arXiv subjects

N. R. Napolitano

Publications and source records attributed to N. R. Napolitano.

At least 91 records · Page 5Linked to original sources

arXiv2013

Planetary Nebula Spectrograph survey of S0 galaxy kinematics. II. Clues to the origins of S0 galaxies

The stellar kinematics of the spheroids and discs of S0 galaxies contain clues to their formation histories. Unfortunately, it is difficult to disentangle the two components and to recover their stellar kinematics in the faint outer parts of the galaxies using conventional absorption line spectroscopy. This paper therefore presents the stellar kinematics of six S0 galaxies derived from observations of planetary nebulae (PNe), obtained using the Planetary Nebula Spectrograph. To separate the kinematics of the two components, we use a maximum-likelihood method that combines the discrete kinematic data with a photometric component decomposition. The results of this analysis reveal that: the discs of S0 galaxies are rotationally supported; however, the amount of random motion in these discs is systematically higher than in comparable spiral galaxies; and the S0s lie around one magnitude below the Tully--Fisher relation for spiral galaxies, while their spheroids lie nearly one magnitude above the Faber--Jackson relation for ellipticals. All of these findings are consistent with a scenario in which spirals are converted into S0s through a process of mild harassment or "pestering," with their discs somewhat heated and their spheroid somewhat enhanced by the conversion process. In such a scenario, one might expect the properties of S0s to depend on environment. We do not see such an effect in this fairly small sample, although any differences would be diluted by the fact that the current location does not necessarily reflect the environment in which the transformation occurred. Similar observations of larger samples probing a broader range of environments, coupled with more detailed modelling of the transformation process to match the wide range of parameters that we have shown can now be measured, should take us from these first steps to the definitive answer as to how S0 galaxies form.

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arXiv2013

The Planetary Nebula Spectrograph survey of S0 galaxy kinematics. Data and Overview

The origins of S0 galaxies remain obscure, with various mechanisms proposed for their formation, likely depending on environment. These mechanisms would imprint different signatures in the galaxies' stellar kinematics out to large radii, offering a method for distinguishing between them. We aim to study a sample of six S0 galaxies from a range of environments, and use planetary nebulae (PNe) as tracers of their stellar populations out to very large radii, to determine their kinematics in order to understand their origins. Using a special-purpose instrument, the Planetary Nebula Spectrograph, we observe and extract PNe catalogues for these six systems*. We show that the PNe have the same spatial distribution as the starlight, that the numbers of them are consistent with what would be expected in a comparable old stellar population in elliptical galaxies, and that their kinematics join smoothly onto those derived at smaller radii from conventional spectroscopy. The high-quality kinematic observations presented here form an excellent set for studying the detailed kinematics of S0 galaxies, in order to unravel their formation histories. We find that PNe are good tracers of stellar kinematics in these systems. We show that the recovered kinematics are largely dominated by rotational motion, although with significant random velocities in most cases.

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arXiv2012

SPIDER - VI. The Central Dark Matter Content of Luminous Early-Type Galaxies: Benchmark Correlations with Mass, Structural Parameters and Environment

We analyze the central dark-matter (DM) content of \sim 4,500 massive (M* \gsim 10^{10} Msun), low-redshift (z<0.1), early-type galaxies (ETGs), with high-quality ugrizYJHK photometry and optical spectroscopy from SDSS and UKIDSS. We estimate the "central" fraction of DM within the K-band effective radius, \Re, using spherically symmetric isotropic galaxy models. We discuss the role of systematics. The main results of the present work are the following: (1) DM fractions increase systematically with both structural parameters and mass proxies, as in previous studies, and decrease with central stellar density. 2) All correlations involving DM fractions are caused by two fundamental ones with galaxy effective radius and central velocity dispersion. These correlations are independent of each other, so that ETGs populate a central-DM plane (DMP), i.e. a correlation among fraction of total-to-stellar mass, effective radius, and velocity dispersion, whose scatter along the total-to-stellar mass axis amounts to \sim 0.15 dex. (3) In general, under the assumption of an isothermal or a constant M/L profile for the total mass distribution, a Chabrier IMF is favoured with respect to a bottom-heavier Salpeter IMF, as the latter produces negative (i.e. unphysical) DM fractions for more than 50% of the galaxies in our sample. For a Chabrier IMF, the DM estimates agree with \LambdaCDM toy-galaxy models based on contracted DM-halo density profiles. We also find agreement with predictions from hydrodynamical simulations. (4) The central DM content of ETGs does not depend significantly on the environment where galaxies reside, with group and field ETGs having similar DM trends. (Abridged)

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arXiv2012

Testing Yukawa-like potentials from f(R)-gravity in elliptical galaxies

We present the first analysis of extended stellar kinematics of elliptical galaxies where a Yukawa--like correction to the Newtonian gravitational potential derived from f(R)-gravity is considered as an alternative to dark matter. In this framework, we model long-slit data and planetary nebulae data out to 7 Re of three galaxies with either decreasing or flat dispersion profiles. We use the corrected Newtonian potential in a dispersion-kurtosis Jeans analysis to account for the mass-anisotropy degeneracy. We find that these modified potentials are able to fit nicely all three elliptical galaxies and the anisotropy distribution is consistent with that estimated if a dark halo is considered. The parameter which measures the "strength" of the Yukawa-like correction is, on average, smaller than the one found previously in spiral galaxies and correlates both with the scale length of the Yukawa-like term and the orbital anisotropy.

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arXiv2012

Population Gradients in the SDSS Galaxy Catalog. The role of merging

We investigate the role of the environment on the colour and stellar population gradients in a local sample of ~3500 central and ~1150 satellite SDSS early-type galaxies (ETGs). The environment is parameterized in terms of the number of satellite galaxies, N_gal in each group. For central galaxies, we find that both optical colour and mass-to-light (M/L) ratio gradients are shallower in central galaxies residing in denser environments (higher N_gal). This trend is driven by metallicity gradients, while age gradients appear to be less dependent on the environment and to have a larger scatter. On the other hand, satellites do not show any differences in terms of the environment. The same results are found if galaxies are classified by central age, and both central and satellite galaxies have shallower gradients if they are older and steeper gradients if younger, satellites being independent of ages. In central galaxies, we show that the observed trends can be explained with the occurrence of dry mergings, which are more numerous in denser environments and producing shallower colour gradients because of more uniform metallicity distributions due to the mixing of stellar populations, while no final clues about merging occurrence can be obtained for satellites. Finally we discuss all systematics on stellar population fitting and their impact on the final results.

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arXiv2011

Stellar mass-to-light ratio gradients in galaxies: correlations with mass

We analyze the stellar mass-to-light ratio (M/L) gradients in a large sample of local galaxies taken from the Sloan Digital Sky Survey, spanning a wide range of stellar masses and morphological types. As suggested by the well known relationship between M/L ratios and colors, we show that M/L gradients are strongly correlated with colour gradients, which we trace to the effects of age variations. Stellar M/L gradients generally follow patterns of variation with stellar mass and galaxy type that were previous found for colour and metallicty gradients. In late-type galaxies M/L gradients are negative, steepening with increasing mass. In early-type galaxies M/L gradients are shallower while presenting a two-fold trend: they decrease with mass up to a characteristic mass of \M* \sim 10^10.3 M_sun and increase at larger masses. We compare our findings with other analyses and discuss some implications for galaxy formation and for dark matter estimates.

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arXiv2011

Secondary infall model and dark matter scaling relations in intermediate redshift early - type galaxies

Scaling relations among dark matter (DM) and stellar quantities are a valuable tool to constrain formation scenarios and the evolution of galactic structures. However, most of the DM properties are actually not directly measured, but derived through model dependent mass mapping procedures. It is therefore crucial to adopt theoretically and observationally well founded models. We use here an updated version of the secondary infall model (SIM) to predict the halo density profile, taking into account the effects of angular momentum, dissipative friction and baryons collapse. The resulting family of halo profiles depends on one parameter only, the virial mass, and nicely fits the projected mass and aperture velocity dispersion of a sample of intermediate redshift lens galaxies. We derive DM related quantities (namely the column density and the Newtonian acceleration) and investigate their correlations with stellar mass, luminosity, effective radius and virial mass.

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arXiv2011

Unravelling the origins of S0 galaxies using maximum likelihood analysis of planetary nebulae kinematics

To investigate the origins of S0 galaxies, we present a new method of analyzing their stellar kinematics from discrete tracers such as planetary nebulae. This method involves binning the data in the radial direction so as to extract the most general possible non-parametric kinematic profiles, and using a maximum likelihood fit within each bin in order to make full use of the information in the discrete kinematic tracers. Both disk and spheroid kinematic components are fitted, with a two-dimensional decomposition of imaging data used to attribute to each tracer a probability of membership in the separate components. Likelihood clipping also allows us to identify objects whose properties are not consistent with the adopted model, rendering the technique robust against contaminants and able to identify additional kinematic features. The method is first tested on an N-body simulated galaxy to assess possible sources of systematic error associated with the structural and kinematic decomposition, which are found to be small. It is then applied to the S0 system NGC~1023, for which a planetary nebula catalogue has already been released and analyzed by (Noordermeer et al., 2008). The correct inclusion of the spheroidal component allows us to show that, contrary to previous claims, the stellar kinematics of this galaxy are indistinguishable from those of a normal spiral galaxy, indicating that it may have evolved directly from such a system via gas stripping or secular evolution. The method also successfully identifies a population of outliers whose kinematics are different from those of the main galaxy; these objects can be identified with a stellar stream associated with the companion galaxy NGC~1023A.

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arXiv2010

The PN.S Elliptical Galaxy Survey: a standard LCDM halo around NGC 4374?

As part of our current programme to test LCDM predictions for dark matter (DM) haloes using extended kinematical observations of early-type galaxies, we present a dynamical analysis of the bright elliptical galaxy NGC 4374 (M84) based on ~450 Planetary Nebulae (PNe) velocities from the PN.Spectrograph, along with extended long-slit stellar kinematics. This is the first such analysis of a galaxy from our survey with a radially constant velocity dispersion profile. We find that the spatial and kinematical distributions of the PNe agree with the field stars in the region of overlap. The velocity kurtosis is consistent with zero at almost all radii. We construct a series of Jeans models, fitting both velocity dispersion and kurtosis to help break the mass-anisotropy degeneracy. Our mass models include DM halos either with shallow cores or with central cusps as predicted by cosmological simulations - along with the novel introduction in this context of adiabatic halo contraction from baryon infall. Both classes of models confirm a very massive dark halo around NGC 4374, demonstrating that PN kinematics data are well able to detect such haloes when present. Considering the default cosmological mass model, we confirm earlier suggestions that bright galaxies tend to have halo concentrations higher than LCDM predictions, but this is found to be solved if either a Salpeter IMF or adiabatic contraction with a Kroupa IMF is assumed. Thus for the first time a case is found where the PN dynamics may well be consistent with a standard dark matter halo. A cored halo can also fit the data, and prefers a stellar mass consistent with a Salpeter IMF. The less dramatic dark matter content found in lower-luminosity "ordinary" ellipticals suggests a bimodality in the halo properties which may be produced by divergent baryonic effects during their assembly histories.

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arXiv2010

Colour and stellar population gradients in galaxies

We discuss the colour, age and metallicity gradients in a wide sample of local SDSS early- and late-type galaxies. From the fitting of stellar population models we find that metallicity is the main driver of colour gradients and the age in the central regions is a dominant parameter which rules the scatter in both metallicity and age gradients. We find a consistency with independent observations and a set of simulations. From the comparison with simulations and theoretical considerations we are able to depict a general picture of a formation scenario.

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arXiv2010

Stellar population gradients from cosmological simulations: dependence on mass and environment in local galaxies

The age and metallicity gradients for a sample of group and cluster galaxies from N-body+hydrodynamical simulation are analyzed in terms of galaxy stellar mass. Dwarf galaxies show null age gradient with a tail of high and positive values for systems in groups and cluster outskirts. Massive systems have generally zero age gradients which turn to positive for the most massive ones. Metallicity gradients are distributed around zero in dwarf galaxies and become more negative with mass; massive galaxies have steeper negative metallicity gradients, but the trend flatten with mass. In particular, fossil groups are characterized by a tighter distribution of both age and metallicity gradients. We find a good agreement with both local observations and independent simulations. The results are also discussed in terms of the central age and metallicity, as well as the total colour, specific star formation and velocity dispersion.

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arXiv2010

Central dark matter trends in early-type galaxies from strong lensing, dynamics and stellar populations

We analyze the correlations between central dark matter (DM) content of early-type galaxies and their sizes and ages, using a sample of intermediate-redshift (z ~ 0.2) gravitational lenses from the SLACS survey, and by comparing them to a larger sample of z ~ 0 galaxies. We decompose the deprojected galaxy masses into DM and stellar components using combinations of strong lensing, stellar dynamics, and stellar populations modeling. For a given stellar mass, we find that for galaxies with larger sizes, the DM fraction increases and the mean DM density decreases, consistently with the cuspy halos expected in cosmological formation scenarios. The DM fraction also decreases with stellar age, which can be partially explained by the inverse correlation between size and age. The residual trend may point to systematic dependencies on formation epoch of halo contraction or stellar initial mass functions. These results are in agreement with recent findings based on local galaxies by Napolitano, Romanowsky & Tortora (2010) and suggest negligible evidence of galaxy evolution over the last ~ 2.5 Gyr other than passive stellar aging.

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arXiv2010

Color and stellar population gradients in galaxies. Correlation with mass

We analyze the color gradients (CGs) of ~50000 nearby SDSS galaxies. From synthetic spectral models based on a simplified star formation recipe, we derive the mean spectral properties, and explain the observed radial trends of the color as gradients of the stellar population age and metallicity (Z). The most massive ETGs (M_* > 10^{11} Msun) have shallow CGs in correspondence of shallow (negative) Z gradients. In the stellar mass range 10^(10.3-10.5) < M_* < 10^(11) Msun, the Z gradients reach their minimum of ~ -0.5 dex^{-1}. At M_* ~ 10^{10.3-10.5} Msun, color and Z gradient slopes suddenly change. They turn out to anti-correlate with the mass, becoming highly positive at the very low masses. We have also found that age gradients anti-correlate with Z gradients, as predicted by hierarchical cosmological simulations for ETGs. On the other side, LTGs have gradients which systematically decrease with mass (and are always more negative than in ETGs), consistently with the expectation from gas infall and SN feedback scenarios. Z is found to be the main driver of the trend of color gradients, especially for LTGs, but age gradients are not negligible and seem to play a significant role too. We have been able to highlight that older galaxies have systematically shallower age and Z gradients than younger ones. Our results for high-mass galaxies are in perfect agreement with predictions based on the merging scenario, while the evolution of LTGs and younger and less massive ETGs seems to be mainly driven by infall and SN feedback. (Abridged)

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arXiv2009

Central mass-to-light ratios and dark matter fractions in early-type galaxies

Dynamical studies of local ETGs and the Fundamental Plane point to a strong dependence of M/L ratio on luminosity (and stellar mass) with a relation of the form $M/L \propto L^γ$. The "tilt" $γ$ may be caused by various factors, including stellar population properties, IMF, rotational support, luminosity profile non-homology and dark matter (DM) fraction. We evaluate the impact of all these factors using a large uniform dataset of local ETGs from Prugniel & Simien (1997). We take particular care in estimating the stellar masses, using a general star formation history, and comparing different population synthesis models. We find that the stellar M/L contributes little to the tilt. We estimate the total M/L using simple Jeans dynamical models, and find that adopting accurate luminosity profiles is important but does not remove the need for an additional tilt component, which we ascribe to DM. We survey trends of the DM fraction within one effective radius, finding it to be roughly constant for galaxies fainter than $M_B \sim -20.5$, and increasing with luminosity for the brighter galaxies; we detect no significant differences among S0s and fast- and slow-rotating ellipticals. We construct simplified cosmological mass models and find general consistency, where the DM transition point is caused by a change in the relation between luminosity and effective radius. A more refined model with varying galaxy star formation efficiency suggests a transition from total mass profiles (including DM) of faint galaxies distributed similarly to the light, to near-isothermal profiles for the bright galaxies. These conclusions are sensitive to various systematic uncertainties which we investigate in detail, but are consistent with the results of dynamics studies at larger radii.

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arXiv2008

Kinematic properties of early-type galaxy haloes using planetary nebulae

We present new planetary nebulae (PNe) positions, radial velocities, and magnitudes for 6 early-type galaxies obtained with the Planetary Nebulae Spectrograph, their two-dimensional velocity and velocity dispersion fields. We extend this study to include an additional 10 early-type galaxies with PNe radial velocity measurements available from the literature, to obtain a broader description of the outer-halo kinematics in early-type galaxies. These data extend the information derived from stellar kinematics to typically up to ~8 Re. The combination of photometry, stellar and PNe kinematics shows: i) good agreement between the PNe number density and the stellar surface brightness in the region where the two data sets overlap; ii) good agreement between PNe and stellar kinematics; iii) that the mean rms velocity profiles fall into two groups: with of the galaxies characterized by slowly decreasing profiles and the remainder having steeply falling profiles; iv) a larger variety of velocity dispersion profiles; v) that twists and misalignments in the velocity fields are more frequent at large radii, including some fast rotators; vi) that outer haloes are characterised by more complex radial profiles of the specific angular momentum-related lambda_R parameter than observed within 1Re; vii) that many objects are more rotationally dominated at large radii than in their central parts; and viii) that the halo kinematics are correlated with other galaxy properties, such as total luminosity, isophotal shape, total stellar mass, V/sigma, and alpha parameter, with a clear separation between fast and slow rotators.

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arXiv2008

Probing the kinematics of early-type galaxy halos using planetary nebulae

We present first results of a study of the halo kinematics for a sample of early type galaxies using planetary nebulae (PNe) as kinematical tracers. PNe allow to extend up to several effective radii (Re) the information from absorption line kinematics (confined to within 1 or 2 Re), providing valuable information and constraints for merger simulations and galaxy formation models. We find that the specific angular momentum per unit mass has a more complex radial dependence when the halo region is taken into account and that the halo velocity dispersion is related to the total galaxy luminosity, isophotal shape, and number of PNe per unit of luminosity

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arXiv2008

The PN.S Elliptical Galaxy Survey: the dark matter in NGC 4494

We present new Planetary Nebula Spectrograph observations of the ordinary elliptical galaxy NGC 4494, resulting in positions and velocities of 255 PNe out to 7 effective radii (25 kpc). We also present new wide-field surface photometry from MMT/Megacam, and long-slit stellar kinematics from VLT/FORS2. The spatial and kinematical distributions of the PNe agree with the field stars in the region of overlap. The mean rotation is relatively low, with a possible kinematic axis twist outside 1 Re. The velocity dispersion profile declines with radius, though not very steeply, down to ~70 km/s at the last data point. We have constructed spherical dynamical models of the system, including Jeans analyses with multi-component LCDM-motivated galaxies as well as logarithmic potentials. These models include special attention to orbital anisotropy, which we constrain using fourth-order velocity moments. Given several different sets of modelling methods and assumptions, we find consistent results for the mass profile within the radial range constrained by the data. Some dark matter (DM) is required by the data; our best-fit solution has a radially anisotropic stellar halo, a plausible stellar mass-to-light ratio, and a DM halo with an unexpectedly low central density. We find that this result does not substantially change with a flattened axisymmetric model. Taken together with other results for galaxy halo masses, we find suggestions for a puzzling pattern wherein most intermediate-luminosity galaxies have very low concentration halos, while some high-mass ellipticals have very high concentrations. We discuss some possible implications of these results for DM and galaxy formation.

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arXiv2008

Gauging the dark matter fraction in a $L_*$ S0 galaxy at z=0.47 through gravitational lensing from deep HST/ACS imaging

We analyze a new gravitational lens, OAC-GL J1223-1239, serendipitously found in a deep I-band image of the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS). The lens is a L_*, edge-on S0 galaxy at z=0.4656. The gravitational arc has a radius of 0.42 arcsec. We have determined the total mass and the dark matter (DM) fraction within the Einstein radius as a function of the lensed source redshift, which is presently unknown. For z ~ 1.3, which is in the middle of the redshift range plausible for the source according to some external constraints, we find the central velocity dispersion to be ~180 km/s. With this value, close to that obtained by means of the Faber-Jackson relation at the lens redshift, we compute a 30% DM fraction within the Einstein radius (given the uncertainty in the source redshift, the allowed range for the DM fraction is 25-35 % in our lensing model). When compared with the galaxies in the local Universe, the lensing galaxy, OAC-GL J1223-1239 seems to fall in the transition regime between massive DM dominated galaxies and lower-mass, DM deficient systems.

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