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Register a new userVEGAS: A VST Early-type GAlaxy Survey.VI. The diffuse light in HCG 86 from the ultra-deep VEGAS images
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Rossella Ragusa
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Context. In this paper we present ultra deep images of the compact group of galaxies HCG 86 as part of the VEGAS survey. Aims. Our main goals are to estimate the amount of intragroup light (IGL), to study the light and color distributions in order to address the main formation process of the IGL component in groups of galaxies. Methods. We derived the azimuthally averaged surface brightness profiles in the g,r and i bands with g - r and r - i average colors and color profiles for all group members. By fitting the light distribution, we have extrapolated the contribution of the stellar halos plus the diffuse light from the brightest component of each galaxy. The results are compared with theoretical predictions. Results. The long integration time and wide area covered make our data deeper than previous literature studies of the IGL in compact groups of galaxies and allow us to produce an extended (~160 kpc) map of the IGL, down to a surface brightness level of about 30 mag/arcsec^2 in the g band. The IGL in HCG 86 is mainly in diffuse form and has average colors of g - r ~ 0.8 mag and r - i ~ 0.4 mag. The fraction of IGL in HCG 86 is ~ 16% of the total luminosity of the group, and this is consistent with estimates available for other compact groups and loose groups of galaxies of similar virial masses. A weak trend is present between the amount of IGL and the early-type to late-type galaxy ratio. Conclusions. By comparing the IGL fraction and colors with those predicted by simulations, the amount of IGL in HCG 86 would be the result of the disruption of satellites at an epoch of z ~ 0.4. At this redshift, observed colors are consistent with the scenario where the main contribution to the mass of the IGL comes from the intermediate-massive galaxies.
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We present the first data release (DR1) of the VST Early-type GAlaxy Survey (VEGAS). This is a deep multi-band (ugri) imaging survey, carried out with the ESO VLT Survey Telescope (VST). To date, using about 90% of the total observing time, VEGAS has already collected 43 targets (groups and clusters of galaxies) covering a total area on the sky of about 95 square degrees. Taking advantage of the wide (1 deg^2) field-of-view of OmegaCAM@VST, the long integration time and the wide variety of targets, VEGAS has proven to be a gold mine to explore the structure of galaxies down to the faintest surface brightness levels of about 27-30 mag/arcsec^2 in the SDSS g band, for the dense clusters of galaxies and for the unexplored poor groups of galaxies. Based on the analysed data, VEGAS allowed us to i) study the galaxy outskirts, detect the intra-cluster light and low-surface brightness features in the intra-cluster/group space, ii) trace the mass assembly in galaxies, by estimating the accreted mass fraction in the stellar halos and provide results that can be directly compared with the predictions of galaxy formation models, iii) trace the spatial distribution of candidate globular clusters, and iv) detect the ultra-diffuse galaxies. With the DR1, we provide the reduced VST mosaics of 10 targets, which have been presented in the VEGAS publications. The data products are available via the ESO Science Portal (see http://www.eso.org/sci/observing/phase3/news.html#VEGAS-DR1).
Many ultra diffuse galaxies (UDGs) have now been identified in clusters of galaxies. However, the number of nearby UDGs suitable for detailed follow-up remain rare. Our aim is to begin to identify UDGs in the environments of nearby bright early-type galaxies from the VEGAS survey. Here we use a deep g band image of the NGC 5846 group, taken as part of the VEGAS survey, to search for UDGs. We found one object with properties of a UDG if it associated with the NGC 5846 group, which seems likely. The galaxy, we name NGC 5846$_$UDG1, has an absolute magnitude of M$_g$ = -14.2, corresponding to a stellar mass of $sim$10$^8$ M$_{odot}$. It also reveals a system of compact sources which are likely globular clusters. Based on the number of globular clusters detected we estimate a halo mass that is greater than 8$times$10$^{10}$ M$_{odot}$ for UDG1.
Using deep g,r,i imaging from the VEGAS survey, we have searched for ultra diffuse galaxies (UDGs) in the IC 1459 group. Assuming they are group members, we identify 9 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 $sim$10$^8$ M$_{odot}$. 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.
Observations of diffuse starlight in the outskirts of galaxies are thought to be a fundamental source of constraints on the cosmological context of galaxy assembly in the $Lambda$CDM model. Such observations are not trivial because of the extreme faintness of such regions. In this work, we investigate the photometric properties of six massive early type galaxies (ETGs) in the VEGAS sample (NGC 1399, NGC 3923, NGC 4365, NGC 4472, NGC 5044, and NGC 5846) out to extremely low surface brightness levels, with the goal of characterizing the global structure of their light profiles for comparison to state-of-the-art galaxy formation models. We carry out deep and detailed photometric mapping of our ETG sample taking advantage of deep imaging with VST/OmegaCAM in the g and i bands. By fitting the light profiles, and comparing the results to simulations of elliptical galaxy assembly, we identify signatures of a transition between relaxed and unrelaxed accreted components and can constrain the balance between in situ and accreted stars. The very good agreement of our results with predictions from theoretical simulations demonstrates that the full VEGAS sample of $sim 100$ ETGs will allow us to use the distribution of diffuse light as a robust statistical probe of the hierarchical assembly of massive galaxies.
This paper is based on the multi-band VST Early-type GAlaxy Survey (VEGAS) with the VLT Survey Telescope (VST). We present new deep photometry of the IC1459 group in g and r band. The main goal of this work is to investigate the photometric properties of the IC1459 group, and to compare our results with those obtained for other galaxy groups studied in VEGAS, in order to provide a first view of the variation of their properties as a function of the evolution of the system. For all galaxies in the IC1459 group, we fit isophotes and extract the azimuthally-averaged surface-brightness profiles, the position angle and ellipticity profiles as a function of the semi-major axis, as well as the average colour profile. In each band, we estimate the total magnitudes, effective radii, mean colour, and total stellar mass for each galaxies in the group. Then we look at the structure of the brightest galaxies and faint features in their outskirts, considering also the intragroup component. The wide field of view, long integration time, high angular resolution, and arcsec-level seeing of OmegaCAM@VST allow us to map the light distribution of IC1459 down to a surface brightness level of 29.26 mag arcsec^{-2} in g band and 28.85 mag arcsec^{-2} in r band, and out to 7-10 Re, and to detect the optical counterpart of HI gas around IC1459. We also explore in depth three low density environments and provide information to understand how galaxies and groups properties change with the group evolution stage. There is a good agreement of our results with predictions of numerical simulations regarding the structural properties of the brightest galaxies of the groups. We suggest that the structure of the outer envelope of the BCGs, the intra-group light and the HI amount and distribution may be used as indicators of the different evolutionary stage and mass assembly in galaxy groups.
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