Galaxy populations in the Hydra i cluster from the VEGAS survey:I. Optical properties of a large sample of dwarf galaxies

At ~50 Mpc, the Hydra I cluster of galaxies is among the closest cluster in the z=0 Universe, and an ideal environment to study dwarf galaxy properties in a cluster environment. We exploit deep imaging data of the Hydra I cluster to construct a new photometric catalog of dwarf galaxies in the cluster core, which is then used to derive properties of the Hydra I cluster dwarf galaxies population as well as to compare with other clusters. Moreover, we investigate the dependency of dwarf galaxy properties on their surrounding environment. The new Hydra I dwarf catalog contains 317 galaxies with luminosity between -18.5<$M_r$<-11.5 mag, a semi-major axis larger than ~200 pc (a=0.84 arcsec), of which 202 are new detections, previously unknown dwarf galaxies in the Hydra I central region. We estimate that our detection efficiency reaches 50% at the limiting magnitude $M_r$=-11.5 mag, and at the mean effective surface brightness $\overline{\mu}_{e,r}$=26.5 mag/$arcsec^2$. We present the standard scaling relations for dwarf galaxies and compare them with other nearby clusters. We find that there are no observational differences for dwarfs scaling relations in clusters of different sizes. We study the spatial distribution of galaxies, finding evidence for the presence of substructures within half the virial radius. We also find that mid- and high-luminosity dwarfs ($M_r$<-14.5 mag) become on average redder toward the cluster center, and that they have a mild increase in $R_e$ with increasing clustercentric distance, similar to what is observed for the Fornax cluster. No clear clustercentric trends are reported with surface brightness and S\'ersic index. Considering galaxies in the same magnitude-bins, we find that for high and mid-luminosity dwarfs ($M_r$<-13.5 mag) the g-r color is redder for the brighter surface brightness and higher S\'ersic n index objects.Comment: Accepted for publication in A&A. 25 pages, 21 figure

The distribution and morphologies of Fornax Cluster dwarf galaxies suggest they lack dark matter

Abstract Due to their low surface brightness, dwarf galaxies are particularly susceptible to tidal forces. The expected degree of disturbance depends on the assumed gravity law and whether they have a dominant dark halo. This makes dwarf galaxies useful for testing different gravity models. In this project, we use the Fornax Deep Survey (FDS) dwarf galaxy catalogue to compare the properties of dwarf galaxies in the Fornax Cluster with those predicted by the Lambda cold dark matter (ΛCDM) standard model of cosmology and Milgromian dynamics (MOND). We construct a test particle simulation of the Fornax system. We then use the Markov Chain Monte Carlo (MCMC) method to fit this to the FDS distribution of tidal susceptibility η (half-mass radius divided by theoretical tidal radius), the fraction of dwarfs that visually appear disturbed as a function of η, and the distribution of projected separation from the cluster centre. This allows us to constrain the η value at which dwarfs should get destroyed by tides. Accounting for an r′-band surface brightness limit of 27.8 magnitudes per square arcsec, the required stability threshold is ηdestr=0.25+0.07−0.03 in ΛCDM and 1.88+0.85−0.53 in MOND. The ΛCDM value is in tension with previous N-body dwarf galaxy simulations, which indicate that ηdestr ≈ 1. Our MOND N-body simulations indicate that ηdestr = 1.70 ± 0.30, which agrees well with our MCMC analysis of the FDS. We therefore conclude that the observed deformations of dwarf galaxies in the Fornax Cluster and the lack of low surface brightness dwarfs towards its centre are incompatible with ΛCDM expectations but well consistent with MOND

Ultra-compact dwarfs beyond the centre of the Fornax galaxy cluster:hints of UCD formation in low-density environments

Abstract Ultra-compact dwarf galaxies (UCDs) were serendipitously discovered by spectroscopic surveys in the Fornax cluster 20 yr ago. Nowadays, it is commonly accepted that many bright UCDs are the nuclei of galaxies that have been stripped. However, this conclusion might be driven by biased samples of UCDs in high-density environments, on which most searches are based. With the deep optical images of the Fornax Deep Survey, combined with public near-infrared data, we revisit the UCD population of the Fornax cluster and search for UCD candidates, for the first time, systematically out to the virial radius of the galaxy cluster. Our search is complete down to magnitude mg = 21 mag or Mg∼ −10.5 mag at the distance of the Fornax cluster. The UCD candidates are identified and separated from foreground stars and background galaxies by their optical and near-infrared colours. This primarily utilizes the u−i/i−Ks diagram and a machine learning technique is employed to incorporate other colour combinations to reduce the number of contaminants. The newly identified candidates (44) in addition to the spectroscopically confirmed UCDs (61), increase the number of known Fornax UCD considerably (105). Almost all of the new UCD candidates are located outside the Fornax cluster core (360 kpc), where all of the known UCDs were found. The distribution of UCDs within the Fornax cluster shows that a population of UCDs may form in low-density environments. This most likely challenges the current models of UCD formation

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

Abstract 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

The SAMI–Fornax dwarfs survey:III. evolution of [α/Fe] in dwarfs, from galaxy clusters to the local group

Abstract Using very deep, high spectral resolution data from the SAMI Integral Field Spectrograph, we study the stellar population properties of a sample of dwarf galaxies in the Fornax Cluster, down to a stellar mass of 10⁷ M⊙, which has never been done outside the Local Group. We use full spectral fitting to obtain stellar population parameters. Adding massive galaxies from the ATLAS3D project, which we re-analysed, and the satellite galaxies of the Milky Way, we obtained a galaxy sample that covers the stellar mass range 10⁴–10¹² M⊙. Using this large range, we find that the mass–metallicity relation is not linear. We also find that the [α/Fe]-stellar mass relation of the full sample shows a U-shape, with a minimum in [α/Fe] for masses between 10⁹ and 10¹⁰ M⊙. The relation between [α/Fe] and stellar mass can be understood in the following way: when the faintest galaxies enter the cluster environment, a rapid burst of star formation is induced, after which the gas content is blown away by various quenching mechanisms. This fast star formation causes high [α/Fe] values, like in the Galactic halo. More massive galaxies will manage to keep their gas longer and form several bursts of star formation, with lower [α/Fe] as a result. For massive galaxies, stellar populations are regulated by internal processes, leading to [α/Fe] increasing with mass. We confirm this model by showing that [α/Fe] correlates with clustercentric distance in three nearby clusters and also in the halo of the Milky Way

The SAMI–Fornax Dwarfs Survey I:sample, observations, and the specific stellar angular momentum of dwarf elliptical galaxies

Abstract Dwarf ellipticals are the most common galaxy type in cluster environments; however, the challenges associated with their observation mean that their formation mechanisms are still poorly understood. To address this, we present deep integral field observations of a sample of 31 low-mass (107.5 &lt; M⋆ &lt; 109.5 M⊙) early-type galaxies in the Fornax cluster with the SAMI instrument. For 21 galaxies, our observations are sufficiently deep to construct spatially resolved maps of the stellar velocity and velocity dispersion — for the remaining galaxies, we extract global velocities and dispersions from aperture spectra only. From the kinematic maps, we measure the specific stellar angular momentum λR of the lowest mass dE galaxies to date. Combining our observations with early-type galaxy data from the literature spanning a large range in stellar mass, we find that λR decreases towards lower stellar mass, with a corresponding increase in the proportion of slowly rotating galaxies in this regime. The decrease of λR with mass in our sample dE galaxies is consistent with a similar trend seen in somewhat more massive spiral galaxies from the CALIFA survey. This suggests that the degree of dynamical heating required to produce dEs from low-mass starforming progenitors may be relatively modest and consistent with a broad range of formation mechanisms

The SAMI–Fornax Dwarfs Survey:II. The Stellar Mass Fundamental Plane and the dark matter fraction of dwarf galaxies

Abstract We explore the kinematic scaling relations of 38 dwarf galaxies in the Fornax Cluster using observations from the SAMI integral field spectrograph. We focus on the Fundamental Plane (FP), defined by the physical properties of the objects (scale length, surface brightness, and velocity dispersion) and the Stellar Mass (Fundamental) Plane, where surface brightness is replaced by stellar mass, and investigate their dynamical-to-stellar-mass ratio. We confirm earlier results that the Fornax dEs are significantly offset above the FP defined by massive, hot stellar systems. For the Stellar Mass (Fundamental) Plane, which shows much lower scatter, we find that young and old dwarf galaxies lie at about the same distance from the plane, all with comparable scatter. We introduce the perpendicular deviation of dwarf galaxies from the Stellar Mass Plane defined by giant early-types as a robust estimate of their DM fraction, and find that the faintest dwarfs are systematically offset above the plane, implying that they have a higher dark matter fraction. This result is confirmed when estimating the dynamical mass of our dEs using a virial mass estimator, tracing the onset of dark matter domination in low mass stellar systems. We find that the position of our galaxies on the Stellar Mass FP agrees with the galaxies in the Local Group. This seems to imply that the processes determining the position of dwarf galaxies on the FP depend on the environment in the same way, whether the galaxy is situated in the Local Group or in the Fornax Cluster

The Fornax Deep Survey with VST:IX. Catalog of sources in the FDS area with an example study for globular clusters and background galaxies

Abstract Context: A possible pathway for understanding the events and the mechanisms involved in galaxy formation and evolution is an in-depth investigation of the galactic and inter-galactic fossil sub-structures with long dynamical timescales: stars in the field and in stellar clusters. Aims: This paper continues the Fornax Deep Survey (FDS) series. Following previous studies dedicated to extended Fornax cluster members, we present the catalogs of compact stellar systems in the Fornax cluster, as well as extended background sources and point-like sources. Methods: We derived ugri photometry of ∼1.7 million sources over the ∼21 square degree area of FDS centered on the bright central galaxy NGC 1399. For a wider area, of ∼27 square degrees extending in the direction of NGC 1316, we provided gri photometry for ∼3.1 million sources. To improve the morphological characterization of sources, we generated multi-band image stacks by coadding the best-seeing gri-band single exposures with a cut at full width at half maximum (FWHM) ≤ 0.″9. We used the multi-band stacks as master detection frames, with a FWHM improved by ∼15% and a FWHM variability from field to field reduced by a factor of ∼2.5 compared to the pass-band with the best FWHM, namely the r-band. The identification of compact sources, in particular, globular clusters (GC), was obtained from a combination of photometric (e.g., colors, magnitudes) and morphometric (e.g., concentration index, elongation, effective radius) selection criteria, also taking as reference the properties of sources with well-defined classifications from spectroscopic or high-resolution imaging data. Results: Using the FDS catalogs, we present a preliminary analysis of GC distributions in the Fornax area. The study confirms and extends further previous results that were limited to a smaller survey area. We observed the inter-galactic population of GCs, a population of mainly blue GCs centered on NGC 1399, extending over ∼0.9 Mpc, with an ellipticity ϵ ∼ 0.65 and a small tilt in the direction of NGC 1336. Several sub-structures extend over ∼0.5 Mpc along various directions. Two of these structures do not cross any bright galaxy; one of them appears to be connected to NGC 1404, a bright galaxy close to the cluster core and particularly poor in GCs. Using the gri catalogs, we analyze the GC distribution over the extended FDS area and do not find any obvious GC sub-structure bridging the two brightest cluster galaxies, namely, NGC 1316 and NGC 1399. Although NGC 1316 is more than twice as bright of NGC 1399 in optical bands, using gri data, we estimate a GC population that is richer by a factor of ∼3−4 around NGC 1399, as compared to NGC 1316, out to galactocentric distances of ∼40’ or ∼230 kpc

Galaxy populations in the Hydra I cluster from the VEGAS survey:I. optical properties of a large sample of dwarf galaxies

Abstract Context: Due to their relatively low stellar mass content and diffuse nature, the evolution of dwarf galaxies can be strongly affected by their environment. Analyzing the properties of the dwarf galaxies over a wide range of luminosities, sizes, morphological types, and environments, we can obtain insights about their evolution. At ∼50 Mpc, the Hydra I cluster of galaxies is among the closest cluster in the z ≃ 0 Universe, and an ideal environment to study dwarf galaxy properties in a cluster environment. Aims: We exploit deep imaging data of the Hydra I cluster to construct a new photometric catalog of dwarf galaxies in the cluster core, which is then used to derive properties of the Hydra I cluster dwarf galaxy population as well as to compare it with other clusters. Moreover, we investigate the dependency of dwarf galaxy properties on their surrounding environment. Methods: The new wide-field g- and r-band images of the Hydra I cluster obtained with the OmegaCAM camera on the VLT Survey Telescope (VST) in the context of the VST Early-type GAlaxy Survey (VEGAS) were used to study the dwarf galaxy population in the Hydra I cluster core down to r-band magnitude Mr = −11.5 mag. We used an automatic detection tool to identify dwarf galaxies from a ∼1 deg² field centered on the Hydra I core, covering almost half of the cluster virial radius. The photometric pipeline was used to estimate the principal photometric parameters for all targets. Scaling relations and visual inspection were used to assess the cluster membership and construct a new dwarf galaxy catalog. Finally, based on the new catalog, we studied the structural (Sérsic index n, effective radius Re, and axis ratio) and photometric (colors and surface brightness) properties of the dwarf galaxies, also investigating how they vary as a function of clustercentric distance. Results: The new Hydra I dwarf catalog contains 317 galaxies with a luminosity between −18.5 &lt; Mr &lt; −11.5 mag, a semi-major axis larger than ∼200 pc (a = 0.84″), of which 202 are new detections, and previously unknown dwarf galaxies in the Hydra I central region. We estimate that our detection efficiency reaches 50% at the limiting magnitude Mr = −11.5 mag, and at the mean effective surface brightness μ̄e,r = 26.5 mag arcsec−2. We present the standard scaling relations for dwarf galaxies, which are color-magnitude, size-luminosity, and Sérsic n-magnitude relations, and compare them with other nearby clusters. We find that there are no observational differences for dwarfs scaling relations in clusters of different sizes. We study the spatial distribution of galaxies, finding evidence for the presence of substructures within half the virial radius. We also find that mid- and high-luminosity dwarfs (Mr < −14.5 mag) become, on average, redder toward the cluster center, and that they have a mild increase in Re with increasing clustercentric distance, similar to what is observed for the Fornax cluster. No clear clustercentric trends are reported for surface brightness and Sérsic index. Considering galaxies in the same magnitude bins, we find that for high and mid-luminosity dwarfs (Mr &lt; −13.5 mag), the g − r color is redder for the brighter surface brightness and higher Sérsic n index objects. This finding is consistent with the effects of harassment and/or partial gas stripping

The Accretion Dicsc in Halfa with OmegaCAM (ADHOC) Survey

We present the first results of the Accretion Discs in Hα with OmegaCAM (ADHOC) survey, which aims to perform a deep and homogeneous photometric study of pre-main sequence (PMS) stars in a number of nearby star-forming regions. We took advantage of the exquisite image quality and wide-field capabilities of OmegaCAM at the VLT Survey Telescope (VST) to perform multi-band (ugri and Hα), deep (iSDSS < 22 mag), homogeneous and wide-field (covering tens of parsecs) observations of eight star-forming regions: the Orion Nebula Cluster, Lupus, Sco-Cen, Haffner 18, Vela OB2, Eta Cha, Chamaeleon and Ophiuchus. Using a robust method to identify PMS stars through their photometric excess in the Hα band, we aim to measure physical parameters (including mass accretion rates) for over 10 000 PMS stars. Direct comparison with low-resolution spectroscopy confirms that the objects with Hα excess emission that are detected photometrically are bona-fide PMS stars. The first results from this study clearly demonstrate the validity of the observational approach to unveiling complex stellar populations in young clusters.Fil: Beccari, Giacomo. European Southern Observatory Chile; ChileFil: Petr-Gotzens, Monika G.. European Southern Observatory Chile; ChileFil: Boffin, Henri M. J.. European Southern Observatory Chile; ChileFil: Jerabkova, Tereza. European Southern Observatory Chile; ChileFil: Romaniello, Martino. European Southern Observatory Chile; ChileFil: Areal, María Belén. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; ArgentinaFil: Carraro, Giovanni. Dipartimento Di Fisica E Astronomia Galileo Galile; ItaliaFil: Celis Peña, Mariela Fernanda. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; ArgentinaFil: De Marchi, Guido De. European Space Research And Technology Centre; Países BajosFil: de Wit, Willem-Jan. European Southern Observatory Chile; ChileFil: Drew, Janet E.. Centre For Astrophysics Research; Reino UnidoFil: Fedele, Davide. Osservatorio Astrofisico Di Arcetri; ItaliaFil: Ferrero, Leticia Virginia. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Córdoba. Observatorio Astronómico de Córdoba; ArgentinaFil: Kalari, Venu M.. Universidad de Santiago de Chile; ChileFil: Manara, Carlo F.. European Southern Observatory Chile; ChileFil: Mardones, Diego. Universidad de Santiago de Chile; ChileFil: Martin, Eduardo L.. Centro de Astrobiologia Madrid; EspañaFil: Meza, Erick. Université Pierre et Marie Curie; FranciaFil: Mieske, Steffen. Universidad de Santiago de Chile; ChileFil: Panagia, Nino. Space Telescope Science Institute; Estados UnidosFil: Testi, Leonardo. European Southern Observatory Chile; ChileFil: Vink, Jorick S.. Armagh Observatory; Reino UnidoFil: Walsh, Jeremy R.. European Southern Observatory Chile; ChileFil: Wright, Nick J.. Keele University.; Reino Unid