arXiv subjects

M. Sarzi

Publications and source records attributed to M. Sarzi.

At least 73 records · Page 4Linked to original sources

arXiv2007

The UV properties of E+A galaxies: constraints on feedback-driven quenching of star formation

We present the first large-scale study of E+A (post-starburst) galaxies that incorporates photometry in the ultraviolet (UV) wavelengths. We find that the starburst that creates the E+A galaxy typically takes place within the last Gyr and creates a high fraction (20-60 percent) of the stellar mass in the remnant over a short timescale (< 0.1 Gyrs). We find a tight correlation between the luminosity of our E+A galaxies and the implied star formation rate (SFR) during the starburst. While low-luminosity E+As (M(z) > -20) exhibit implied SFRs of less than 50 solar masses per year, their luminous counterparts (M(z) < -22) shows SFRs greater than 300 and as high as 2000 solar masses per year, suggesting that luminous and ultra-luminous infrared galaxies in the low-redshift Universe could be the progenitors of massive nearby E+A galaxies. We perform a comprehensive study of the characteristics of the quenching that truncates the starburst in E+A systems.We find that, for galaxies less massive than 10^10 MSun, the quenching efficiency decreases as the galaxy mass increases. However, for galaxies more massive than 10^10 MSun, this trend is reversed and the quenching efficiency increases with galaxy mass. Noting that the mass threshold at which this reversal occurs is in excellent agreement with the mass above which AGN become significantly more abundant in nearby galaxies, we use simple energetic arguments to show that the bimodal behaviour of the quenching efficiency is consistent with AGN and supernovae (SN) being the principal sources of negative feedback above and below M ~ 10^10 MSun respectively. (abridged)

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arXiv2007

Supermassive black holes from OASIS and SAURON integral-field kinematics

Supermassive black holes are a key element in our understanding of how galaxies form. Most of the progress in this very active field of research is based on just ~30 determinations of black hole mass, accumulated over the past decade. We illustrate how integral-field spectroscopy, and in particular our OASIS modeling effort, can help improve the current situation.

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arXiv2007

Recent Star Formation in Nearby Early-type Galaxies

Motivated by recent progress in the study of early-type galaxies owing to technological advances, the launch of new space telescopes and large ground-based surveys, we attempt a short review of our current understanding of the recent star-formation activity in such intriguing galactic systems.

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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

Towards a new classification of early-type galaxies: an integral-field view

In this proceeding we make use of the two-dimensional stellar kinematics of a representative sample of E and S0 galaxies obtained with the SAURON integral-field spectrograph to reveal that early-type galaxies appear in two broad flavours, depending on whether they exhibit clear large-scale rotation or not. We measure the level of rotation via a new parameter LambdaR and use it as a basis for a new kinematic classification that separates early-type galaxies into slow and fast rotators. With the aid of broad-band imaging we will reinforce this finding by comparing our kinematic results to the photometric properties of these two classes.

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arXiv2007

The kinematics of Core and Cusp galaxies: comparing HST imaging and integral-field observations

In this proceeding we look at the relationship between the photometric nuclear properties of early-type galaxies from Hubble Space Telescope imaging and their overall kinematics as observed with the SAURON integral-field spectrograph. We compare the inner slope of their photometric profiles and the Slow/Fast rotator classes, defined by the amplitude of a newly defined LambdaR parameter, to show that slow rotators tend to be more massive systems and display shallower inner profiles and fast rotators steper ones. It is important to remark, however, that there is not a one-to-one relationship between the two photometric and kinematic groups.

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arXiv2007

Star formation and stellar populations across nuclear rings in galaxies

(Abridged) We present a study of the optical spectra of a sample of eight star-forming nuclear rings and the nuclei of their host galaxies. The spectra were obtained with the ISIS spectrograph on the William Herschel Telescope and cover a wide range in wavelength, enabling the measurement of several stellar absorption features and gas emission lines. We compared the strength of the absorption lines to a variety of population synthesis models for the star-formation history in the nuclear rings, including also the contribution of the older bulge and disc stellar components. We find that the stars in our sample of nuclear rings have most likely formed over a prolonged period of time characterised by episodic bursts of star-formation activity. Constant star formation is firmly ruled out by the data, whereas a one-off formation event is an unlikely explanation for a common galactic component such as nuclear rings. We have used emission-line measurements to constrain the physical conditions of the ionised gas within the rings. Emission in all nuclear rings originates from HII-regions with electron densities typical for these kinds of objects, and that the rings are characterised by values for the gas metallicity ranging from slightly below to just above solar. As 20% of nearby spiral galaxies hosts nuclear rings that are currently forming massive stars, our finding of an episodic star formation history in nuclear rings implies that a significant population remains to be identified of young nuclear rings that are not currently in a massive star formation phase.

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arXiv2007

The SAURON project -- X. The orbital anisotropy of elliptical and lenticular galaxies: revisiting the (V/sigma,epsilon) diagram with integral-field stellar kinematics

We analyse the orbital distribution of E and S0 galaxies using SAURON integral-field stellar kinematics. We construct the anisotropy (V/sigma,epsilon) diagram, for the 48 E/S0 galaxies from the SAURON survey. For a subsample of 24 galaxies consistent with axisymmetry, we use three-integral axisymmetric Schwarzschild dynamical models to recover the detailed orbital distribution and we find good agreement with the anisotropy derived from the (V/sigma,epsilon) diagram. In the companion Paper IX we show that the early-type galaxies can be subdivided into two classes of systems with or without a significant amount of specific stellar angular momentum. Here we show that the two classes have different distributions on the (V/sigma,epsilon) diagram. The slow rotators are more common among the most massive systems and are generally classified as E from photometry alone. Those in our sample tend to be fairly round (epsilon<0.3), but can have significant kinematical misalignments, indicating that as a class they are moderately triaxial, and span a range of anisotropies (delta<0.3). The fast rotators are generally fainter and are classified either E or S0. They can appear quite flattened (epsilon<0.7), do not show significant kinematical misalignments (unless barred or interacting), indicating they are nearly axisymmetric, and span an even larger range of anisotropies (delta<0.5). These results are confirmed when we extend our analysis to 18 additional E/S0 galaxies observed with SAURON. All these results support the idea that fast rotators are nearly oblate and contain disk-like components. The role of gas must have been important for their formation. The slow rotators are weakly triaxial. Current collisionless merger models seem unable to explain their detailed observed properties. (Abridged)

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arXiv2006

On the origin and fate of ionised-gas in early-type galaxies: the SAURON perspective

By detecting ionised-gas emission in 75% of the cases, the SAURON integral-field spectroscopic survey has further demonstrated that early-type galaxies often display nebular emission. Furthermore, the SAURON data have shown that such emission comes with an intriguing variety of morphologies, kinematic behaviours and line ratios. Perhaps most puzzling was the finding that round and slowly rotating objects generally display uncorrelated stellar and gaseous angular momenta, consistent with an external origin for the gas, whereas flatter and fast rotating galaxies host preferentially co-rotating gas and stars, suggesting internal production of gas. Alternatively, a bias against the internal production of ionised gas and against the acquisition of retrograde material may be present in these two kinds of objects, respectively. In light of the different content of hot gas in these systems, with slowly rotating objects being the only systems capable of hosting massive X-ray halos, we suggest that a varying importance of evaporation of warm gas in the hot interstellar medium can contribute to explain the difference in the relative behaviour of gas and stars in these two kinds of objects. Namely, whereas in X-ray bright and slowly rotating galaxies stellar-loss material would quickly evaporate in the hot medium, in X-ray faint and fast rotating objects such material would be allowed to lose angular momentum and settle in a disk, which could also obstruct the subsequent acquisition of retrograde gas. Evidence for a connection between warm and hot gas phases, presumably driven by heat conduction, is presented for four slowly rotating galaxies with CHANDRA observations.

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arXiv2006

Star formation in the central regions of galaxies

Massive star formation in the central regions of spiral galaxies plays an important role in the dynamical and secular evolution of their hosts. Here, we summarise a number of recent investigations of the star formation history and the physical conditions of the gas in circumnuclear regions, to illustrate not only the detailed results one can achieve, but also the potential of using state-of-the-art spectroscopic and analysis techniques in researching the central regions of galaxies in general. We review how the star formation history of nuclear rings confirms that they are long-lived and stable configurations. Gas flows in from the disk, through the bar, and into the ring, where successive episodes of massive star formation occur. Analysing the ring in NGC 7742 in particular, we determine the physical conditions of the line emitting gas using a combination of ionisation and stellar population modelling, concluding that the origin of the nuclear ring in this non-barred galaxy lies in a recent minor merger with a small gas-rich galaxy.

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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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arXiv2006

Stars and Gas in the Inner Parts of Galaxies seen in SAURON Integral Field Observations

We give two examples of spiral galaxies that show non-circular gas motions in the inner kiloparsecs, from SAURON integral field spectroscopy. We use harmonic decomposition of the velocity field of the ionized gas to study the underlying mass distribution, employing linear theory. The higher order harmonic terms and the main kinematic features of the observed data are consistent with an analytically constructed simple bar model. We also present maps of a number of strong absorption lines in M 100, derive simple stellar populations and correlate them with features in the gas kinematics.

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arXiv2006

The SAURON project - VIII. OASIS/CFHT integral-field spectroscopy of elliptical and lenticular galaxy centres

We present high spatial resolution integral-field spectroscopy of 28 elliptical (E) and lenticular (S0) galaxies from the SAURON representative survey obtained with the OASIS spectrograph during its operation at the CFHT. These seeing-limited observations explore the central 8"x10" (typically one kiloparsec diameter) regions of these galaxies using a spatial sampling four times higher than SAURON (0.27" vs. 0.94" spatial elements), resulting in almost a factor of two improvement in the median PSF. These data allow accurate study of the central regions to complement the large-scale view provided by SAURON. Here we present the stellar and gas kinematics, stellar absorption-line strengths and nebular emission-line strengths for this sample. We also characterise the stellar velocity maps using the "kinemetry" technique, and derive maps of the luminosity-weighted stellar age, metallicity and abundance ratio via stellar population models. We give a brief review of the structures found in our maps, linking also to larger-scale structures measured with SAURON. We present two previously unreported kinematically-decoupled components (KDCs) in the centres of NGC3032 and NGC4382. We compare the intrinsic size and luminosity-weighted stellar age of all the visible KDCs in the full SAURON sample, and find two types of components: kiloparsec-scale KDCs, which are older than 8 Gyr, and are found in galaxies with little net rotation; and compact KDCs, which have intrinsic diameters of less than a few hundred parsec, show a range of stellar ages from 0.5 - 15 Gyr (with 5/6 younger than 5 Gyr), are found exclusively in fast-rotating galaxies, and are close to counter-rotating around the same axis as their host. (abridged)

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arXiv2006

Minor Merger Origin for the Circumnuclear Starburst in NGC 7742

We present an emission-line diagnostic analysis of integral-field spectroscopic observations that cover the central kiloparsec of NGC 7742. This Sa galaxy hosts a spectacular nuclear starburst ring and nuclear regions characterized by low-ionization emission. The gas in the ring rotates in the opposite sense to the stars in the galaxy, suggesting a recent merging or acquisition event. The combination of integral-field measurements for the H-alpha + [NII] emission lines from DensePak and the H-beta and [OIII] emission from Sauron allow the construction of diagnostic diagrams that highlight the transition from star formation in the nuclear ring to excitation by high-velocity shocks or by a central AGN towards the center. DensePak measurements for the [SII] line ratio reveal very low gas densities in the nuclear ring, Ne < 100 per cm^3, characteristic of massive HII regions. Comparison with MAPPINGS III models for starbursts with low gas densities show that the ring is of roughly solar metallicity. This suggests that the gas in the nuclear ring originated in a stellar system capable of substantially enriching the gas metallicity through sustained star formation. We suggest that NGC 7742 cannibalised a smaller galaxy rich in metal-poor gas, and that star formation episodes in the ring have since increased the metallicity to its present value.

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arXiv2006

Neutral hydrogen in nearby elliptical and lenticular galaxies: the continuing formation of early-type galaxies

(Abridged) We present deep WSRT observations of neutral hydrogen in 12 nearby elliptical and lenticular galaxies. The selected objects come from a representative sample of nearby galaxies earlier studied at optical wavelengths with the integral-field spectrograph Sauron. We detect HI - both in regular discs as well as in clouds and tails off-set from the host galaxy - in 70% of the galaxies. This detection rate is much higher than in previous, shallower single-dish surveys, and is similar to that for the ionised gas. The results suggest that at faint detection levels the presence of HI is a relatively common characteristic of field early-type galaxies. The observed total HI masses range between a few times 10^6 Msun to just over 10^9 Msun. The presence of regular disc-like structures is a situation as common as HI in offset clouds and tails around early-type galaxies. All galaxies where HI is detected also contain ionised gas, whereas no HI is found around galaxies without ionised gas. Galaxies with regular HI discs tend to have strong emission from ionised gas. In these cases, the similar kinematics of the neutral hydrogen and ionised gas suggest that they form one structure. We do not find a clear trend between the presence of HI and the global age of the stellar population or the global dynamical characteristics of the galaxies. Our observations support the idea that gas accretion is common and does not happen exclusively in peculiar early-type galaxies. For the majority of the cases the gas is likely acquired through merging, but the lack of correlation with the stellar population age suggests that smooth, cold accretion could be an alternative scenario, at least in some galaxies. In either cases, the data suggest that early-type galaxies continue to build their mass up to the present.

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arXiv2006

Central stellar populations of early-type galaxies in low-density environments

We have investigated the properties of a volume and magnitude limited sample of nearby early type galaxies that were carefully selected from the AAO two degree field galaxy redshift survey. We used images from the DSS to confirm the E/S0 morphologies, and augmented this sample with field galaxies from Colbert et al. 2001. We present spectroscopic observations of 22 galaxies from the combined sample, from which central velocity dispersions and the Lick stellar population indices were measured. After carefully correcting the spectra for nebular emission we derived luminosity-weighted ages, metallicities, and alpha-element abundance ratios. We compare these isolated galaxies with samples of early-type galaxies in the Virgo and Coma clusters, and also with the sample of galaxies in low-density regions of Kuntschner et al. (2002). We find that galaxies in low-density environments are younger and have a greater spread of ages compared to cluster galaxies. They also show a wider range of metallicities at a given velocity dispersion than cluster galaxies, which display only super-solar metallicities. On average cluster, as well as, isolated galaxies show non-solar abundance ratios in alpha-elements, suggesting that, independent of galactic environment, star formation occurred on short time-scales. We reason that early-type galaxies in low-density environments experienced merging-induced star-formation episodes over a longer and more recent period of time compared to a cluster environment, and speculate that a considerable fraction of their stars formed out of low-metallicity halo gaseous material during the slow growth of a stellar disk between merging events.

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arXiv2006

The SAURON project - VII. Integral-field absorption and emission-line kinematics of 24 spiral galaxy bulges

(Abridged) We present observations of the stellar and gas kinematics for a representative sample of 24 Sa galaxies obtained with our custom-built integral-field spectrograph SAURON operating on the William Herschel Telescope. Our maps typically cover the bulge dominated region. We find a significant fraction of kinematically decoupled components (12/24), many of them displaying central velocity dispersion minima. They are mostly aligned and co-rotating with the main body of the galaxies, and are usually associated with dust discs and rings detected in unsharp-masked images. Almost all the galaxies in the sample (22/24) contain significant amounts of ionised gas which, in general, is accompanied by the presence of dust. The kinematics of the ionised gas is consistent with circular rotation in a disc co-rotating with respect to the stars. The distribution of mean misalignments between the stellar and gaseous angular momenta in the sample suggest that the gas has an internal origin. The [OIII]/Hbeta ratio is usually very low, indicative of current star formation, and shows various morphologies (ring-like structures, alignments with dust lanes or amorphous shapes). The star formation rates in the sample are comparable with that of normal disc galaxies. Low gas velocity dispersion values appear to be linked to regions of intense star formation activity. We interpret this result as stars being formed from dynamically cold gas in those regions. In the case of NGC5953, the data suggest that we are witnessing the formation of a kinematically decoupled component from cold gas being acquired during the ongoing interaction with NGC5954.

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arXiv2006

SAURON observations of Sa bulges: the formation of a kinematically decoupled core in NGC5953

We present results from our ongoing effort to understand the nature and evolution of nearby galaxies using the SAURON integral-field spectrograph. In this proceeding we focus on the study of the particular case formed by the interacting galaxies NGC5953 and NGC5954. We present stellar and gas kinematics of the central regions of NGC5953. We use a simple procedure to determine the age of the stellar populations in the central regions and argue that we may be witnessing the formation of a kinematically decoupled component from cold gas being acquired during the ongoing interaction with NGC5954.

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