arXiv subjects

T. Contini

Publications and source records attributed to T. Contini.

216 records · Page 12Linked to original sources

arXiv2003

Early results from the VIMOS VLT Deep Survey

The VIMOS VLT Deep Survey (VVDS) is underway to study the evolution of galaxies, large scale structures and AGNs, from the measurement of more than 100000 spectra of faint objects. We present here the results from the first epoch observations of more than 20000 spectra. The main challenge of the program, the redshift measurements, is described, in particular entering the ``redshift desert'' in the range $1.5<z<3$ for which only very weak features are observed in the observed wavelength range. The redshift distribution of a magnitude limited sample brighter than $I_{AB}=24$ is presented for the first time, showing a peak at a low redshift $z\sim0.7$, and a tail extending all the way above $z=4$. The evolution of the luminosity function out to $z=1.5$ is presented, with the LF of blue star forming galaxies carrying most of the evolution, with L* changing by more than two magnitudes for this sub-sample.

astro-ph↗

arXiv2003

Properties of Faint Distant Galaxies as seen through Gravitational Telescopes

This paper reviews the most recent developments related to the use of lensing clusters of galaxies as Gravitational Telescopes in deep Universe studies. We summarize the state of the art and the most recent results aiming at studying the physical properties of distant galaxies beyond the limits of conventional spectroscopy. The application of photometric redshift techniques in the context of gravitational lensing is emphasized for the study of both lensing structures and the background population of lensed galaxies. A presently ongoing search for the first building blocks of galaxies behind lensing clusters is presented and discussed.

astro-ph↗

arXiv2002

Physical properties of two low-luminosity z ~ 1.9 galaxies behind the lensing cluster AC 114

We present VLT/ISAAC near-infrared spectroscopy of two gravitationally-lensed z ~ 1.9 galaxies, A2 and S2, located behind the cluster AC 114. Thanks to large magnification factors, we have been successful in detecting rest-frame optical emission lines in star-forming galaxies 1 to 2 magnitudes fainter than in previous studies of Lyman break galaxies (LBGs) at z ~ 3. From the Ha luminosity, we estimate star formation rates (SFRs) which are 7 to 15 times higher than those inferred from the UV continuum flux at 1500 ang without dust extinction correction. The behavior of S2 and A2 in terms of O/H and N/O abundance ratios are very different, and they are also different from typical LBGs at z ~ 3. S2 is a low-metallicity object (Z ~ 0.03 Zsun) with a low N/O ratio, similar to those derived in the most metal-poor nearby HII galaxies. In contrast, A2 is a high-metallicity galaxy (Z ~ 1.3 Zsun) with a high N/O abundance ratio, similar to those derived in the most metal-rich starburst nucleus galaxies. The virial masses, derived from emission-line widths, are 0.5 and 2.4 x 10^10 Msun, for S2 and A2 respectively. Thanks to the gravitational amplification, the line profiles of S2 are spatially resolved, leading to a velocity gradient of +- 240 km/s, which yields a dynamical mass of ~ 1.3 x 10^10 Msun within the inner 1 kpc radius. Combining these new data with the sample of LBGs at z ~ 3, we conclude that these three galaxies exhibit different physical properties in terms of SFRs, abundance and mass-to-light ratios, and reddening. High-redshift galaxies of different luminosities could thus have quite different star formation histories (abridged version).

astro-ph↗

arXiv2002

Physical Properties of Low-Luminosity Galaxies at z ~ 2

We report the results obtained from ISAAC/VLT near-IR spectroscopy of two low-luminosity z ~ 1.9 galaxies located in the core of the lensing cluster AC114. The amplification factor allowed to obtain, for the first time, physical properties (SFR, abundance ratios, mass, etc) of star-forming galaxies, 1 to 2 magnitudes fainter than in previous studies of Lyman-break galaxies at z ~ 3.

astro-ph↗

arXiv2002

Chemical evolution of starburst galaxies: How does star formation proceed?

We compute chemical evolution models to constrain the mode and the history of star formation in starburst galaxies as a whole, i.e. over a large range of mass and metallicity. To this end, we investigate the origin of the dispersion observed in the evolution of both nitrogen-to-oxygen abundance ratio and galaxy luminosity as a function of metallicity for a large sample of starburst galaxies. We find that the variation of the star formation efficiency, in the framework of continuous star formation models, produce a scatter equivalent to what is observed in the N/O versus O/H diagram for low-mass HII galaxies only. However, continous star formation models are unable to reproduce i) the scatter observed for massive starburst and UV-selected galaxies in the N/O versus O/H relation, and ii) the scatter in the luminosity versus O/H scaling relation observed for the whole sample of starburst galaxies. The dispersion associated with the distribution of N/O as a function of metallicity, for both low-mass and massive galaxies, is well explained in the framework of bursting star formation models. It is interpreted as a consequence of the time-delay between the ejection of nitrogen and that of oxygen into the ISM. These models also reproduce the spread observed in the luminosity-metallicity relation. Metal-rich spiral galaxies differ from metal-poor ones by a higher star formation efficiency and starburst frequency. Low-mass galaxies experienced a few bursts of star formation whereas massive spiral galaxies experienced numerous and extended powerful starbursts (abridged version).

astro-ph↗

arXiv2001

Chemical abundances in an UV-selected sample of galaxies

We discuss the chemical properties of a sample of UV-selected intermediate-redshift (0 < z < 0.4) galaxies in the context of their physical nature and star formation history. This work represents an extension of our previous studies (Treyer et al. 1998; Sullivan et al. 2000, 2001) on UV-selected galaxies. We revisit the optical spectra of these galaxies and perform further emission-line measurements restricting the analysis to those spectra with the full set of emission lines required to derive chemical abundances. Our final sample consists of 68 galaxies with heavy element abundance ratios and both UV and CCD B-band photometry. Diagnostics based on emission-line ratios show that all but one of the galaxies in our sample are powered by hot, young stars rather than by an AGN. Oxygen-to-hydrogen (O/H) and nitrogen-to-oxygen (N/O) abundance ratios are compared to those for various local and intermediate-redshift samples. Our UV-selected galaxies span a wide range of oxygen abundances, from 0.1 Zsun to solar, intermediate between low-mass HII galaxies and massive starburst nuclei. For a given oxygen abundance, most have strikingly low N/O values. Moreover, UV-selected and HII galaxies systematically deviate from the usual metallicity-luminosity relation in the sense of being more luminous by 2-3 magnitudes. Adopting the "delayed-release" chemical evolution model, we propose our UV-selected sources are observed at a special stage in their evolution, following a powerful starburst which enriched their ISM in oxygen and temporarily lowered their mass-to-light ratios. We discuss briefly the implications of our conclusions on the nature of similarly-selected high-redshift galaxies.

astro-ph↗

arXiv2001

Chemical Properties of Starburst Galaxies Near and Far: Clues to Galaxy Evolution

The determination of chemical abundances in star-forming galaxies and the study of their evolution on cosmological timescales are powerful tools for understanding galaxy formation and evolution. This contribution presents the latest results in this domain. We show that detailed studies of chemical abundances in UV-selected, HII and starburst nucleus galaxies, together with the development of new chemical evolution models, put strong constraints on the evolutionary stage of these objects in terms of star formation history. Finally, we summarize our current knowledge on the chemical properties of intermediate- and high-redshift galaxies. Although the samples are still too small for statistical studies, these results give insight into the nature and evolution of distant star-forming objects and their link with present-day galaxies.

astro-ph↗

arXiv2000

Starbursts in barred spiral galaxies IV: On young bars and the formation of abundance gradients

The oxygen (O/H) and N/O abundance ratios along the bar of 16 barred spiral starburst galaxies are determined using long-slit spectroscopy. The abundance gradients and the spatial distribution of the ionized gas along the bar are used to understand the role of bars in starburst galaxies. The oxygen abundance gradients are steeper than in normal barred galaxies, while the intersects are low. This excludes the possibility that these are chemically evolved galaxies rejuvenated by the effect of a bar. The nitrogen-to-oxygen abundance gradients are flatter than the oxygen ones. But N/O intersects are high, which rules out the possibility that a large quantity of gas was recently funneled by a bar toward the center of a young galaxy. Bars, therefore, cannot be at the origin of the bursts in the nuclei of our sample galaxies. The oxygen and N/O abundance gradients are generally stronger in the bar than in the disk and are linked together by a linear relation. This can be fully explained in terms of star formation history: the gradients build up from the inside out, becoming stronger as the oxygen and N/O abundances increase in the bulge while staying low in the disk. In many of the sample galaxies, star formation occurs at one or both ends of the bar. The low level of chemical enrichment in these regions suggests that they recently experienced bar-triggered star formation: this is the only visible effect of bars. Our analysis shows that bars probably appeared very recently (a few 10^7 years) in the starburst galaxies, which are relatively "young" galaxies still in the process of formation.(Abridged)

astro-ph↗

arXiv2000

The formation of bars and disks in Markarian starburst galaxies

We have proposed in a companion paper (Considere et al. 2000) that bars appeared recently in massive starburst nucleus galaxies. We now test this hypothesis on an extended sample of barred and unbarred Markarian starburst galaxies, using several samples of normal galaxies as control samples. In support of this hypothesis, we show that the proportion of barred galaxies is much lower in Markarian starburst galaxies than in normal galaxies. In addition to this deficiency of bars, we find that Markarian starburst galaxies have smaller disks than normal galaxies, and that the disks of unbarred starburst galaxies are smaller, on average, than barred ones. Finally, we show that the Markarian starburst galaxies do not seem to follow the local Tully--Fisher relation. Various alternatives are examined to explain the deficiency of bars and the small disk dimensions in Markarian starburst galaxies. One possibility, which is in agreement with the young bar hypothesis, is that the formation of disks happens after the formation of bulges and that bars appear only later, when enough gas has been accreted in the disk.

astro-ph↗

arXiv1999

The Chemical Evolution of Galaxies by Successive Starbursts

We propose an evolutionary scenario by successive bursts of star formation to reproduce the chemical properties of massive nearby Starburst Nucleus Galaxies (SBNGs). The N/O abundance ratios in SBNGs are 0.2 dex higher than in normal HII regions observed in the disks of late-type spirals. The variation of the N/O ratio as a function of metallicity follows a "primary + secondary" relation, but the increase of nitrogen does not appear as a continuous process. Assuming that nitrogen is produced by intermediate-mass stars, we show that our observations are consistent with a model where the bulk of nitrogen in SBNGs was formed during past sequences of bursts of star formation which probably started 2 or 3 Gyrs in the past.

astro-ph↗

arXiv1999

The abundance of nitrogen in starburst nucleus galaxies

We show that the excess of nitrogen emission observed in a large sample of Starburst Nucleus Galaxies (SBNGs) can only be explained at a given metallicity by an overabundance of nitrogen with respect to normal HII regions in the disks of late-type spirals. The N/O ratios in the SBNGs are comparable to the values found in the bulges of normal early-type spirals, which suggests that what we observe could be the main production of nitrogen in the bulges of these galaxies. The variation of the N/O ratio as a function of metallicity in SBNGs follows a primary + secondary relation, but the increase of nitrogen does not appear as a continuous process. In SBNGs, nitrogen is probably produced by different populations of intermediate-mass stars, which were formed during past sequences of bursts of star formation. This assumption pushes the origin of the main bursts 2-3 Gyrs back in the past. On a cosmological scale, this time interval corresponds to redshifts z = 0.2-0.3, where a significant increase of star formation activity occurred. The origin of the SBNG phenomenon would thus have cosmological implications, it would be related to a more active phase of star formation in the Universe sometime in its recent past.

astro-ph↗

arXiv1999

Starbursts in barred spiral galaxies. V. Morphological analysis of bars

We have measured the bar lengths and widths of 125 barred galaxies observed with CCDs. The dependence of bar strength (identified with bar axis ratio) on morphological type, nuclear activity, central and mid-bar surface brightness is investigated. The properties of the bars are best explained if the sample is divided into early- (< SBbc) and late-type galaxies, and into active (starburst, Seyfert or LINER) and normal galaxies. We find that galaxies with very long bars are mostly active and that normal late-type galaxies have a distinct behavior from the three other groups of galaxies. We confirm earlier findings that active late-type galaxies tend to have both stronger and longer bars than normal ones. An important result of this paper is that early-type galaxies do not share this behavior : they all tend to have strong bars, whether they are active or not. We also find correlations between bar strength and relative surface brightness in the middle and at the edge of the bar, which are not followed by normal late-type galaxies. These results are interpreted in the light of recent numerical simulations and paradigms about galaxy evolution. They suggest that normal late-type galaxies represent the first stage of galaxy evolution, and that bars in early- and late-type galaxies do not have the same properties because they have a different origin.

astro-ph↗

arXiv1998

The PDS starburst galaxies

(Abridged) We discuss the nature of the galaxies found in the Pico dos Dias Survey (PDS) for young stellar objects. The PDS galaxies were selected from the IRAS Point Source catalog. They have flux density of moderate or high quality at 12, 25 and 60 $μ$m and spectral indices in the ranges $-3.00 \leq α(25,12) \leq +0.35$ and $-2.50 \leq α(60,25) \leq +0.85$. These criteria allowed the detection of 382 galaxies, which are a mixture of starburst and Seyfert galaxies. The starburst galaxies show an excess of FIR luminosity and their IRAS colors are significantly different from those of Seyfert galaxies -- 99% of the starburst galaxies in our sample have a spectral index $α(60,25) < -1.9$. As opposed to Seyfert galaxies, very few PDS starbursts are detected in X-rays. In the infrared, the starburst galaxies form a continuous sequence with normal galaxies. But they generally can be distinguished from normal galaxies by their spectral index $α(60,25) > -2.5$. This color cut--off also marks a change in the dominant morphologies of the galaxies: the normal IRAS galaxies are preferentially late--type spirals (Sb and later), while the starbursts are more numerous among early--type spirals (earlier than Sbc). No difference is found between the starbursts detected in the FIR and those detected on the basis of UV excess. The PDS starburst galaxies represent the FIR luminous branch of the UV-bright starburst nucleus galaxies, with mean FIR luminosity $\log({\rm L}_{\rm IR}/{\rm L}_\odot) = 10.3 \pm 0.5$ and redshifts smaller than 0.1. They form a complete sample limited in flux in the FIR at $2\times10^{-10}$ erg cm$^{-2}$ s$^{-1}$.

astro-ph↗

arXiv1997

Starbursts in Barred Spiral Galaxies. III. Definition of a homogeneous sample of Starburst Nucleus Galaxies

This paper presents optical long-slit spectroscopic observations of 105 barred Markarian IRAS galaxies. These observations are used to determine the spectral type of emission-line regions in the nucleus and along the bar of the galaxies, in order to define a homogeneous sample of Starburst Nucleus Galaxies (SBNGs). Our selection criteria have been very efficient for selecting star-forming galaxies, since our sample of 221 emission-line regions includes 82% nuclear or extranuclear starbursts. The contamination by Seyferts is low (9%). The remaining galaxies (9%) are objects with ambiguous classification (HII or LINER). The dust content and Halpha luminosity increase towards the nuclei of the galaxies. No significant variation of the electron density is found between nuclear and bar HII regions. However, the mean Halpha luminosity and electron density in the bar are higher than in typical disk HII regions. We investigate different mechanisms for explaining the excess of nitrogen emission observed in our starburst nuclei. There is no evidence for the presence of a weak hidden active galactic nucleus in our starburst galaxies. The cause of this excess is probably a selective enrichment of nitrogen in the nuclei of the galaxies, following a succession of short and intense bursts of star formation. Our sample of SBNGs, located at a mean redshift of 0.015, has moderate Halpha (10^41 erg/s) and far infrared (10^10 Lsun) luminosities. The types are distributed equally among early- and late-type giant spirals with a slight preference for Sbc/Sc types because of their barred morphology. The majority (62%) of SBNGs are isolated with no sign of gravitational interaction. In terms of distance, luminosity and level of interaction, SBNGs are intermediate between HII galaxies and luminous infrared galaxies.

astro-ph↗

arXiv1997

The Chemical Evolution of Starburst Nucleus Galaxies

The metallicities derived from spectroscopic observations of a sample of Starburst Nucleus Galaxies (SBNGs) are compared to those of several other types of galaxies (normal giant galaxies, Irregular and HII galaxies) drawn from the literature. The SBNGs are deficient in metals with respect to normal galaxies of same morphological type, suggesting that - SBNGs are galaxies still in the process of formation. Breaking the SBNGs into early-types (Sb and earlier) and late-types reveals that the former seem to follow the same linear luminosity-metallicity relation as the irregular and elliptical galaxies, whereas the latter and the giant spirals show comparable (0.2 and 0.3 dex) excess abundances with respect to the linear relation. This difference between the two types of SBNGs is consistent with the predictions of the model of hierarchical formation of galaxies: the early-type SBNGs are building their bulges by successive mergers of small stellar and gaseous systems, while the late-type SBNGs are mostly accreting gas to form a disk.

astro-ph↗

arXiv1997

Starbursts in Barred Spiral Galaxies. II. Molecular and Optical Study of Three Wolf-Rayet Galaxies

We have searched for dense molecular gas in three barred spiral galaxies with young starbursts, NGC 3049, 5430 and 6764, which are known Wolf-Rayet galaxies. We detected HCN in the latter two, and CS was marginally detected in NGC 6764. The dense molecular gas contents of the three galaxies are compared to those of other galaxies and to other indicators of star formation. The HCN luminosities (relative to the CO and far infrared ones) in these galaxies with very young starbursts are consistent with those observed in galaxies with older starbursts and in normal galaxies, and so are our upper limits to the CS intensities (relative to CO). The starburst ages evaluated from our spectrophotometric observations are in the range 3.4 to 6.0 Myr. A circum-nuclear ring is apparent on our images of NGC 5430, the galaxy with the oldest central starburst; this galaxy also has the widest molecular lines. The central star formation rates derived from the Halpha luminosity are consistent with those expected from the global FIR luminosities, and are correlated with the HCN luminosities. Finally, an independent estimate of the H_2 column density is obtained by optical spectrophotometry; it leads to a H_2 column density to CO intensity ratio which is about 2 to 3 times lower than the standard value, because the CO intensities of the three galaxies are higher than average, relative to their far infrared fluxes.

astro-ph↗

arXiv1996

A Correlation Between CO Linewidth and Starburst Age in Barred Spiral Galaxies

New CO(1-0) and CO(2-1) profiles complemented by data from the literature are used to obtain CO linewidths for 29 barred spiral galaxies with young nuclear starbursts. The ages of the starbursts were estimated from optical spectroscopy and recent evolutionary synthesis models. The CO linewidths and the starburst ages are correlated : galaxies with young (4-6 Myr) starbursts display narrow (< 100 km/s) CO line while those with older starbursts show broader CO lines. We discuss several scenarios of the gas dynamics during the nuclear starbursts' evolution to interpret the correlation.

astro-ph↗

arXiv1995

Comparison of the dense molecular gas in the LINER galaxy NGC 6764 and in the Wolf-Rayet galaxy NGC 5430

We compare the dense molecular gas content in two barred spiral galaxies, NGC 6764 (classified LINER) and NGC 5430 (a Wolf-Rayet galaxy). We find a significant difference in the proportion of dense molecular gas between the two galaxies. CS(3-2) is detected in NGC 6764 but not in NGC 5430, even though the intensities of CO(2-1) and HCN(1-0) are higher in the latter galaxy. The non detection in NGC 5430 indicates that the CS abundance in that galaxy is unusually low, or that HCN is subthermally excited. To complement these observations, we discuss the ionization source in the nucleus of the LINER galaxy NGC 6764.

astro-ph↗