arXiv subjects

R. Ragusa

Publications and source records attributed to R. Ragusa.

24 records · Page 2Linked to original sources

arXiv2025

Euclid: Early Release Observations -- The Intracluster Light of Abell 2390

Intracluster light (ICL) provides a record of the dynamical interactions undergone by clusters, giving clues on cluster formation and evolution. Here, we analyse the properties of ICL in the massive cluster Abell 2390 at redshift z=0.228. Our analysis is based on the deep images obtained by the Euclid mission as part of the Early Release Observations in the near-infrared (Y, J, H bands), using the NISP instrument in a 0.75 deg$^2$ field. We subtracted a point--spread function (PSF) model and removed the Galactic cirrus contribution in each band after modelling it with the DAWIS software. We then applied three methods to detect, characterise, and model the ICL and the brightest cluster galaxy (BCG): the CICLE 2D multi-galaxy fitting; the DAWIS wavelet-based multiscale software; and a mask-based 1D profile fitting. We detect ICL out to 600 kpc. The ICL fractions derived by our three methods range between 18% and 36% (average of 24%), while the BCG+ICL fractions are between 21% and 41% (average of 29%), depending on the band and method. A galaxy density map based on 219 selected cluster members shows a strong cluster substructure to the south-east and a smaller feature to the north-west. Based on colours, the ICL (out to about 400 kpc) seems to be built by the accretion of small systems (M ~ $10^{9.5}$ solar mass), or from stars coming from the outskirts of Milky Way-type galaxies (M ~ $10^{10}$ solar mass). Though Abell 2390 does not seem to be undergoing a merger, it is not yet fully relaxed, since it has accreted two groups that have not fully merged with the cluster core. We estimate that the contributions to the inner 300 kpc of the ICL of the north-west and south-east subgroups are 21% and 9% respectively.

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arXiv2024

Retrieval of the physical parameters of galaxies from WEAVE-StePS-like data using machine learning

The WHT Enhanced Area Velocity Explorer (WEAVE) is a new, massively multiplexing spectrograph. This new instrument will be exploited to obtain high S/N spectra of $\sim$25000 galaxies at intermediate redshifts for the WEAVE Stellar Population Survey (WEAVE-StePS). We test machine learning methods for retrieving the key physical parameters of galaxies from WEAVE-StePS-like spectra using both photometric and spectroscopic information at various S/Ns and redshifts. We simulated $\sim$105000 galaxy spectra assuming SFH with an exponentially declining star formation rate, covering a wide range of ages, stellar metallicities, sSFRs, and dust extinctions. We then evaluated the ability of the random forest and KNN algorithms to correctly predict such parameters assuming no measurement errors. We checked how much the predictive ability deteriorates for different S/Ns and redshifts, finding that both algorithms still accurately estimate the ages and metallicities with low bias. The dispersion varies from 0.08-0.16 dex for ages and 0.11-0.25 dex for metallicity, depending on the redshift and S/N. For dust attenuation, we find a similarly low bias and dispersion. For the sSFR, we find a very good constraining power for star-forming galaxies, log sSFR$\gtrsim$ -11, where the bias is $\sim$ 0.01 dex and the dispersion is $\sim$ 0.10 dex. For more quiescent galaxies, with log sSFR$\lesssim$ -11, we find a higher bias, 0.61-0.86 dex, and a higher dispersion, $\sim$ 0.4 dex, for different S/Ns and redshifts. Generally, we find that the RF outperforms the KNN. Finally, the retrieved sSFR was used to successfully classify galaxies as part of the blue cloud, green valley, or red sequence. We demonstrate that machine learning algorithms can accurately estimate the physical parameters of simulated galaxies even at relatively low S/N=10 per angstrom spectra with available ancillary photometric information.

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arXiv2024

MHONGOOSE discovery of a gas-rich low-surface brightness galaxy in the Dorado Group

We present the discovery of a low-mass gas-rich low-surface brightness galaxy in the Dorado Group, at a distance of 17.7 Mpc. Combining deep MeerKAT 21-cm observations from the MeerKAT HI Observations of Nearby Galactic Objects: Observing Southern Emitters (MHONGOOSE) survey with deep photometric images from the VST Early-type Galaxy Survey (VEGAS) we find a stellar and neutral atomic hydrogen (HI) gas mass of $M_\star = 2.23\times10^6$ M$_\odot$ and $M_{\rm HI}=1.68\times10^6$ M$_\odot$, respectively. This low-surface brightness galaxy is the lowest mass HI detection found in a group beyond the Local Universe ($D\gtrsim 10$ Mpc). The dwarf galaxy has the typical overall properties of gas-rich low surface brightness galaxies in the Local group, but with some striking differences. Namely, the MHONGOOSE observations reveal a very low column density ($\sim 10^{18-19}$ cm$^{-2}$) HI disk with asymmetrical morphology possibly supported by rotation and higher velocity dispersion in the centre. There, deep optical photometry and UV-observations suggest a recent enhancement of the star formation. Found at galactocentric distances where in the Local Group dwarf galaxies are depleted of cold gas (at $390$ projected-kpc distance from the group centre), this galaxy is likely on its first orbit within the Dorado group. We discuss the possible environmental effects that may have caused the formation of the HI disk and the enhancement of star formation, highlighting the short-lived phase (a few hundreds of Myr) of the gaseous disk, before either SF or hydrodynamical forces will deplete the gas of the galaxy.

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arXiv2024

Attenuation proxy hidden in surface brightness-colour diagrams. A new strategy for the LSST era

Large future sky surveys, such as the LSST, will provide optical photometry for billions of objects. This paper aims to construct a proxy for the far ultraviolet attenuation (AFUVp) from the optical data alone, enabling the rapid estimation of the star formation rate (SFR) for galaxies that lack UV or IR data. To mimic LSST observations, we use the deep panchromatic optical coverage of the SDSS Photometric Catalogue DR~12, complemented by the estimated physical properties for the SDSS galaxies from the GALEX-SDSS-WISE Legacy Catalog (GSWLC) and inclination information obtained from the SDSS DR7. We restricted our sample to the 0.025-0.1 z-spec range and investigated relations among surface brightness, colours, and dust attenuation in the far UV range for star-forming galaxies obtained from the spectral energy distribution (SED). {Dust attenuation is best correlated with (u-r) colour and the surface brightness in the u band ($\rm μ_{u}$). We provide a dust attenuation proxy for galaxies on the star-forming main sequence, which can be used for the LSST or any other type of broadband optical survey. The mean ratio between the catalogue values of SFR and those estimated using optical-only SDSS data with the AFUVp prior calculated as $Δ$SFR=log(SFR$_{\tiny{\mbox{this work}}}$/SFR$_{\tiny{}\texttt{GSWLC}}$) is found to be less than 0.1~dex, while runs without priors result in an SFR overestimation larger than 0.3~dex. The presence or absence of theAFUVp has a negligible influence on the stellar mass estimation (with $Δ$M$_{star}$ in the range from 0 to $-0.15$ dex). Forthcoming deep optical observations of the LSST Deep Drilling Fields, which also have multi-wavelength data, will enable one to calibrate the obtained relation for higher redshift galaxies and, possibly, extend the study towards other types of galaxies, such as early-type galaxies off the main sequence.

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arXiv2023

The AGN fuelling/feedback cycle in nearby radio galaxies - V. The cold atomic gas of NGC 3100 and its group

We present Australia Compact Telescope Array (ATCA) 21-cm observations of the nearby low-excitation radio galaxy (LERG) NGC 3100. This is the brightest galaxy of a loose group and hosts a young ($\sim 2$ Myr) radio source. The ATCA observations reveal for the first time the presence of neutral hydrogen (HI) gas in absorption in the centre of this radio galaxy, and in emission in two low-mass galaxies of the group and in a diffuse dark cloud in the proximity of NGC 3100. The sensitivity to low-column density gas ($N_{\rm HI}\sim 10^{19}$ cm$^{-2}$) allows us to reveal asymmetries in the periphery of most the HI-detected galaxies, suggesting that tidal interactions may be on-going. The diffuse cloud does not show a stellar counterpart down to $27$ mag/arcsec$^2$ and could be the remnant of these interactions. The analysis of the HI absorption complex in NGC 3100 indicates that the atomic phase of the hydrogen is distributed as its molecular phase (observed at arcsecond resolution through several carbon monoxide emission lines). We suggest that the interactions occurring within the group are causing turbulent cold gas clouds in the intra-group medium to be slowly accreted towards the centre of NGC 3100. This caused the recent formation of the cold circum-nuclear disk which is likely sustaining the young nuclear activity.

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arXiv2022

Dorado and its member galaxies. III. Mapping star formation with FUV imaging from UVIT

We are investigating the SF in galaxies of the actively evolving Dorado group where signatures of interactions and merging events are revealed by optical and radio observations in both ETGs and LTGs. Our previous Ha+[NII] study, probing ~10 Myrs timescales, suggested that SF is still ongoing in ETGs. In this work, we use far-UV (FUV) imaging to map recent SF on times scales of about 100 Myrs. We used the Ultraviolet telescope UVIT on board Astrosat to image the Dorado backbone galaxies previously observed in Ha+[NII], with the far-UV filter FUV.CaF2 (1300-1800A). The sample includes NGC1536, NGC1546, NGC1549, [CMI2001]4136-01, NGC1553, IC2058, PGC75125,NGC1566, NGC1596 and NGC1602. FUV.CaF2 emission is revealed in all galaxies, tracing young stellar populations in rings and showing tidal distortions. The Sersic index, derived by fitting the luminosity profiles, is always n<3 suggesting that the FUV.CaF2 emission originates from a disk also in ETGs. The SFR ranges from 0.004+-0.001 M_sol yr^{-1} of [CMI2001]4136-01 to 2.455+-0.027 M_sol yr^{-1} of NGC 1566. Most of the recent SF is found at the periphery of the Dorado group where most of LTGs are located. For these galaxies, the ratio SFR_Ha/SFR_FUV.CaF2 is close to 1, except for the edge-on IC 2058, similarly to previously reported relations for Local Volume samples. For ETGs, however, SFR_Ha is about 15 times higher than SFR_FUV. The Dorado's ETGs define a separate locus in SFR_FUV, SFR_Ha space with respect to the LTGs, which is well represented by the relation log (SFR_FUV.CaF2) = 0.70xlog (SFR_{Ha})-1.26.The disk structure of the FUV.CaF2 emitting populations discovered in all the ETGs implies dissipative processes and wet merging events. The systematic discrepancy between SFRs derived from Ha and FUV fluxes suggests that rejuvenation episodes in ETGs cannot sustain constant SF over ~100 Myrs timescales.

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