arXiv subjects

E. Dalla Bontà

Publications and source records attributed to E. Dalla Bontà.

At least 37 records · Page 2Linked to original sources

arXiv2018

On the observational diagnostics to separate classical and disk-like bulges

Flattened bulges with disk-like properties are considered to be the end product of secular evolution processes at work in the inner regions of galaxies. On the contrary, classical bulges are characterized by rounder shapes and thought to be similar to low-luminosity elliptical galaxies. We aim at testing the variety of observational diagnostics which are commonly adopted to separate classical from disk-like bulges in nearby galaxies. We select a sample of eight unbarred lenticular galaxies to be morphologically and kinematically undisturbed with no evidence of other components than bulge and disk. We analyze archival data of broad-band imaging from SDSS and integral-field spectroscopy from the ATLAS$^{\rm 3D}$ survey to derive the photometric and kinematic properties, line-strength indices, and intrinsic shape of the sample bulges. We argue that the bulge Sérsic index is a poor diagnostics to discriminate different bulge types. We find that the combination of line-strength with either kinematic or photometric diagnostics does not provide a clear separation for half of the sample bulges. We include for the first time the intrinsic three-dimensional shape of bulges as a possible discriminant of their nature. All bulges turn out to be thick oblate spheroids, but only one has a flattening consistent with that expected for outer disks. We conclude that bulge classification may be difficult even adopting all observational diagnostics proposed so far and that classical and disk-like bulges could be more confidently identified by considering their intrinsic shape.

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arXiv2017

The intrinsic shape of bulges in the CALIFA survey

The intrinsic shape of galactic bulges in nearby galaxies provides crucial information to separate bulge types. We intended to derive accurate constraints to the intrinsic shape of bulges to provide new clues on their formation mechanisms and set new limitations for future simulations. We retrieved the intrinsic shape of a sample of CALIFA bulges using a statistical approach. Taking advantage of GalMer numerical simulations of binary mergers we estimated the reliability of the procedure. Analyzing the $i$-band mock images of resulting lenticular remnants, we studied the intrinsic shape of their bulges at different galaxy inclinations. Finally, we introduced a new ($B/A$, $C/A$) diagram to analyze possible correlations between the intrinsic shape and the properties of bulges. We tested the method on simulated lenticular remnants, finding that for galaxies with inclinations $25^{\circ} < θ< 65^{\circ}$ we can safely derive the intrinsic shape of their bulges. We found that our CALIFA bulges tend to be nearly oblate systems (66%), with a smaller fraction of prolate spheroids (19%) and triaxial ellipsoids (15%). The majority of triaxial bulges are in barred galaxies (75%). Moreover, we found that bulges with low Sérsic indices or in galaxies with low bulge-to-total luminosity ratios form a heterogeneous class of objects; additionally, also bulges in late-type galaxies or in less massive galaxies have no preference in being oblate, prolate, or triaxial. On the contrary, bulges with high Sérsic index, in early-type galaxies, or in more massive galaxies are mostly oblate systems. We concluded that various evolutionary pathways may coexist in galaxies, with merging events and dissipative collapse being the main mechanisms driving the formation of the most massive oblate bulges and bar evolution reshaping the less massive triaxial bulges.

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arXiv2017

Stringent limits on the masses of the supermassive black holes in seven nearby galaxies

We present new stringent limits on the mass $M_{bh}$ of the central supermassive black hole for a sample of 7 nearby galaxies. Our $M_{bh}$ estimates are based on the dynamical modeling of the central width of the nebular emission lines measured over subarcsecond apertures with the Hubble Space Telescope. The central stellar velocity dispersion $σ_c$ of the sample galaxies is derived from new long-slit spectra from ground-based observations and the bulge effective radius is obtained from a two-dimensional photometric decomposition of the i-band images from the Sloan Digital Sky Survey. The derived stringent $M_{bh}$ limits run parallel and above the $M_{bh}-σ_c$ relation with no systematic trend depending on the galaxy distance or morphology. This gives further support to previous findings suggesting that the nuclear gravitational potential is remarkably well traced by the width of the nebular lines when the gas is centrally peaked. With our investigation, the number of galaxies with stringent $M_{bh}$ limits obtained from nebular-line width increases to 114 and can be used for studying the scaling relations between $M_{bh}$ and properties of their host galaxies.

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arXiv2017

No evidence for small disk-like bulges in a sample of late-type spirals

About 20% of low-redshift galaxies are late-type spirals with a small or no bulge component. Although they are the simplest disk galaxies in terms of structure and dynamics, the role of the different physical processes driving their formation and evolution is not yet fully understood. We investigated whether small bulges of late-type spirals follow the same scaling relations traced by ellipticals and large bulges and if they are disk-like or classical bulges. We derived the photometric and kinematic properties of 9 nearby late-type spirals. To this aim, we analyzed the surface brightness distribution from the i-band images of the Sloan Digital Sky Survey and obtained the structural parameters of the galaxies from a two-dimensional photometric decomposition. We measured the line-of-sight stellar velocity distribution within the bulge effective radius from the long-slit spectra taken with high spectral resolution at the Telescopio Nazionale Galileo. We used the photometric and kinematic properties of the sample bulges to study their location in the Fundamental Plane, Kormendy, and Faber-Jackson relations defined for ellipticals and large bulges. We found that our sample bulges satisfy some of the photometric and kinematic prescriptions for being considered disk-like bulges such as small sizes and masses with nearly exponential light profiles, small bulge-to-total luminosity ratios, low stellar velocity dispersions, and ongoing star formation. However, each of them follows the same scaling relations of ellipticals, massive bulges, and compact early-type galaxies so they cannot be classified as disk-like systems. We find a single population of galaxy spheroids that follow the same scaling relations, where the mass seems to lead to a smooth transition in the photometric and kinematic properties from less massive bulges to more massive bulges and ellipticals.

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arXiv2017

Space Telescope and Optical Reverberation Mapping Project. V. Optical Spectroscopic Campaign and Emission-Line Analysis for NGC 5548

We present the results of an optical spectroscopic monitoring program targeting NGC 5548 as part of a larger multi-wavelength reverberation mapping campaign. The campaign spanned six months and achieved an almost daily cadence with observations from five ground-based telescopes. The H$β$ and He II $λ$4686 broad emission-line light curves lag that of the 5100 $Å$ optical continuum by $4.17^{+0.36}_{-0.36}$ days and $0.79^{+0.35}_{-0.34}$ days, respectively. The H$β$ lag relative to the 1158 $Å$ ultraviolet continuum light curve measured by the Hubble Space Telescope is roughly $\sim$50% longer than that measured against the optical continuum, and the lag difference is consistent with the observed lag between the optical and ultraviolet continua. This suggests that the characteristic radius of the broad-line region is $\sim$50% larger than the value inferred from optical data alone. We also measured velocity-resolved emission-line lags for H$β$ and found a complex velocity-lag structure with shorter lags in the line wings, indicative of a broad-line region dominated by Keplerian motion. The responses of both the H$β$ and He II $λ$4686 emission lines to the driving continuum changed significantly halfway through the campaign, a phenomenon also observed for C IV, Ly $α$, He II(+O III]), and Si IV(+O IV]) during the same monitoring period. Finally, given the optical luminosity of NGC 5548 during our campaign, the measured H$β$ lag is a factor of five shorter than the expected value implied by the $R_\mathrm{BLR} - L_\mathrm{AGN}$ relation based on the past behavior of NGC 5548.

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arXiv2017

Kinematic and stellar population properties of the counter-rotating components in the S0 galaxy NGC 1366

Context. Many disk galaxies host two extended stellar components that rotate in opposite directions. The analysis of the stellar populations of the counter-rotating components provides constraints on the environmental and internal processes that drive their formation. Aims. The S0 NGC 1366 in the Fornax cluster is known to host a stellar component that is kinematically decoupled from the main body of the galaxy. Here we successfully separated the two counter-rotating stellar components to independently measure the kinematics and properties of their stellar populations. Methods. We performed a spectroscopic decomposition of the spectrum obtained along the galaxy major axis and separated the relative contribution of the two counter-rotating stellar components and of the ionized-gas component. We measured the line-strength indices of the two counter-rotating stellar components and modeled each of them with single stellar population models that account for the α/Fe overabundance. Results. We found that the counter-rotating stellar component is younger, has nearly the same metallicity, and is less α/Fe enhanced than the corotating component. Unlike most of the counter-rotating galaxies, the ionized gas detected in NGC 1366 is neither associated with the counter-rotating stellar component nor with the main galaxy body. On the contrary, it has a disordered distribution and a disturbed kinematics with multiple velocity components observed along the minor axis of the galaxy. Conclusions. The different properties of the counter-rotating stellar components and the kinematic peculiarities of the ionized gas suggest that NGC 1366 is at an intermediate stage of the acquisition process, building the counter-rotating components with some gas clouds still falling onto the galaxy.

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arXiv2016

Stellar populations in the bulges of isolated galaxies

We present photometry and long-slit spectroscopy for 12 S0 and spiral galaxies selected from the Catalogue of Isolated Galaxies. The structural parameters of the sample galaxies are derived from the Sloan Digital Sky Survey i-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 Sèrsic bulge, an exponential disc, and a Ferrers bar characterized by elliptical and concentric isophotes with constant ellipticity and position angles. The rotation curves and velocity dispersion profiles of the stellar component are measured from the spectra obtained along the major axis of galaxies. The radial profiles of the Hβ, Mg and Fe line-strength indices are derived too. Correlations between the central values of the Mg 2 and Fe line-strength indices and the velocity dispersion are found. The mean age, total metallicity and total α/Fe enhancement of the stellar population in the centre and at the radius where the bulge gives the same contribution to the total surface brightness as the remaining components are obtained using stellar population models with variable element abundance ratios. We identify intermediate-age bulges with solar metallicity and old bulges with a large spread in metallicity. Most of the sample bulges display super-solar α/Fe enhancement, no gradient in age and negative gradients of metallicity and α/Fe enhancement. These findings support a formation scenario via dissipative collapse where environmental effects are remarkably less important than in the assembly of bulges of galaxies in groups and clusters.

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arXiv2016

The role of quenching time in the evolution of the mass-size relation of passive galaxies from the WISP survey

We analyze how passive galaxies at z $\sim$ 1.5 populate the mass-size plane as a function of their stellar age, to understand if the observed size growth with time can be explained with the appearance of larger quenched galaxies at lower redshift. We use a sample of 32 passive galaxies extracted from the Wide Field Camera 3 Infrared Spectroscopic Parallel (WISP) survey with spectroscopic redshift 1.3 $\lesssim$ z $\lesssim$ 2.05, specific star-formation rates lower than 0.01 Gyr$^{-1}$, and stellar masses above 4.5 $\times$ 10$^{10}$ M$_\odot$. All galaxies have spectrally determined stellar ages from fitting of their rest-frame optical spectra and photometry with stellar population models. When dividing our sample into young (age $\leq$ 2.1 Gyr) and old (age $>$ 2.1 Gyr) galaxies we do not find a significant trend in the distributions of the difference between the observed radius and the one predicted by the mass-size relation. This result indicates that the relation between the galaxy age and its distance from the mass-size relation, if it exists, is rather shallow, with a slope alpha $\gtrsim$ -0.6. At face value, this finding suggests that multiple dry and/or wet minor mergers, rather than the appearance of newly quenched galaxies, are mainly responsible for the observed time evolution of the mass-size relation in passive galaxies.

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arXiv2016

The young nuclear stellar disc in the SB0 galaxy NGC 1023

Small kinematically-decoupled stellar discs with scalelengths of a few tens of parsec are known to reside in the centre of galaxies. Different mechanisms have been proposed to explain how they form, including gas dissipation and merging of globular clusters. Using archival Hubble Space Telescope imaging and ground-based integral-field spectroscopy, we investigated the structure and stellar populations of the nuclear stellar disc hosted in the interacting SB0 galaxy NGC 1023. The stars of the nuclear disc are remarkably younger and more metal rich with respect to the host bulge. These findings support a scenario in which the nuclear disc is the end result of star formation in metal enriched gas piled up in the galaxy centre. The gas can be of either internal or external origin, i.e. from either the main disc of NGC 1023 or the nearby satellite galaxy NGC 1023A. The dissipationless formation of the nuclear disc from already formed stars, through the migration and accretion of star clusters into the galactic centre is rejected.

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arXiv2015

Study of the stellar population properties in the discs of ten spiral galaxies

We investigated the properties of the stellar populations in the discs of a sample of ten spiral galaxies. Our analysis focused on the galaxy region where the disc contributes more than 95 per cent of total surface brightness in order to minimise the contamination of the bulge and bar. The luminosity-weighted age and metallicity were obtained by fitting the galaxy spectra with a linear combination of stellar population synthesis models, while the total overabundance of α-elements over iron was derived by measuring the line-strength indices. Most of the sample discs display a bimodal age distribution and they are characterised by a total [α/Fe] enhancement ranging from solar and supersolar. We interpreted the age bimodality as due to the simultaneous presence of both a young (Age$\,\leq\,4$ Gyr) and an old (Age$\,>\,$4 Gyr) stellar population. The old stellar component usually dominates the disc surface brightness and its light contribution is almost constant within the observed radial range. For this reason, no age gradient is observed in half of the sample galaxies. The old component is slightly more metal poor than the young one. The metallicity gradient is negative and slightly positive in the old and young components, respectively. These results are in agreement with an inside-out scenario of disc formation and suggest a reduced impact of the radial migration on the stellar populations of the disc. The young component could be the result of a second burst of star formation in gas captured from the environment.

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arXiv2015

Stellar populations of the bulges of four spiral galaxies

Key information to understand the formation and evolution of disk galaxies are imprinted in the stellar populations of their bulges. This paper has the purpose to make available new measurements of the stellar population properties of the bulges of four spiral galaxies. Both the central values and radial profiles of the line strength of some of the most common Lick indices are measured along the major- and minor- axis of the bulge-dominated region of the sample galaxies. The corresponding age, metallicity, and α/Fe ratio are derived by using the simple stellar population synthesis model predictions. The central values and the gradients of the stellar population properties of ESO-LV1890070, ESO-LV4460170, and ESO-LV 5140100 are consistent with previous findings for bulges of spiral galaxies. On the contrary, the bulge of ESO-LV 4500200 shows peculiar chemical properties possibly due to the presence of a central kinematically-decoupled component. The negative metallicity gradient found in our bulges sample indicates a relevant role for the dissipative collapse in bulge formation. However, the shallow gradients found for the age and α/Fe ratio suggests that merging can not be completely ruled out for the sample bulges. This is confirmed by the properties of ESO-LV 4500200 which can hardly be explained without invoking the capture of external material.

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arXiv2013

The Size of the Narrow-Line Emitting Region in the Seyfert 1 Galaxy NGC 5548 from Emission-Line Variability

The narrow [O III] 4959, 5007 emission-line fluxes in the spectrum of the well-studied Seyfert 1 galaxy NGC 5548 are shown to vary with time. From this we show that the narrow line-emitting region has a radius of only 1-3 pc and is denser (n ~ 10^5 cm^{-3}) than previously supposed. The [O III] line width is consistent with virial motions at this radius given previous determinations of the black hole mass.Since the [O III] emission-line flux is usually assumed to be constant and is therefore used to calibrate spectroscopic monitoring data, the variability has ramifications for the long-term secular variations of continuum and emission-line fluxes, though it has no effect on shorter-term reverberation studies. We present corrected optical continuum and broad Hbeta emission-line light curves for the period 1988 to 2008.

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arXiv2013

The external origin of the polar gaseous disk of the S0 galaxy IC 5181

Galaxies accrete material from the environment through acquisition and merging events. We study the nearby S0 galaxy IC 5181 to address the origin of the ionized-gas component orbiting the galaxy on polar orbit ionized gas of IC 5181 from broad and narrow-band imaging. We measure the ionized-gas and stellar kinematics and the line strengths of the Lick indices of the stellar component along both the major and minor axis. The age, metallicity, and [alpha/Fe] enhancement of the stellar populations are derived using single stellar population models with variable element abundance ratios. The ionized-gas metallicity is obtained from the equivalent width of the emission lines. IC 5181 is a morphologically undisturbed S0 galaxy with a classical bulge made by old stars with super solar metallicity and overabundance. Stellar age and metallicity decrease in the disk region. The galaxy hosts a geometrically and kinematically decoupled component of ionized gas. It is elongated along the galaxy minor axis and in orthogonal rotation with respect to the galaxy disk. We interpret the kinematical decoupling as suggestive of a component of gas, which is not related to the stars and having an external origin. It was accreted by IC 5181 on polar orbits from the surrounding environment.

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arXiv2013

Searching for nuclear stellar discs in simulations of star cluster mergers

The nuclei of galaxies often host small stellar discs with scale-lengths of a few tens of parsecs and luminosities up to 10^7 Lsun. To investigate the formation and properties of nuclear stellar discs (NSDs), we look for their presence in a set of N-body simulations studying the dissipationless merging of multiple star clusters in galactic nuclei. A few tens of star clusters with sizes and masses comparable to those of globular clusters observed in the Milky Way are accreted onto a pre-existing nuclear stellar component: either a massive super star cluster or a rapidly rotating, compact disc with a scale-length of a few parsecs, mimicing the variety of observed nuclear structures. Images and kinematic maps of the simulation time-steps are then built and analysed as if they were real and at the distance of the Virgo cluster. We use the Scorza-Bender method to search for the presence of disc structures via photometric decomposition. In one case the merger remnant has all the observed photometric and kinematic properties of NSDs observed in real galaxies. This shows that current observations are consistent with most of the NSD mass being assembled from the migration and accretion of star clusters into the galactic centre. In the other simulation instead, we detect an elongated structure from the unsharp masked image, that does not develop the photometric or kinematic signature of a NSD. Thus, in the context of searches for a disc structure, the Scorza-Bender method is a robust and necessary tool.

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arXiv2013

Constraining the luminosity function of faint undetected i-dropout galaxies

We present a new technique to quantify the light contribution coming from the faint high redshift ($z\sim6$) galaxies below the detection threshold of imaging data, set conventionally at S/N=4.5. We illustrate the technique with an application to Hubble Space Telescope Advanced Camera for Surveys images in the F775W and F850LP filters of the Ultra Deep Field parallel field NICP12. The aim of this analysis is to extend by a few magnitudes the faint end of the luminosity function at $z\sim6$. After masking all the detected sources in the field we apply a Fast Fourier Transform to obtain the spatial power spectrum of the background signal. The power spectrum permits us to separate the background noise signal, the residuals due to the data reduction of the wide field, and the overall signal produced by faint galaxies. The ratio of the signal in the i_775 and z_850 bands is used to estimate the contribution of the faint i-dropout objects. We rely on extensive Monte Carlo simulations to characterize various sources of uncertainty and quantify the number of faint i-dropout galaxies in the field. The analysis allows us to put constraints on the luminosity function at $z\sim6$ down to z_850= 30 mag, 2.5 mag fainter than with standard techniques on the same data. The data are consistent with a faint end slope of the luminosity function of $α= -1.9$. Assuming a specific set of values for the clumping factor, escape fraction, and spectral energy distribution, we find that the $z\sim6$ undetected galaxies down to z_850=30 mag could have driven cosmic reionization.

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arXiv2012

Fast and slow rotators in the densest environments: a FLAMES/GIRAFFE IFS study of galaxies in Abell 1689 at z=0.183

We present FLAMES/GIRAFFE integral field spectroscopy of 30 galaxies in the massive cluster Abell 1689 at z = 0.183. Conducting an analysis similar to that of ATLAS3D, we extend the baseline of the kinematic morphology-density relation by an order of magnitude in projected density and show that it is possible to use existing instruments to identify slow and fast rotators beyond the local Universe. We find 4.5 +- 1.0 slow rotators with a distribution in magnitude similar to those in the Virgo cluster. The overall slow rotator fraction of our Abell 1689 sample is 0.15 +- 0.03, the same as in Virgo using our selection criteria. This suggests that the fraction of slow rotators in a cluster is not strongly dependent on its density. However, within Abell 1689, we find that the fraction of slow rotators increases towards the centre, as was also found in the Virgo cluster.

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arXiv2012

Gas-phase metallicity of 27 galaxies at intermediate redshift

The purpose of this work is to make available new gas-phase oxygen abundance measurements for a serendipitous sample of 27 galaxies with redshift 0.35<z<0.52. We measured the equivalent widths of the [O II]λ3727, Hβ, and [O III]λλ4959, 5007 emission lines observed in the galaxy spectra obtained with the Visible Multi-Object Spectrograph mounted at the Very Large Telescope. For each galaxy, we derived the metallicity-sensitive emission lines ratio R23, ionization-sensitive emission lines ratio O32, and gas-phase oxygen abundance 12+log(O/H). The values of gas-phase oxygen abundance 12+log(O/H) we obtained for the sample galaxies are consistent with previous findings for galaxies at intermediate redshift.

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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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