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M. Sarzi

Publications and source records attributed to M. Sarzi.

At least 55 records · Page 3Linked to original sources
arXiv2012

AGN Feedback Driven Molecular Outflow in NGC 1266

NGC 1266 is a nearby field galaxy observed as part of the ATLAS3D survey (Cappellari et al. 2011). NGC 1266 has been shown to host a compact (< 200 pc) molecular disk and a mass-loaded molecular outflow driven by the AGN (Alatalo et al. 2011). Very Long Basline Array (VLBA) observations at 1.65 GHz revealed a compact (diameter < 1.2 pc), high bright- ness temperature continuum source most consistent with a low-level AGN origin. The VLBA continuum source is positioned at the center of the molecular disk and may be responsible for the expulsion of molecular gas in NGC 1266. Thus, the candidate AGN-driven molecular outflow in NGC 1266 supports the picture in which AGNs do play a significant role in the quenching of star formation and ultimately the evolution of the red sequence of galaxies.

astro-ph.CO↗
arXiv2012

Quenching of Star Formation in Molecular Outflow Host NGC 1266

We detail the rich molecular story of NGC 1266, its serendipitous discovery within the ATLAS3D survey (Cappellari et al. 2011) and how it plays host to an AGN-driven molecular outflow, potentially quenching all of its star formation (SF) within the next 100 Myr. While major mergers appear to play a role in instigating outflows in other systems, deep imaging of NGC 1266 as well as stellar kinematic observations from SAURON, have failed to provide evidence that NGC 1266 has recently been involved in a major interaction. The molecular gas and the instantaneous SF tracers indicate that the current sites of star formation are located in a hypercompact disk within 200 pc of the nucleus (Fig. 1; SF rate ~ 2 Msuns/yr). On the other hand, tracers of recent star formation, such as the Hβ absorption map from SAURON and stellar population analysis show that the young stars are distributed throughout a larger area of the galaxy than current star formation. As the AGN at the center of NGC 1266 continues to drive cold gas out of the galaxy, we expect star formation rates to decline as the star formation is ultimately quenched. Thus, NGC 1266 is in the midst of a key portion of its evolution and continued studies of this unique galaxy may help improve our understanding of how galaxies transition from the blue to the red sequence (Alatalo et al. 2011).

astro-ph.CO↗
arXiv2011

The SAURON Project - XIX. Optical and near-infrared scaling relations of nearby elliptical, lenticular and Sa galaxies

[Abridged] We present ground-based MDM V-band and Spitzer/IRAC 3.6um-band photometric observations of the 72 representative galaxies of the SAURON Survey. In combination with the SAURON stellar velocity dispersion measured within an effective radius (se), this allows us to explore the location of our galaxies in the main scaling relations. We investigate the dependence of these relations on our recent kinematical classification of early-type galaxies (i.e. Slow/Fast Rotators) and the stellar populations. Slow Rotator and Fast Rotator E/S0 galaxies do not populate distinct locations in the scaling relations, although Slow Rotators display a smaller intrinsic scatter. Surprisingly, extremely young objects do not display the bluest (V-[3.6]) colours in our sample, as is usually the case in optical colours. This can be understood in the context of the large contribution of TP-AGB stars to the infrared, even for young populations, resulting in a very tight (V-[3.6]) - se relation that in turn allows us to define a strong correlation between metallicity and velocity dispersion. Many Sa galaxies appear to follow the Fundamental Plane defined by E/S0 galaxies. Galaxies that appear offset from the relations correspond mostly to objects with extremely young populations, with signs of on-going, extended star formation. We correct for this effect in the Fundamental Plane, by replacing luminosity with stellar mass using an estimate of the stellar mass-to-light ratio, so that all galaxies are part of a tight, single relation. The new estimated coefficients are consistent in both photometric bands and suggest that differences in stellar populations account for about half of the observed tilt with respect to the virial prediction. After these corrections, the Slow Rotator family shows almost no intrinsic scatter around the best-fit Fundamental Plane.

astro-ph.CO↗
arXiv2011

Molecular Gas and Star Formation in Local Early-Type Galaxies

The molecular gas content of local early-type galaxies is constrained and discussed in relation to their evolution. First, as part of the Atlas3D survey, we present the first complete, large (260 objects), volume-limited single-dish survey of CO in normal local early-type galaxies. We find a surprisingly high detection rate of 22%, independent of luminosity and at best weakly dependent on environment. Second, the extent of the molecular gas is constrained with CO synthesis imaging, and a variety of morphologies is revealed. The kinematics of the molecular gas and stars are often misaligned, implying an external gas origin in over a third of the systems, although this behaviour is drastically diffferent between field and cluster environments. Third, many objects appear to be in the process of forming regular kpc-size decoupled disks, and a star formation sequence can be sketched by piecing together multi-wavelength information on the molecular gas, current star formation, and young stars. Last, early-type galaxies do not seem to systematically obey all our usual prejudices regarding star formation, following the standard Schmidt-Kennicutt law but not the far infrared-radio correlation. This may suggest a greater diversity in star formation processes than observed in disk galaxies. Using multiple molecular tracers, we are thus starting to probe the physical conditions of the cold gas in early-types.

astro-ph.CO↗
arXiv2011

Investigating the merger origin of Early-Type Galaxies using ultra-deep optical images

The mass assembly of galaxies leaves various imprints on their surroundings, such as shells, streams and tidal tails. The frequency and properties of these fine structures depend on the mechanism driving the mass assembly: e.g. a monolithic collapse, rapid cold-gas accretion followed by violent disk instabilities, minor mergers or major dry / wet mergers. Therefore, by studying the outskirts of galaxies, one can learn about their main formation mechanism. I present here our on-going work to characterize the outskirts of Early-Type Galaxies (ETGs), which are powerful probes at low redshift of the hierarchical mass assembly of galaxies. This work relies on ultra-deep optical images obtained at CFHT with the wide-field of view MegaCam camera of field and cluster ETGs obtained as part of the Atlas-3D and NGVS projects. State of the art numerical simulations are used to interpret the data. The images reveal a wealth of unknown faint structures at levels as faint as 29 mag arcsec-2 in the g-band. Initial results for two galaxies are presented here.

astro-ph.CO↗
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.

astro-ph.CO↗
arXiv2010

A Census of Nuclear Stellar Disks in Early-type Galaxies

Nuclear Stellar Disks (NSDs), of a few tens to hundreds of parsec across, are a common and yet poorly studied feature of early-type galaxies. Still, such small disks represent a powerful tool to constrain the assembling history of galaxies, since they can be used to trace to the epoch when galaxies experienced their last major merger event. By studying the fraction and stellar age of NSDs it is thus possible to test the predictions for the assembly history of early-type galaxies according the current hierarchical paradigm for galaxy formation. In this paper we have produced the most comprehensive census of NSDs in nearby early-type galaxies by searching for such disks in objects within 100 Mpc and by using archival images from the Hubble Space Telescope. We found that NSDs are present in approximately 20% of early-type galaxies, and that the fraction of galaxies with NSDs does not depend on their Hubble type nor on their galactic environment, whereas the incidence of NSDs appears to decline in the most massive systems. Furthermore, we have separated the light contribution of twelve such disks from that of their surrounding stellar bulge in order to extract their physical properties. This doubles the number of decomposed NSDs and although the derived values for their central surface brightness and scale-length are consistent with previous studies they also give a hint of possible different characteristics due to different formation scenario between nuclear disks and other kinds of large galactic disks.

astro-ph.CO↗
arXiv2010

Formation of slowly rotating early-type galaxies via major mergers: a Resolution Study

We study resolution effects in numerical simulations of gas-rich and gas-poor major mergers, and show that the formation of slowly-rotating elliptical galaxies often requires a resolution that is beyond the present-day standards to be properly modelled. Our sample of equal-mass merger models encompasses various masses and spatial resolutions, ranging from about 200pc and 10^5 particles per component, typical of some recently published major merger simulations, to up to 32pc and 10^3 M_sun in simulations using 2.4 x 10^7 collisionless particles and 1.2 x 10^7 gas particles, among the highest resolutions reached so far for gas-rich major merger of massive disc galaxies. We find that the formation of fast-rotating early-type galaxies, that are flattened by a significant residual rotation, is overall correctly reproduced at all such resolutions. However, the formation of slow-rotating early-type galaxies, which have a low residual angular momentum and are supported mostly by anisotropic velocity dispersions, is strongly resolution-dependent. The evacuation of angular momentum from the main stellar body is largely missed at standard resolution, and systems that should be slow rotators are then found to be fast rotators. The effect is most important for gas-rich mergers, but is also witnessed in mergers with an absent or modest gas component. The effect is robust with respect to our initial conditions and interaction orbits, and originates in the physical treatment of the relaxation process during the coalescence of the galaxies. Our findings show that a high-enough resolution is required to accurately model the global properties of merger remnants and the evolution of their angular momentum. The role of gas-rich mergers of spiral galaxies in the formation of slow-rotating ellipticals may therefore have been underestimated.

astro-ph.CO↗
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.

astro-ph↗
arXiv2009

The SAURON project - XV. Modes of star formation in early-type galaxies and the evolution of the red sequence

We combine SAURON integral field data of a representative sample of local early-type, red sequence galaxies with Spitzer/IRAC imaging in order to investigate the presence of trace star formation in these systems. With the Spitzer data, we identify galaxies hosting low-level star formation, as traced by PAH emission, with measured star formation rates that compare well to those estimated from other tracers. This star formation proceeds according to established scaling relations with molecular gas content, in surface density regimes characteristic of disk galaxies and circumnuclear starbursts. We find that star formation in early-type galaxies happens exclusively in fast-rotating systems and occurs in two distinct modes. In the first, star formation is a diffuse process, corresponding to widespread young stellar populations and high molecular gas content. The equal presence of co- and counter-rotating components in these systems strongly implies an external origin for the star-forming gas, and we argue that these star formation events may be the final stages of (mostly minor) mergers that build up the bulges of red sequence lenticulars. In the second mode of star formation, the process is concentrated into well-defined disk or ring morphologies, outside of which the host galaxies exhibit uniformly evolved stellar populations. This implies that these star formation events represent rejuvenations within previously quiescent stellar systems. Evidence for earlier star formation events similar to these in all fast rotating early-type galaxies suggests that this mode of star formation may be common to all such galaxies, with a duty cycle of roughly 1/10, and likely contributes to the embedded, co-rotating inner stellar disks ubiquitous in this population.

astro-ph.CO↗
arXiv2009

An HI view of the on-going assembly of early-type galaxies: present and future observations

We present a preliminary analysis of the HI properties of early-type galaxies in the ATLAS3D sample. Using WSRT data for ~100 galaxies outside the Virgo cluster and data from the Alfalfa project for galaxies inside Virgo, we discuss the dependence of HI properties on environment. We detect HI in about half of the galaxies outside Virgo. For these systems, the HI morphology and kinematics change as a function of environment, going from regular, rotating systems around isolated galaxies to progressively more disturbed structures for galaxies with neighbours or in groups. In denser environment, inside Virgo, nearly none of the galaxies contains HI. We discuss future work in this field which will be enabled by next-generation, pre-SKA radio instruments. We present a simulated Apertif HI observation of an ATLAS3D early-type galaxy, showing how its appearance and detection level vary as a function of redshift.

astro-ph.CO↗
arXiv2009

Galaxy Zoo: The properties of merging galaxies in the nearby Universe - local environments, colours, masses, star-formation rates and AGN activity

Following the study of Darg et al. (2009; hereafter D09a) we explore the environments, optical colours, stellar masses, star formation and AGN activity in a sample of 3003 pairs of merging galaxies drawn from the SDSS using visual classifications from the Galaxy Zoo project. While D09a found that the spiral-to-elliptical ratio in (major) mergers appeared higher than that of the global galaxy population, no significant differences are found between the environmental distributions of mergers and a randomly selected control sample. This makes the high occurrence of spirals in mergers unlikely to be an environmental effect and must, therefore, arise from differing time-scales of detectability for spirals and ellipticals. We find that merging galaxies have a wider spread in colour than the global galaxy population, with a significant blue tail resulting from intense star formation in spiral mergers. Galaxies classed as star-forming using their emission-line properties have average star-formation rates approximately doubled by the merger process though star formation is negligibly enhanced in merging elliptical galaxies. We conclude that the internal properties of galaxies significantly affect the time-scales over which merging systems can be detected (as suggested by recent theoretical studies) which leads to spirals being `over-observed' in mergers. We also suggest that the transition mass $3\times10^{10}{M}_{\astrosun}$, noted by \citet{kauffmann1}, below which ellipticals are rare could be linked to disc survival/destruction in mergers.

astro-ph.GA↗
arXiv2009

Galaxy Zoo: the fraction of merging galaxies in the SDSS and their morphologies

We present the largest, most homogeneous catalogue of merging galaxies in the nearby universe obtained through the Galaxy Zoo project - an interface on the world-wide web enabling large-scale morphological classification of galaxies through visual inspection of images from the Sloan Digital Sky Survey (SDSS). The method converts a set of visually-inspected classifications for each galaxy into a single parameter (the `weighted-merger-vote fraction,' $f_m$) which describes our confidence that the system is part of an ongoing merger. We describe how $f_m$ is used to create a catalogue of 3003 visually-selected pairs of merging galaxies from the SDSS in the redshift range $0.005 < z <0.1$. We use our merger sample and values of $f_m$ applied to the SDSS Main Galaxy Spectral sample (MGS) to estimate that the fraction of volume-limited ($M_r < -20.55$) major mergers ($1/3 < {M}^*_1/{M}^*_2 < 3$) in the nearby universe is $1 - 3 \times C%$ where $C \sim 1.5$ is a correction factor for spectroscopic incompleteness. Having visually classified the morphologies of the constituent galaxies in our mergers, we find that the spiral-to-elliptical ratio of galaxies in mergers is higher by a factor $\sim 2$ relative to the global population. In a companion paper, we examine the internal properties of these merging galaxies and conclude that this high spiral-to-elliptical ratio in mergers is due to a longer time-scale over which mergers with spirals are detectable compared to mergers with ellipticals.

astro-ph.GA↗
arXiv2008

The High-Mass End of the Black Hole Mass Function: Mass Estimates in Brightest Cluster Galaxies

We present Hubble Space Telescope imaging and spectroscopic observations of three Brightest Cluster Galaxies, Abell 1836-BCG, Abell 2052-BCG, and Abell 3565-BCG, obtained with the Wide Field and Planetary Camera 2, the Advanced Camera for Surveys and the Space Telescope Imaging Spectrograph. The data provide detailed information on the structure and mass profile of the stellar component, the dust optical depth, and the spatial distribution and kinematics of the ionized gas within the innermost region of each galaxy. Dynamical models, which account for the observed stellar mass profile and include the contribution of a central supermassive black hole (SBH), are constructed to reproduce the kinematics derived from the Halpha and [N II](lambda 6548,6583) emission lines. Secure SBH detection with M_bh=3.61(+0.41,-0.50)x10^9 M_sun and M_bh=1.34(+0.21,-0.19)x10^9 M_sun, respectively, are obtained for Abell 1836-BCG and Abell 3565-BCG, which show regular rotation curves and strong central velocity gradients. In the case of Abell 2052-BCG, the lack of an orderly rotational motion prevents a secure determination, although an upper limit of M_bh < 4.60x10^9 M_sun can be placed on the mass of the central SBH. These measurements represent an important step forward in the characterization of the high-mass end of the SBH mass function.

astro-ph↗
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)

astro-ph↗
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.

astro-ph↗
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.

astro-ph↗