic parameters (Sersic 1968) to the isophotal profiles produced by AutoProf. The procedure was run on cutout images with size of ∼ 9 × 9 arcmim^2 around each of our candidate dwarfs, both in the g- and r-bands, adopting as reference image the g, and then running forced photometry on the r-band frame. For each dwarf candidate we derived both structural (effective radius Re, and Sersic index n) and photometric (e.g., effective surface brightness µeff) properties. The resulting parameters are reported in Table C.1, together with their uncertainties, which we decided to conservatively round-off to 0.1 on all the fitted parameters. Due to their extremely faint surface brightness, structural and photometric properties could not be retrieved in four of the 27 identified dwarfs. Along with these properties, we also estimated the total integrated magnitude and the (g − r) color within one effective radius, adopting the mean Re of the g- and r-bands as radius. All magnitudes and colors are corrected for Galactic extinction using values from Schlafly & Finkbeiner (2011). Then, we inspected the color-magnitude diagram and compare the properties of the dwarfs selected in this work with the relation retrieved by Misgeld et al. (2008): (g−r) = −0.0314× Mr +0.145, σ = 0.15. We found that the 80% of our sample is within 1σ from the relation and the full catalog is included within 2σ, suggesting that the identified dwarfs are located at roughly the same distance of the group. To derive the total magnitude, mT, we decided to adopt the following procedure. First, we derived mX,T (with X being g or r) by using the asymptotic value of the growth curve derived from the fitted isophotes. We also derive mT using the equation meq.,T = µe − 2.5log10(2πReff^2) from Caon et al. (1994) and Graham & Colless (1997), where µe is the effective surface brightness and Reff is the effective radius. Finally, as a third option, we integrate the light profile up to the radius corresponding to a surface brightness value of ∼ 27 mag/arcsec^2 which we considered as our photometry limiting radius, beyond which we are contaminated by the background. The reason for having three different approaches is due to the peculiarities of the profiles for some dwarfs, which did not allow using the simple curve of growth approach straightforwardly. 
For galaxies exhibiting a flat asymptotic curve of growth, we assumed the asymptotic value as the total magnitude mT (Case A). In cases where the curve of growth was affected by the presence of bright contaminants (stars or galaxies), we assumed an upper limit to the magnitude meq.,T from the equation above (Case B). Finally, in cases where the behavior of the curve of growth was ambiguous, and the results from meq.,T were substantially different from the expected magnitudes based on the broad asymptotic value of the curve of growth (e.g. due to the presence of a plateau region immediately followed by an increase of the flux) we opted for the magnitude estimated using the third method (Case C). For Case A galaxies the uncertainty was determined by taking the semi-difference between the maximum and minimum magnitude estimates among the three methods used. For Case B, we only have upper limits, while for Case C we conservatively assumed an uncertainty of 0.5 mag.