arXiv subjects

F. Bertola

Publications and source records attributed to F. Bertola.

At least 19 recordsLinked to original sources

arXiv2012

Polar bulges and polar nuclear discs: the case of NGC 4698

The early-type spiral NGC 4698 is known to host a nuclear disc of gas and stars which is rotating perpendicularly with respect to the galaxy main disc. In addition, the bulge and main disc are characterised by a remarkable geometrical decoupling. Indeed they appear elongated orthogonally to each other. In this work the complex structure of the galaxy is investigated by a detailed photometric decomposition of optical and near-infrared images. The intrinsic shape of the bulge was constrained from its apparent ellipticity, its twist angle with respect to the major axis of the main disc, and the inclination of the main disc. The bulge is actually elongated perpendicular to the main disc and it is equally likely to be triaxial or axisymmetric. The central surface brightness, scalelength, inclination, and position angle of the nuclear disc were derived by assuming it is infinitesimally thin and exponential. Its size, orientation, and location do not depend on the observed passband. These findings support a scenario in which the nuclear disc is the end result of the acquisition of external gas by the pre-existing triaxial bulge on the principal plane perpendicular to its shortest axis and perpendicular to the galaxy main disc. The subsequent star formation either occurred homogeneously all over the extension of the nuclear disc or through an inside-out process that ended more than 5 Gyr ago.

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arXiv2010

Multiband photometric decomposition of nuclear stellar disks

Small and bright stellar disks with scale lengths of few tens of parsec are known to reside in the center of galaxies. They are believed to have formed in a dissipational process as the end result of star formation in gas either accreted in a merging (or acquisition) event or piled up by the secular evolution of a nuclear bar. Only few of them have been studied in detail to date. Using archival Hubble Space Telescope (HST) imaging, we investigated the photometric parameters of the nuclear stellar disks hosted by three early-type galaxies in the Virgo cluster, NGC 4458, NGC4478, and NGC4570. We aimed at constraining the process of formation of their stars. The central surface brightness, scale length, inclination, and position angle of the nuclear disks were derived by adopting the photometric decomposition method introduced by Scorza & Bender and assuming the disks to be infinitesimally thin and exponential. The location, orientation, and size of the nuclear disks is the same in all the images obtained with the Wide Field Planetary Camera 2 and Advanced Camera for Survey and available in the HST Science Archive. The scale length, inclination, and position angle of each disk are constant within the errors in the observed U, B, V, and I passbands, independently of their values and of the properties of the host spheroid. We interpret the absence of color gradients in the stellar population of the nuclear disks as the signature that star formation homogeneously occurred all through their extension. A inside-out formation scenario is, instead, expected to produce color gradients and therefore is ruled out.

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arXiv2010

Upper Limits on the Masses of 105 Supermassive Black Holes from Hubble Space Telescope/Space Telescope Imaging Spectrograph Archival Data

Based on the modeling of the central emission-line width measured over sub-arcsecond apertures with the Hubble Space Telescope, we present stringent upper bounds on the mass of the central supermassive black hole, MBH, for a sample of 105 nearby galaxies (D<100Mpc) spanning a wide range of Hubble types (E-Sc) and values of the central stellar velocity dispersion, sigma (58-419km/s). For the vast majority of the objects the derived MBH upper limits run parallel and above the well-known MBH-sigma relation independently of the galaxy distance, suggesting that our nebular line-width measurements trace rather well the nuclear gravitational potential. For values of sigma between 90 and 220km/s the 68% of our upper limits falls immediately above the MBH-sigma relation without exceeding the expected MBH values by more than a factor 4.1. No systematic trends or offsets are observed in this sigma range as a function of the galaxy Hubble type or with respect to the presence of a bar. For 6 of our 12 MBH upper limits with sigma<90km/s our line-width measurements are more sensitive to the stellar contribution to the gravitational potential, either due to the presence of a nuclear stellar cluster or because of a greater distance compared to the other galaxies at the low-sigma end of the MBH-sigma relation. Conversely, our MBH upper bounds appear to lie closer to the expected MBH in the most massive elliptical galaxies with values of sigma above 220km/s. Such a flattening of the MBH-sigma relation at its high-sigma end would appear consistent with a coevolution of supermassive black holes and galaxies driven by dry mergers, although better and more consistent measurements for sigma and K-band luminosity are needed for these kind of objects before systematic effects can be ruled out.

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arXiv2009

The ultraviolet flare at the center of the elliptical galaxy NGC 4278

A large fraction of otherwise normal galaxies shows a weak nuclear activity. One of the signatures of the low-luminosity active galactic nuclei (LLAGNs) is the ultraviolet variability which was serendipitously discovered in the center of some low-ionization nuclear emission-line region (LINER) galaxies. There is a pressing need to acquire better statistics about UV flaring and variability in galaxy nuclei, both in terms of the number and monitoring of targets. The Science Data Archive of the Hubble Space Telescope was queried to find all the elliptical galaxies with UV images obtained in different epochs with the Wide Field Planetary Camera 2 (WFPC2) and possibly with nuclear spectra obtained with the Space Telescope Imaging Spectrograph (STIS) in the region of the Halpha emission line. These data were found only for the elliptical radiogalaxy NGC 4278. The UV flux of the nuclear source of NGC 4278 was measured by means of aperture photometry on the WFPC2/F218W images obtained between June 1994 and January 1995. The mass of the central supermassive black hole (SBH) was estimated by measuring the broad components of the emission lines observed in the STIS/G750M spectrum and assuming that the gas is uniformly distributed in a sphere. The nucleus of NGC 4278 hosts a barely resolved but strongly variable UV source. Its UV luminosity increased by a factor of 1.6 in a period of 6 months. The amplitude and scale time of the UV flare in NGC 4278 are remarkably similar to those of the brightest UV nuclear transients which were earlier found in other LLAGNs. The mass of the SBH was found to be in the range between 7x10^7 and 2x10^9 M_sun. This is in agreement with previous findings based on different assumptions about the gas distribution and with the predictions based on the galaxy velocity dispersion.

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arXiv2009

The V_c-sigma_0 relation in Low Surface Brightness galaxies: including the light concentration

It has been found that there is a strong correlation between the circular velocity V_c and the central stellar velocity dispersion σ_0 of galaxies. In this respect, low surface brightness galaxies (LSB) follow a different relation when compared to Elliptical and high surface brightness (HSB) galaxies. The intrinsic scatter of the V_c-σ_0 is partially due to the different concentration of the light distribution. In this work we measure the C_28 concentration parameter for a sample of 17 LSB finding that the C_28 parameter does not account for the different behavior in the $V_c - σ_0$ for this class of objects.

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arXiv2008

Could the Fermi-LAT detect gamma-rays from dark matter annihilation in the dwarf galaxies of the Local Group?

The detection of gamma-rays from dark matter (DM) annihilation is among the scientific goals of the Fermi Large Area Telescope (formerly known as GLAST) and Cherenkov telescopes. In this paper we investigate the existence of realistic chances of such a discovery selecting some nearby dwarf spheroidal galaxies (dSph) as a target. We study the detectability with the Fermi-LAT of the gamma-ray flux from DM annihilation in Draco, Ursa Minor, Carina, and Sextans, for which the state-of-art DM density profiles were available. We assume the DM is made of Weakly Interacting Massive Particles such as the Lightest Supersymmetric Particle (LSP) and compute the expected gamma-ray flux for optimistic choices of the unknown underlying particle physics parameters. We then compute the boost factors due to the presence of DM clumps and of a central supermassive black hole. Finally, we compare our predictions with the Fermi-LAT sensitivity maps. We find that the dSph galaxies shine above the Galactic smooth halo: e.g., the Galactic halo is brighter than the Draco dSph only for angles smaller than 2.3 degrees above the Galactic Center. We also find that the presence of a cusp or a constant density core in the DM mass density profile does not produce any relevant effect in the gamma-ray flux due to the fortunate combination of the geometrical acceptance of the Fermi-LAT detector and the distance of the galaxies and that no significant enhancement is given by the presence of a central black hole or a population of sub-subhalos. We conclude that, even for the most optimistic scenario of particle physics, the gamma-ray flux from DM annihilation in the dSph galaxies of the LG would be too low to be detected with the Fermi-LAT.

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arXiv2008

Early-type galaxies in the near-infrared: 1.5-2.4\mum spectroscopy

Near-infrared (hereafter NIR) data may provide complementary information to the traditional optical population synthesis analysis of unresolved stellar populations because the spectral energy distribution of the galaxies in the 1-2.5\mum range is dominated by different types of stars than at optical wavelengths. Furthermore, NIR data are subjected to less absorption and hence could constrain the stellar populations in dust-obscured galaxies. We want to develop observational constraints on the stellar populations of unresolved stellar systems in the NIR. To achieve this goal we need a benchmark sample of NIR spectra of ``simple'' early-type galaxies, to be used for testing and calibrating the outputs of population synthesis models. We obtained low-resolution (R~1000) long-slit spectra between 1.5 and 2.4\mum for 14 nearby early-type galaxies using SofI at NTT and higher resolution (R~3000) long-slit spectra, centered at the MgI at ~1.51\mum for a heterogeneous sample of 5 nearby galaxies observed with ISAAC at VLT. We defined spectral indices for CO, NaI, CaI and MgI features and measured the strengths of these features in the sample galaxies. We defined a new global NIR metallicity index, suitable for abundance measurements in low-resolution spectra. Finally, we present an average NIR spectrum of an early-type galaxy, built from a homogenized subset of our sample. The NIR spectra of the sample galaxies show great similarity and the strength of some features does correlate with the iron abundance [Fe/H] and optical metal features of the galaxies. The data suggest that the NIR metal features, in combination with a hydrogen absorption feature may be able to break the age-metallicity degeneracy just like the Mg and Fe features in the optical wavelength range.

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arXiv2008

Stellar Populations of Bulges in 14 Cluster Disc Galaxies

Photometry and long-slit spectroscopy are presented for 14 S0 and spiral galaxies of the Fornax, Eridanus and Pegasus cluster, and NGC 7582 group. The structural parameters of the galaxies are derived from the R-band images by performing a two-dimensional photometric decomposition of the surface-brightness distribution. This is assumed to be the sum of the contribution of a bulge and disc component characterized by elliptical and concentric isophotes with constant (but possibly different) ellipticity and position angles. The rotation curves and velocity dispersion profiles are measured from the spectra obtained along the major axis of galaxies. The radial profiles of the Hb, Mg, and Fe line-strength indices are presented too. Correlations between the central values of Mgd, , Hb, and sigma are found. The age, metallicity and alpha/Fe enhancement of the stellar population in the center and at the radius where bulge and disc give the same contribution to the total surface brightness are obtained using stellar population models with variable element abundance ratios. Three classes of bulges are identified. The youngest bulges (~2 Gyr) with ongoing star formation, intermediate-age bulges (4-8 Gyr) have solar metallicity, and old bulges (~10 Gyr) have high metallicity. Most of the sample bulges display solar alpha/Fe enhancement, no gradient in age, and a negative gradient of metallicity. The presence of negative gradient in the metallicity radial profile favors a scenario with bulge formation via dissipative collapse. (abridged)

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arXiv2008

Structure and dynamics of galaxies with a low surface brightness disc. I. The stellar and ionised-gas kinematics

(abridged) Photometry and long-slit spectroscopy are presented for a sample of 6 galaxies with a low surface brightness stellar disc and a bulge. The stellar and ionised-gas kinematics were measured along the major and minor axis in half of the sample galaxies, whereas the other half was observed only along two diagonal axes. Spectra along two diagonal axes were obtained also for one of the objects with major and minor axis spectra. The kinematic measurements extend in the disc region out to a surface-brightness level mu_R~24mag/arcsec^2 reaching in all cases the flat part of the rotation curve. The stellar kinematics turns out to be more regular and symmetric than the ionised-gas kinematics, which often shows the presence of non-circular, off-plane, and non-ordered motions. This raises the question about the reliability of the use of the ionised gas as the tracer of the circular velocity in the modeling of the mass distribution, in particular in the central regions of low surface brightness galaxies.

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arXiv2008

The pseudobulge of NGC 1292

The photometric and kinematic properties of Sb NGC 1292 suggest it hosts a pseudobulge. The properties of the stellar population of such a pseudobulge are consistent with a slow buildup within a scenario of secular evolution.

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arXiv2008

Detection of non-ordered central gas motions in a sample of four low surface brightness galaxies

We present integral-field spectroscopy of the ionized gas in the central regions of four galaxies with a low surface brightness disk taken with the Visible Multi Object Spectrograph at the Very Large Telescope and aimed at testing the accuracy in the determination of the central logarithmic slope $α$ of the mass density radial profile $ρ(r) \propto r^α$ in this class of objects. For all the sample galaxies we subtracted from the observed velocity field the best-fit model of gas in circular motions and derived the residuals. Only ESO-LV 5340200 is characterized by a regular velocity field. We extracted the velocity curves of this galaxy along several position angles, in order to estimate the uncertainty in deriving the central gradient of the total mass density from long-slit spectroscopy. We report the detection of strong non-ordered motions of the ionized gas in three out of four sample galaxies. The deviations have velocity amplitudes and spatial scales that make not possible to disentangle between cuspy and core density radial profiles.

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arXiv2007

Upper limits on the mass of supermassive black holes from STIS archival data

The growth of supermassive black holes (SMBHs) appears to be closely linked with the formation of spheroids. There is a pressing need to acquire better statistics on SMBH masses, since the existing samples are preferentially weighted toward early-type galaxies with very massive SMBHs. With this motivation we started a project aimed at measuring upper limits on the mass of the SMBHs that can be present in the center of all the nearby galaxies (D<100 Mpc) for which STIS/G750M spectra are available in the HST archive. These upper limits will be derived by modeling the central emission-line widths ([NII], Halpha and [SII]) observed over an aperture of ~0.1" (R<50 pc). Here we present our preliminary results for a subsample of 76 bulges.

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arXiv2007

The search for inner polar disks with integral field spectroscopy: the case of NGC 2855 and NGC 7049

The presence of non-circular and off-plane gas motions is frequently observed in the inner regions of disk galaxies. We have measured with integral-field spectroscopy the surface-brightness distribution and kinematics of the ionized gas in NGC 2855 and NGC 7049. These two early-type spiral galaxies were selected to possibly host a kinematically-decoupled gaseous component in orthogonal rotation with respect to the galaxy disk. We have modeled the ionized-gas kinematics and distribution of both galaxies assuming that the gaseous component is distributed either on two orthogonally-rotating disks or in a single and strongly warped disk. In both galaxies the velocity field and distribution of the inner gas are consistent with the presence of an inner polar disk. In NGC 2855 it correponds to the innermost and strongly warped portion of the main disk. In NGC 7049 it is a central and geometrically-decoupled disk, which is nested in the main disk.

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arXiv2006

NGC5719/13: interacting spirals forming a counter-rotating stellar disc

(Context) When a galaxy acquires material from the outside, it is likely that the resulting angular momentum of the accreted material is decoupled from that of the pre-existing galaxy. The presence of stars counter-rotating with respect to other stars and/or gas represents an extreme case of decoupling. (Aims) NGC5719, an almost edge-on Sab galaxy with a prominent skewed dust lane, shows a spectacular on-going interaction with its face-on Sbc companion NGC5713. Observations of such interacting systems provide insight into the processes at work in assembling and reshaping galaxies. (Methods) Studies were made of the distribution and kinematics of neutral hydrogen in the NGC5719/13 galaxy pair and the ionised gas and stellar kinematics along the major axis of NGC5719. (Results) Two HI tidal bridges that loop around NGC5719 and connect to NGC5713, and two HI tidal tails departing westward from NGC5713 were detected. There is a correspondence between the HI condensations and the location of clumps of young stars within and outside the disc of NGC5719. The low-mass satellite PGC135857 at the tip of the northern tail was detected in HI, and is likely a by-product of the interaction. The neutral and ionised hydrogen in the disc of NGC5719 are counter-rotating with respect to the main stellar disc. The counter-rotating stellar disc contains about 20% of the stars in the system, and has the same radial extension as the main stellar disc. This is the first interacting system in which a counter-rotating stellar disc has been detected. (Conclusions) The data support a scenario where HI from the large reservoir available in the galaxy's surroundings was accreted by NGC5719 onto a retrograde orbit and subsequently fuelled the in-situ formation of the counter-rotating stellar disc.

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arXiv2006

Upper limits on the mass of supermassive black holes from HST/STIS archival data

The growth of supermassive black holes (SBHs) appears to be closely linked with the formation of spheroids. There is a pressing need to acquire better statistics on SBH masses, since the existing samples are preferentially weighted toward early-type galaxies with very massive SBHs. With this motivation we started a project aimed at measuring upper limits on the mass of the SBHs in the center of all the nearby galaxies (D<100 Mpc) for which STIS/G750M spectra are available in the HST archive. These upper limits will be derived by modeling the central emission-line widths observed in the Halpha region over an aperture of ~0.1''. Here we present our results for a subsample of 20 S0-Sb galaxies within 20 Mpc.

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arXiv2005

The relation between circular velocity and central velocity dispersion in low surface brightness galaxies

We analyzed a sample of high and low surface brightness (HSB and LSB) disc galaxies and elliptical galaxies to investigate the correlation between the circular velocity (Vc) and the central velocity dispersion (sigma). We better defined the previous Vc-sigma correlation for HSB and elliptical galaxies, especially at the lower end of the sigma values. Elliptical galaxies with Vc based on dynamical models or directly derived from the HI rotation curves follow the same relation as the HSB galaxies in the V-sigma plane. On the contrary, the LSB galaxies follow a different relation, since most of them show either higher Vc (or lower sigma) with respect to the HSB galaxies. This argues against the relevance of baryon collapse in the radial density profile of the dark matter haloes of LSB galaxies. Moreover, if the Vc-sigma relation is equivalent to one between the mass of the dark matter halo and that of the supermassive black hole, these results suggest that the LSB galaxies host a supermassive black hole with a smaller mass compared to HSB galaxies of equal dark matter halo. On the other hand, if the fundamental correlation of SMBH mass is with the halo Vc, then LSBs should have larger black hole masses for given bulge sigma.

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arXiv2005

NGC 4435: a bulge dominated galaxy with an unforeseen low mass central black hole

We present the ionised gas kinematics of the SB0 galaxy NGC 4435 from spectra obtained with the Space Telescope Imaging Spectrograph. This galaxy has been selected on the basis of its ground-based spectroscopy, for displaying a position velocity diagram consistent with the presence of a circumnuclear Keplerian disc rotating around a supermassive black hole (SMBH). We obtained the Halpha and [NII](6583) kinematics kinematics in the galaxy nucleus along the major axis and two parallel offset positions. We built a dynamical model of the gaseous disc taking into account the whole bidimensional velocity field and the instrumental set-up. For the mass of the central SMBH we found an upper limit of 7.5e6 Msun at 3sigma level. This indicates that the mass of SMBH of NGC 4435 is lower than the one expected from the Mbh-sigma (5e7 Msun) and near-infrared Mbh- Lbulge (4e7 Msun) relationships.

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arXiv2005

Spiral galaxies with a central plateau in the gas velocity curve along the major axis

We present the minor-axis kinematics of ionized gas and stars for a sample of 5 spiral galaxies, which are characterized by either a zero or a shallow gas velocity gradient along their major axis. The asymmetric velocity profiles observed along the minor axis of NGC 4064 and NGC 4189 can be explained as due to the presence of a bar. This is also the case of NGC 4178 where the innermost portion of the gaseous disk is nearly face on. In NGC 4424 and NGC 4941, we measured non-zero gas velocities only in the central regions along the minor axis and gas velocities drop to zero at larger radii. This kinematic feature is suggestive of the presence of an orthogonally-rotating gaseous component which is confined in the innermost regions (i.e. an inner polar disk) to be confirmed with integral-field spectroscopy.

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