Multiwavelength study of interacting and peculiar galaxies

I present a multiwavelength study of a sample of peculiar galaxies in order to constraint the physics of interacting objects, to study how the physical processes affect the structure of galaxies, and to derive some hints on the formation and evolution history of such galaxies. One of the major open issues in modern cosmology is to understand how galaxies formed and evolved. Its likely that the formation of galaxies was dominated by two processes: the assembly of luminous and dark matter through accretion and merger, and the conversion of baryonic and non-baryonic matter into stars. This is the reason why the study of galaxy interactions has received an increasing attention both on the observational and the theoretical sides. Several theoretical works based on numerical simulations, have tested different plausible scenarios for the origin of peculiar galaxies, such as ``Tidal accretion'', ``Cold accretion'' and ``Merging''. For this reason, the sample of galaxies to use was selected in order to reproduce all these mechanisms. The first peculiar object that I studied is the minor axis dust lane galaxy NGC1947. I performed a detailed study of the main properties of this galaxy (Spavone et al 2009). In NGC1947 are present components with different angular momentum, infact gas and dust rotate along the minor axis while stars rotate along the major one. This is a clear evidence that it cannot be the result of a single protogalactic cloud collapse, but rather the result of an interaction event. I performed a detailed study of the main properties of this galaxy and compared them with the prediction of simulations. Putting together all this evidences it was difficult to disentangle in a non ambiguous way the two possible scenarios, even though some aspects can help us in understanding. First of all, the galaxy does not present clear signs of interaction, such as tidal tails and so on, and this leads to the conclusion that the merger occurred about 10 Gyrs ago, a fact which is not consistent with the fact that, according to my estimate, the last burst of star formation occurred 1 Gyr ago. So I can say that the accretion scenario is favoured. The second object in my sample is the Polar Ring Galaxy (PRG) NGC4650A. I used high resolution NIR and optical spectroscopy along the North and South side of the polar disk, to measure the metallicity of the HII regions in the polar disk of this galaxy because, if it formed from the accretion of external cold gas from cosmic web filaments, we expect metallicities similar to those of late type galaxies, while if the metallicities are similar to those of early type galaxies, the accretion from a gas rich donor is favoured. I estimated the metallicity by using both direct and empirical methods, the Stellar Formation Rate (SFR), and the metallicity gradient along the disk. The average metallicity for the polar disk of NGC4650A turned out to be $Z = 0.2 Z_{\odot}$, which is lower than the typical values found in spiral galaxies, and is instead consistent with the metallicities predicted for the formation of disks by cold accretion processes ($Z \sim\ 1/10 Z_{\odot}$), due to the accretion of pristine gas in the cold streams. Moreover, also the absence of any metallicity gradient is consistent with the infall of metal-poor gas from outside which is still forming the disk (see Spavone et al 2010 for details). As a follow up of this work, I obtained observing time at the TNG telescope to observe the PRGs UGC7576 and UGC9796. I performed the study of the chemical abundances also for these galaxies in order to constrain their formation history (Spavone et al. submitted). Both PRGs have metallicities (respectively 0.4 $Z_{\odot}$ and 0.1 $Z_{\odot}$) lower than that observed in spiral galaxies of the same total luminosity and, given their present star formation rate, this values is again consistent with the predictions of the cold accretion mechanism for disk formation. UGC7576 is an isolated galaxy and the absence of close companions led to exclude both the tidal accretion from a donor galaxy and the merging with another galaxy. UGC9796 is instead in a group and has 5 close companions with an amount of HI gas comparable with that of UGC9796. The merging scenario is however ruled out because in order to produce a massive polar disk such as those observed in UGC9796 is required a merging with high mass ratios (7:1 or 10:1) and this would destroy the ordered motion of the central galaxy, transforming it into an elliptical-like, not rotationally supported galaxy. In conclusion, for this object, both the tidal accretion and the cold accretion seem to be plausible scenarios. Finally, to analyze also another type of merging process, I am studying the pair of interacting galaxies known as CSL-1. By using high resolution spectroscopy (FORS1@VLT) and imaging (HST) I am studying the morphology, light distribution and structural parameters of this system, to test the dry-merger scenario (cf. ``A prototype dry-merger caught in the act'', M. Paolillo , G. Covone, C. Nipoti, M. Spavone, M. Capaccioli, G. Longo, A.Cimatti, L. Ciotti, in preparation). In order to investigate also the minor merging processes I also studied the photometric and kinematical properties of a compact group of galaxies belonging to the Hickson's catalogue. The group analyzed in this work was HCG62, one of the nearest group in the celestial Southern hemisphere, which was selected by cross correlating the available X-ray and optical data. Galaxies in compact groups are in a very dense configuration in the sky, having a mean separation comparable with their dimensions and a very low velocity dispersion. Taking into account that theories on formation and evolution of galaxies predict that the intensity and frequency of interactions strongly depend on the density of the environment, compact groups may be considered the ideal place where to test interaction processes, such as dynamical friction, tidal interaction, collisions, merging and so on. The main goal of this part of my work was to derive some hints on the formation and evolutionary history of compact groups, in order to address the possible scenario for the formation of structure in the Universe, and to determine the evolutionary status of the studied objects. To this aim, I performed a detailed study of the kinematical properties of HCG62, that revealed the presence of many peculiarities in the dominant galaxy of the group, NGC4778, such as the presence of a kinematically decoupled and counter-rotating core (KDC), or kinematical profiles strongly perturbed, also in the outer regions of this galaxy. Moreover, I also performed an analysis of the photometric properties of the whole group, to look for correlations between kinematical and photometric peculiarities. The absence of such correlations in HCG62, can be explained by stating that weak interactions do not perturb the rotation curves but produce morphological deformations in the outer regions, while the so called minor mergers perturb the rotation curves in the inner regions, without producing morphological peculiarities. The results obtained in this work are in good agreement with similar studies performed on the same group and with its observed X-ray properties (Spavone et al 2006). I was also Co-Investigator in two accepted proposals to observe, with the TNG telescope, a sample of Shakhbazian galaxy groups, with the aim of building a larger statistical sample and obtain redshift informations which are lacking for most Shakhbazian groups, and are needed to establish on firm grounds their physical nature. Groups of galaxies have been extensively studied in the past decades. Despite this effort, their evolution is yet not well understood. Loose groups are almost certainly still collapsing and are therefore crucial to uncover the formation processes shaping cosmic structures. As it was already mentioned, in compact groups a few member galaxies are compressed in a small volume of space with low relative velocities. Early theoretical studies suggested that in such high density environments the low velocity dispersion of compact galaxy groups would favor strong interactions and mergers, leading to rapid evolution (within $\sim 10^9$ yrs) into a single massive merger remnant. The best studied sample of compact groups to date is the one included in the Hickson catalogue; this sample however, is biased towards extremely high values of matter density and therefore it allows to investigate only the ``close-to-final'' stage of the complex dynamical evolution of groups. The density range bridging the field to these almost coalesced structures is still poorly explored, mainly due to the difficulties encountered in constructing reliable samples of `physically bound', low multiplicity groups. Shakhbazian Groups of galaxies (SHKGs) in spite of having been originally selected as ``compact groups of compact galaxies'', have been shown to sample a large range of spatial densities. To properly characterize the properties of these groups and their evolutionary path, I obtained more accurate redshift determination for a sample of 10 SHK, so almost doubling the sample of SHK groups, with detailed spectroscopic data. Main goals of this work are: i) obtain redshift estimates for groups without literature data, sampling the different sub-populations; ii) confirm the galaxy membership of the groups, which is currently based on photometric estimates for most of the objects; iii) study the dynamical status of the group and derive dynamical mass estimates; iv) study the stellar population of the member galaxies through comparison with population synthesis models, and the degree of activity from emission line measurements

Surface Photometry and Metallicity of the Polar Ring Galaxy A0136-0801

We present a photometric and spectroscopic study of the polar ring galaxy A0136-0801 in order to constrain its formation history. Near-Infrared (NIR) and optical imaging data are used to extract surface brightness and color profiles of the host galaxy and the wide polar structure in A0136-0801. The host galaxy dominates the light emission in all bands; the polar structure is more luminous in the optical bands and is three times more extended than the main spheroid. The average stellar population in the spheroid is redder than in the polar structure and we use their (B-K) vs. (J-K) colors to constraint the ages of these populations using stellar population synthesis models. The inferred ages are 3-5 Gyrs for the spheroid and 1-3 Gyrs for the polar structure. We then use long slit spectra along the major axis of the polar structure to derive the emission line ratios and constrain the oxygen abundance, metallicity and star formation rate in this component. We find 12+log(O/H) = 8.33 +- 0.43 and Z ~ 0.32 Zsun, using emission line ratios. These values are used, together with the ratio of the baryonic masses of the host galaxy and polar structure, to constraint the possible models for the formation scenario. We conclude that the tidal accretion of gas from a gas rich donor or the disruption of a gas-rich satellite are formation mechanisms that may lead to systems with physical parameters in agreement with those measured for A0136-0801.Comment: 17 pages, 9 figures. Accepted for publication in MNRA

Deep Photometry of Galaxies in the VEGAS Survey: The Case of NGC 4472

The VST-VEGAS project is aimed at observing and studying a rich sample of nearby early-type galaxies in order to systematically characterize their properties over a wide baseline of sizes and out to the faint outskirts where data are rather scarce so far. The external regions of galaxies more easily retain signatures about the formation and evolution mechanisms which shaped them, as their relaxation time are longer, and they are more weakly influenced by processes such as mergers, secular evolution, central black hole activity, and supernova feedback on the ISM, which tend to level age and metallicity gradients. The collection of a wide photometric dataset of a large number of galaxies in various environmental conditions, may help to shed light on these questions. To this end VEGAS exploits the potential of the VLT Survey Telescope (VST) which provides high quality images of 1 deg2 field of view in order to satisfy both the requirement of high resolution data and the need of studying nearby, and thus large, objects. We present a detailed study of the surface photometry of the elliptical galaxy NGC4472 and of smaller ETGs in its field, performed by using new g and i bands images to constrain the formation history of this nearby giant galaxy, and to investigate the presence of very faint substructures in its surroundings

An Ultra diffuse Galaxy in the NGC 5846 group from the VEGAS survey

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.Comment: 5 pages, 4 figures, accepted for publication in A&

VEGAS: A VST Early-type GAlaxy Survey. III. Mapping the galaxy structure, interactions and intragroup light in the NGC 5018 group

Most of the galaxies in the Universe at present day are in groups, which are key to understanding the galaxy evolution. In this work we present a new deep mosaic of 1.2 x 1.0 square degrees of the group of galaxies centered on NGC 5018, acquired at the ESO VLT Survey Telescope. We use u, g, r images to analyse the structure of the group members and to estimate the intra-group light. Taking advantage of the deep and multiband photometry and of the large field of view of the VST telescope, we studied the structure of the galaxy members and the faint features into the intra-group space and we give an estimate of the intragroup diffuse light in the NGC 5018 group of galaxies. We found that ~ 41% of the total g-band luminosity of the group is in the form of intragroup light (IGL). The IGL has a (g - r) color consistent with those of other galaxies in the group, indicating that the stripping leading to the formation of IGL is ongoing. From the study of this group we can infer that there are at least two different interactions involving the group members: one between NGC 5018 and NGC 5022, which generates the tails and ring-like structures detected in the light, and another between NGC 5022 and MCG-03-34-013 that have produced the HI tail. A minor merging event also happened in the formation history of NGC 5018 that have perturbed the inner structure of this galaxy.Comment: 21 pages, 15 figures. Accepted for publication in Ap

The Intra-Group Baryons in the LEO I Pair From the VST Early-Type GAlaxy Survey

In this paper we present the deep, wide-field and multi-band imaging of the LEO I pair NGC 3379-NGC 3384, from the VST Early-type GAlaxy Survey (VEGAS). The main goal of this study is to map the intra-group baryons in the pair, in the form of diffuse light and globular clusters (GCs). Taking advantage from the large covered area, which extends for ∼ 3.9 square degrees around the pair, and the long integration time, we can map the light distribution out to ∼ 63 kpc and down to ∼ 30 mag/arcsec2 in the g band and ∼ 29 mag/arcsec2 in the r band, deeper than previous data available for this target. The map of the intra-group light (IGL) presents two very faint (μg ∼ 28-29 mag/arcsec2) streams protruding from the brightest group member NGC 3379 and elongated toward North-West and South. We estimate that the fraction of the stellar halo around NGC 3379 plus the intra-group light is (Formula presented.) in both g and r bands, with an average color g-r = 0.75 ± 0.04 mag. The color distribution of the GCs appears multi-modal, with two dominant peaks at (u-r) = 1.8 mag and (u-r) = 2.1 mag, respectively. The GC population stretches from North-East to South-West and from North-West to South of the pair, in the last case overlapping with the streams of IGL, as well as the PNe distribution found by Hartke et al. (Hartke et al., A&amp;A, 2020, 642, A46) and Hartke et al. (Hartke et al., 2022). Since these structures are elongated in the direction of the two nearby galaxies M96 and NGC 3338, they could be the remnant of a past gravitational interactions with the pair.</p

Does the virial mass drive the intra-cluster light? The relationship between the ICL and Mvir_{vir} from VEGAS

In this Letter we revisit the relationship between the fraction of the intra-cluster light (ICL) and both the virial mass and the fraction of Early Type Galaxies in the host halo. This is based on a statistically significant and homogeneous sample of 22 groups and clusters of galaxies in the local Universe ($z \leq 0.05$), obtained with the VST Early-type GAlaxy Survey (VEGAS). Taking advantage of the long integration time and large area of the VEGAS images, we are able to map the galaxy outskirts and ICL down to $\mu_g$ $\geq$ 29-30 mag/arcsec$^2$ and out to hundreds of kpc. With this data-set, we have expanded the sample of ICL measurements, doubling the previous measures available from the literature for z $\leq$ 0.05. The main result of this work is the lack of any significant trend between the fraction of ICL and the virial mass of the host environment, covering a wide range of virial masses ( $\sim$ $10^{12.5} \leq M_{vir} \leq 10^{15.5} M_{\odot}$), in agreement with some theoretical studies. Since the new data points are all derived with the same methodology and from the same observational setup, and all have comparable depth, the large observed scatter indicates an intrinsic variation in the ICL fraction.On the other hand, there is a weak relation between the fraction of ICL and the fraction of Early Type Galaxies in the host halo, where a larger fraction of ICL is found in groups and clusters of galaxies dominated by earlier morphological types, indicating a connection between the ICL and the dynamical state of the host system.Comment: Letter to the editor, accepted for publication in A&A. 8 pages, 3 figure

The Fornax Deep Survey with the VST XII. Low surface brightness dwarf galaxies in the Fornax cluster

Context. Low surface brightness (LSB) dwarf galaxies in galaxy clusters are an interesting group of objects as their contribution to the galaxy luminosity function and their evolutionary paths are not yet clear. Increasing the completeness of our galaxy catalogs is crucial for understanding these galaxies, which have effective surface brightnesses below 23 mag arcsec−2 (in optical). Progress is continuously being made via the performance of deep observations, but detection depth and the quantification of the completeness can also be improved via the application of novel approaches in object detection. For example, the Fornax Deep Survey (FDS) has revealed many faint galaxies that can be visually detected from the images down to a surface brightness level of 27 mag arcsec−2, whereas traditional detection methods, such as using Source Extractor (SE), fail to find them. Aims. In this work we use a max-tree based object detection algorithm (Max-Tree Objects, MTO) on the FDS data in order to detect previously undetected LSB galaxies. After extending the existing Fornax dwarf galaxy catalogs with this sample, our goal is to understand the evolution of LSB dwarfs in the cluster. We also study the contribution of the newly detected galaxies to the faint end of the luminosity function. Methods. We test the detection completeness and parameter extraction accuracy of MTO using simulated and real images. We then apply MTO to the FDS images to identify LSB candidates. The identified objects are fitted with 2D Sérsic models using GALFIT and classified as imaging artifacts, likely cluster members, or background galaxies based on their morphological appearance, colors, and structure. Results. With MTO, we are able to increase the completeness of our earlier FDS dwarf catalog (FDSDC) 0.5–1 mag deeper in terms of total magnitude and surface brightness. Due to the increased accuracy in measuring sizes of the detected objects, we also add many small galaxies to the catalog that were previously excluded as their outer parts had been missed in detection. We detect 265 new LSB dwarf galaxies in the Fornax cluster, which increases the total number of known dwarfs in Fornax to 821. Using the whole cluster dwarf galaxy population, we show that the luminosity function has a faint-end slope of α = −1.38 ± 0.02. We compare the obtained luminosity function with different environments studied earlier using deep data but do not find any significant differences. On the other hand, the Fornax-like simulated clusters in the IllustrisTNG cosmological simulation have shallower slopes than found in the observational data. We also find several trends in the galaxy colors, structure, and morphology that support the idea that the number of LSB galaxies is higher in the cluster center due to tidal forces and the age dimming of the stellar populations. The same result also holds for the subgroup of large LSB galaxies, so-called ultra-diffuse galaxies

Photometric Redshifts in the W-CDF-S and ELAIS-S1 Fields Based on Forced Photometry from 0.36 -- 4.5 Microns

The W-CDF-S and ELAIS-S1 fields will be two of the LSST Deep Drilling fields, but the availability of spectroscopic redshifts within these two fields is still limited on deg^2 scales. To prepare for future science, we use EAZY to estimate photometric redshifts (photo-zs) in these two fields based on forced-photometry catalogs. Our photo-z catalog consists of ~0.8 million sources covering 4.9 deg^2 in W-CDF-S and ~0.8 million sources covering 3.4 deg^2 in ELAIS-S1, among which there are ~0.6 (W-CDF-S) and ~0.4 (ELAIS-S1) million sources having signal-to-noise-ratio (SNR) > 5 detections in more than 5 bands. By comparing photo-zs and available spectroscopic redshifts, we demonstrate the general reliability of our photo-z measurements. Our photo-z catalog is publicly available at \doi{10.5281/zenodo.4603178}.Comment: 6 pages, 2 figure