arXiv subjects

Nicola R. Napolitano

Publications and source records attributed to Nicola R. Napolitano.

83 records · Page 5Linked to original sources

arXiv2015

VEGAS: A VST Early-type GAlaxy Survey. I. Presentation, wide-field surface photometry, and substructures in NGC 4472

We present the VST Early-type GAlaxy Survey (VEGAS), which is designed to obtain deep multiband photometry in g, r, i, of about one hundred nearby galaxies down to 27.3, 26.8, and 26 mag/arcsec^2 respectively, using the ESO facility VST/OmegaCAM. The goals of the survey are 1) to map the light distribution up to ten effective radii, r_e, 2) to trace color gradients and surface brightness fluctuation gradients out to a few r_e for stellar population characterization, and 3) to obtain a full census of the satellite systems (globular clusters and dwarf galaxies) out to 20% of the galaxy virial radius. The external regions of galaxies retain signatures of the formation and evolution mechanisms that shaped them, and the study of nearby objects enables a detailed analysis of their morphology and interaction features. To clarify the complex variety of formation mechanisms of early-type galaxies (ETGs), wide and deep photometry is the primary observational step, which at the moment has been pursued with only a few dedicated programs. The VEGAS survey has been designated to provide these data for a volume-limited sample with exceptional image quality. In this commissioning photometric paper we illustrate the capabilities of the survey using g- and i-band VST/OmegaCAM images of the nearby galaxy NGC 4472 and of smaller ETGs in the surrounding field. Our surface brightness profiles reach rather faint levels and agree excellently well with previous literature. Genuine new results concern the detection of an intracluster light tail in NGC 4472 and of various substructures at increasing scales. We have also produced extended (g-i) color profiles. The VST/OmegaCAM data that we acquire in the context of the VEGAS survey provide a detailed view of substructures in the optical emission from extended galaxies, which can be as faint as a hundred times below the sky level.

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arXiv2015

The SLUGGS Survey: Multi-population dynamical modelling of the elliptical galaxy NGC 1407 from stars and globular clusters

We perform in-depth dynamical modelling of the luminous and dark matter (DM) content of the elliptical galaxy NGC 1407. Our strategy consists of solving the spherical Jeans equations for three independent dynamical tracers: stars, blue GCs and red GCs in a self-consistent manner. We adopt a maximum-likelihood Markov-Chain Monte Carlo fitting technique in the attempt to constrain the inner slope of the DM density profile (the cusp/core problem), and the stellar initial mass function (IMF) of the galaxy. We find the inner logarithmic slope of the DM density profiles to be $γ= 0.6\pm0.4$, which is consistent with either a DM cusp ($γ= 1$) or with a DM core $(γ= 0)$. Our findings are consistent with a Salpeter IMF, and marginally consistent with a Kroupa IMF. We infer tangential orbits for the blue GCs, and radial anisotropy for red GCs and stars. The modelling results are consistent with the virial mass--concentration relation predicted by $Λ$CDM simulations. The virial mass of NGC 1407 is $\log$ $M_{\rm vir} = 13.3 \pm 0.2 M_{\odot}$, whereas the stellar mass is $\log M_* = 11.8 \pm 0.1 M_{\odot}$. The overall uncertainties on the mass of NGC 1407 are only 5 per cent at the projected stellar effective radius. We attribute the disagreement between our results and previous X-ray results to the gas not being in hydrostatic equilibrium in the central regions of the galaxy. The halo of NGC 1407 is found be DM dominated, with a dynamical mass-to-light ratio of $M/L=260_{-100} ^{+174} M_{\odot}/L_{\odot, B}$. However, this value can be larger up to a factor of 3 depending on the assumed prior on the DM scale radius.

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arXiv2014

The SLUGGS Survey: Breaking degeneracies between dark matter, anisotropy and the IMF using globular cluster subpopulations in the giant elliptical NGC 5846

We study the mass and anisotropy distribution of the giant elliptical galaxy NGC 5846 using stars, as well as the red and blue globular cluster (GC) subpopulations. We break degeneracies in the dynamical models by taking advantage of the different phase space distributions of the two GC subpopulations to unambiguously constrain the mass of the galaxy and the anisotropy of the GC system. Red GCs show the same spatial distribution and behaviour as the starlight, whereas blue GCs have a shallower density profile, a larger velocity dispersion and a lower kurtosis, all of which suggest a different orbital distribution. We use a dispersion-kurtosis Jeans analysis and find that the solutions of separate analyses for the two GC subpopulations overlap in the halo parameter space. The solution converges on a massive dark matter halo, consistent with expectations from $Λ$CDM and WMAP7 cosmology in terms of virial mass ($\log M_{DM} \sim13.3 M_{sun}$) and concentration ($c_{vir}\sim8$). This is the first such analysis that solves the dynamics of the different GC subpopulations in a self-consistent manner. Our method improves the uncertainties on the halo parameter determination by a factor of two and opens new avenues for the use of elliptical galaxy dynamics as tests of predictions from cosmological simulations. The implied stellar mass-to-light ratio derived from the dynamical modelling is fully consistent with a Salpeter initial mass function (IMF) and rules out a bottom light IMF. The different GC subpopulations show markedly distinct orbital distributions at large radii, with red GCs having an anisotropy parameter $β\sim0.4$ outside $\sim3R_e$, and the blue GCs having $β\sim0.15$ at the same radii, while centrally ($\sim1R_e$) they are both isotropic. We discuss the implications of our findings within the two-phase formation scenario for early-type galaxies.

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arXiv2013

Unifying static analysis of gravitational structures with a scale-dependent scalar field gravity as an alternative to dark matter

Aims. We investigated the gravitational effects of a scalar field within scalar-tensor gravity as an alternative to dark matter. Motivated by chameleon, symmetron and f(R)-gravity models, we studied a phenomenological scenario where the scalar field has both a mass (i.e. interaction length) and a coupling constant to the ordinary matter which scale with the local properties of the considered astrophysical system. Methods. We analysed the feasibility of this scenario using the modified gravitational potential obtained in its context and applied it to the galactic and hot gas/stellar dynamics in galaxy clusters and elliptical/spiral galaxies respectively. This is intended to be a first step in assessing the viability of this new approach in the context of "alternative gravity" models. Results. The main results are: 1. the velocity dispersion of elliptical galaxies can be fitted remarkably well by the suggested scalar field, with model significance similar to a classical Navarro-Frenk-White dark halo profile; 2. the analysis of the stellar dynamics and the gas equilibrium in elliptical galaxies has shown that the scalar field can couple with ordinary matter with different strengths (different coupling constants) producing and/or depending on the different clustering state of matter components; 3. elliptical and spiral galaxies, combined with clusters of galaxies, show evident correlations among theory parameters which suggest the general validity of our results at all scales and a way toward a possible unification of the theory for all types of gravitational systems we considered. All these results demonstrate that the proposed scalar field scenario can work fairly well as an alternative to dark matter.

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arXiv2012

An inventory of the stellar initial mass function in early-type galaxies

Given a flurry of recent claims for systematic variations in the stellar initial mass function (IMF), we carry out the first inventory of the observational evidence using different approaches. This includes literature results, as well as our own new findings from combined stellar-populations synthesis (SPS) and Jeans dynamical analyses of data on $\sim$~4500 early-type galaxies (ETGs) from the SPIDER project. We focus on the mass-to-light ratio mismatch relative to the Milky Way IMF, \dimf, correlated against the central stellar velocity dispersion, \sigs. We find a strong correlation between \dimf\ and \sigs, for a wide set of dark matter (DM) model profiles. These results are robust if a uniform halo response to baryons is adopted across the sample. The overall normalization of \dimf, and the detailed DM profile, are less certain, but the data are consistent with standard cold-DM halos, and a central DM fraction that is roughly constant with \sigs. For a variety of related studies in the literature, using SPS, dynamics, and gravitational lensing, similar results are found. Studies based solely on spectroscopic line diagnostics agree on a Salpeter-like IMF at high \sigs, but differ at low \sigs. Overall, we find that multiple independent lines of evidence appear to be converging on a systematic variation in the IMF, such that high-\sigs\ ETGs have an excess of low-mass stars relative to spirals and low-\sigs\ ETGs. Robust verification of super-Salpeter IMFs in the highest-\sigs\ galaxies will require additional scrutiny of scatter and systematic uncertainties. The implications for the distribution of DM are still inconclusive.

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arXiv2012

Dark matter and alternative recipes for the missing mass

Within the standard cosmological scenario the Universe is found to be filled by obscure components (dark matter and dark energy) for ~95% of its energy budget. In particular, almost all the matter content in the Universe is given by dark matter, which dominates the mass budget and drives the dynamics of galaxies and clusters of galaxies. Unfortunately, dark matter and dark energy have not been detected and no direct or indirected observations have allowed to prove their existence and amount. For this reason, some authors have suggested that a modification of Einstein Relativity or the change of the Newton's dynamics law (within a relativistic and classical framework, respectively) could allow to replace these unobserved components. We will start discussing the role of dark matter in the early-type galaxies, mainly in their central regions, investigating how its content changes as a function of the mass and the size of each galaxy and few considerations about the stellar Initial mass function have been made. In the second part of the paper we have described, as examples, some ways to overcome the dark matter hypothesis, by fitting to the observations the modified dynamics coming out from general relativistic extended theories and the MOdyfied Newtonian dynamics (MOND).

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arXiv2010

The central dark matter content of early-type galaxies: scaling relations and connections with star formation histories

We examine correlations between the masses, sizes, and star formation histories for a large sample of low-redshift early-type galaxies, using a simple suite of dynamical and stellar populations models. We confirm an anti-correlation between size and stellar age, and survey for trends with the central content of dark matter (DM). An average relation between central DM density and galaxy size of ~ Reff^-2 provides the first clear indication of cuspy DM haloes in these galaxies -- akin to standard LCDM haloes that have undergone adiabatic contraction. The DM density scales with galaxy mass as expected, deviating from suggestions of a universal halo profile for dwarf and late-type galaxies. We introduce a new fundamental constraint on galaxy formation by finding that the central DM fraction decreases with stellar age. This result is only partially explained by the size-age dependencies, and the residual trend is in the opposite direction to basic DM halo expectations. Therefore we suggest that there may be a connection between age and halo contraction, and that galaxies forming earlier had stronger baryonic feedback which expanded their haloes, or else lumpier baryonic accretion that avoided halo contraction. An alternative explanation is a lighter initial mass function for older stellar populations.

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arXiv2005

The line-of-sight velocity distributions of intracluster planetary nebulae in the Virgo cluster core

Radial velocities of 40 intracluster planetary nebulae (ICPNe) in the Virgo cluster were obtained with the new multi-fiber FLAMES spectrograph on UT2 at VLT. For the first time, the lambda 4959 AA line of the [OIII] doublet is seen in a large fraction (50%) of ICPNe spectra, and a large fraction of the photometric candidates with m(5007) <\= 27.2 is spectroscopically confirmed. ICPNe with the velocity dispersion of the Virgo cluster are found in our CORE field 1 deg from M87. These may have originated from tidal mass loss of smaller galaxies in the M87 subcluster halo. In a field 0.25 deg from M87, we see an extended stellar halo of M87 in approximate dynamical equilibrium, but with few ICPNe. Finally, in a field near M84/M86, the ICPNe velocities are highly correlated with the galaxy velocities, showing that any well-mixed intracluster population is yet to form. Overall, the measured velocity distributions confirm the non-uniform dynamical structure and on-going assembly of the Virgo cluster.(Based on data collected with the FLAMES spectrograph at the UT2 of the VLT at Cerro Paranal, Chile, operated by ESO, during observing run 71.B-0147(A))

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arXiv2003

A Dearth of Dark Matter in Ordinary Elliptical Galaxies

The kinematics of the outer parts of three intermediate-luminosity elliptical galaxies have been studied using the Planetary Nebula Spectrograph. The galaxies' velocity dispersion profiles are found to decline with radius; dynamical modeling of the data indicates the presence of little if any dark matter in these galaxies' halos. This surprising result conflicts with findings in other galaxy types, and poses a challenge to current galaxy formation theories.

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arXiv2003

Narrow band survey for intragroup light in the Leo HI cloud. Constraints on the galaxy background contamination in imaging surveys for intracluster planetary nebulae

We have observed emission line objects located in a 0.26 deg^2 field in the M96 (Leo) group, coincident with the intergalactic HI cloud. The emission line objects were selected using the same procedure as used for the search for intracluster planetary nebulae in the Virgo cluster, and their m_5007 luminosity function has a bright cut-off ~1.2 magnitude fainter than for the PNLF associated with the elliptical galaxies in the M96 group.Therefore the vast majority of these emission line objects are compatible with not being intragroup planetary nebulae at the Leo group distance of 10 Mpc. Spectroscopic follow-up of two emission line objects in this Leo field showed that indeed these do not have the [OIII] doublet expected for a real PN. The brighter source is identified as a starburst object at redshift z = 3.128, because of a second emission in the near infrared, identified as FeII (λ2220 AA). From these data we derive three main results: (i) from the absence of PN we can determine a more stringent upper limit to the surface brightness in any old stellar population associated with the Leo HI cloud. (ii) This translates to an upper limit of 1.6% for the fraction of luminosity in a diffuse intragroup component in the densest 3 x 2 deg area of the Leo group,relative to the light in galaxies. (iii) Using this Leo field as a blank field, we derive an average fraction of 13.6% background emission line objects that enter in surveys of Virgo intracluster PN. The small fraction confirms the validity of the selection criteria for intracluster PN candidates in Virgo.

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arXiv2001

Planetary nebulae as mass tracers of their parent galaxies: biases in the estimate of the kinematical quantities

Multi-object and multi-fiber spectrographs on 4 and 8 meter telescopes make it possible to use extragalactic planetary nebulae (PNe) in the outer halos of early-type galaxies as kinematical tracers, where classical techniques based on integrated stellar light fail. Until now, published PNe radial velocity samples are small, with only a few tens of radial velocity measurements (except for a few cases like NGC 5128 or M31), causing uncertainties in the mass and angular momentum estimates based on these data. To quantify these uncertainties, we have made equilibrium models for spherical galaxies, with and without dark matter, and via Montecarlo simulations we produce radial velocity samples with different sizes. We then apply, to these discrete radial velocity fields, the same standard kinematical analysis as it is commonly done with small samples of observed PNe radial velocities. By comparison of the inferred quantities with those computed from the analytical model, we test for systematic biases and establish a robust procedure to infer the angular momentum distribution and radial velocity dispersion profiles from such samples.

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