arXiv subjects

F. Combes

Publications and source records attributed to F. Combes.

At least 487 records · Page 27Linked to original sources

arXiv2000

Molecular Gas in Galaxies

Knowledge of the molecular component of the ISM is fundamental to understand star formation. The H2 component appears to dominate the gas mass in the inner parts of galaxies, while the HI component dominates in the outer parts. Observation of the CO and other lines in normal and starburst galaxies have questioned the CO-to-H2 conversion factor, and detection of CO in dwarfs have shown how sensitive the conversion f actor is to metallicity. Our knowledge has made great progress in recent years, because of sensitivity and spatial resolution improvements. Large-scale CO maps of nearby galaxies are now available, which extend our knowledge on global properties, radial gradients, and spiral structure of the molecular ISM. Millimetric interferometers reveal high velocity gradients in galaxy nuclei, and formation of embedded structures, like bars within bars. Galaxy interactions are very effective to enhance gas concentrations and trigger starbursts. Nuclear disks or rings are frequently observed, that concentrate the star formation activity. Since the density of starbursting galaxies is strongly increasing with redshift, the CO lines and the mm dust emission are a privileged tool to follow evolution of galaxies and observe the ISM dynamics at high redshift: they could give an answer about the debated question of the star-formation history, since many massive remote starbursts could be dust-enshrouded.

astro-ph↗

arXiv2000

N-body simulations of self-gravitating gas in stationary fragmented state

The interstellar medium is observed in a hierarchical fractal structure over several orders of magnitude in scale. Aiming to understand the origin of this structure, we carry out numerical simulations of molecular cloud fragmentation, taking into account self-gravity, dissipation and energy input. Self-gravity is computed through a tree code, with fully or quasi periodic boundary conditions. Energy dissipation is introduced through cloud-cloud ineslatic collisions. Several schemes are tested for the energy input. It appears that energy input from galactic shear allows to achieve a stationary clumped state for the gas, avoiding final collapse. When a stationary turbulent cascade is established, it is possible to derive meaningful statistical studies on the data such as the fractal dimension of the mass distribution.

astro-ph↗

arXiv2000

Tidal Tails Around 20 Galactic Globular Cluster: Observational Evidence for Gravitational Disk/Bulge Shocking

Large-field multi-color images of 20 galactic globular clusters are used to investigate the presence of tidal tails around these stellar systems. Field and cluster stars are sorted with the help of color-magnitude diagrams, and star-count analysis is performed on the selected cluster stars in order to increase the signal-to-noise ratio of their surface density. We study the overdensities of these stars using the wavelet transform of the star counts in order to filter the background density noise and to detect the weak structures, at large scale, formed by the numerous stars previously members of the clusters. We associate these stellar overdensities with the stars evaporated from the clusters because of dynamical relaxation and/or tidal stripping from the clusters by the galactic gravitational field. Most of the globular clusters in our sample display strong evidence of tidal interactions with the galactic plane in the form of large and extended deformations. These tidal tails exhibit projected directions preferentially towards the galactic center. All the clusters observed, which do not suffer from strong observational biases, present such tidal tails, tracing their dynamical evolution (evaporation, tidal shocking, tidal torquing, and bulge shocking) in the Galaxy. The clusters exhibit different regimes of mass loss rate, detected using the radial density slope in the outer parts of the clusters. For NGC 5139 ($ω$ Centauri), we estimate, taking into account the possible presence of mass segregation in its outer parts, that about 0.6 to 1% of its mass has been lost during the crossing (Abridged)

astro-ph↗

arXiv2000

First Detection of Molecular Gas in the Shells of CenA

Shells are faint arc-like stellar structures, which have been observed around early type galaxies and are thought to be the result of an interaction. HI gas has recently been detected in shells, a surprising result in view of the theoretical predictions that most of the gas should decouple from stars and fall into the nucleus in such interactions. Here we report the first detection of molecular gas (CO) in shells, found 15kpc away from the center of NGC5128 (CenA), a giant elliptical galaxy that harbors an active nucleus (AGN). The ratio between CO and HI emission in the shells is the same as that found in the central regions, which is unexpected given the metallicity gradient usually observed in galaxies. We propose that the dynamics of the gas can be understood within the standard picture of shell formation if one takes into account that the interstellar medium is clumpy and hence not highly dissipative. The observed metal enrichment could be due to star formation induced by the AGN jet in the shells. Furthermore our observations provide evidence that molecular gas in mergers may be spread out far from the nuclear regions.

astro-ph↗

arXiv2000

Search for molecular gas in HVCs through HCO+ absorption

High-Velocity Clouds (HVCs) have radial velocities that cannot be explained by the global Galactic rotation; their distances remain mostly unknown, and their true nature and origin are still a mystery. Some of them could be of galactic origin, or belong to tidal streams drawn by the Milky-Way/ Magellanic Clouds interaction, or could even be intergalactic clouds infalling onto the Local Group. In the latter hypothesis, they play a major role in the hierarchical formation scenario of the Milky-Way and are connected to the Lyman-limit absorption systems. In any case, the determination of their physical state (density, temperature, internal structure, abundances, excitation) will help to discriminate between current theories on their origin and nature. A recent UV measurement (Richter et al 1999) has discovered for the first time in a HVC the molecular phase that was previously searched for, without success, through CO emission. Previous non detections could be due either to metallicity problems, or insufficient excitation (because of low density). Low-excitation molecular gas may, however, be detectable though absorption. Here we report on a sensitive search for HCO+(1-0) absorption lines in front of 27 quasars, already known to be strong millimetric continuum sources. Except for one tentative case, no detection was obtained in most HVCs, although HCO+(1-0) was clearly detected towards galactic low-velocity clouds. We discuss the implications of this result.

astro-ph↗

arXiv1999

H2 in Galaxies

The bulk of the molecular gas in spiral galaxies is under the form of cold H2, that does not radiate and is only suspected through tracer molecules, such as CO. All tracers are biased, and in particular H2 could be highly underestimated in low metallicity regions. Our knowledge is reviewed of the H2 content of galaxies, according to their types, environment, or star-forming activities. The HI and CO components are generally well-mixed (spiral arms, vertical distribution), although their radial distributions are radically different, certainly due to radial abundance gradients. The hypothesis of H2 as dark matter is discussed, as well as the implications on galaxy dynamics, or the best perspectives for observational tests.

astro-ph↗

arXiv1999

N-body simulations of globular cluster tides

We present N-body simulations of globular clusters, in orbits around the Galaxy, in order to study quantitatively and geometrically the tidal effects they encounter. The clusters are modelised with multi-mass King-Michie models (Michie 1963), including mass segregation at initial conditions. The Galaxy is modelled as realistic as possible, with three components: bulge, disk and dark halo. The main finding is that there exist two giant tidal tails around the globuler cluster in permanence along its orbit, whatever this orbit. The length of these tails is of the order of 5 tidal radii, or greater. The escaped stars are distributed radially as a power law in density, with a slope of -4. The tails present substructures, or clumps, that are the relics of the strongest shocks. Due to the compressive disk-shocking, the clusters display a prolate shape which major axis is precessing around the z axis. The tails are preferentially formed by the lowest mass stars, as expected, so that the tidal truncation increases mass segregation. Internal rotation of the cluster increases the mass loss. The flattening of dark matter cannot influence significantly the dynamics of the clusters. The orientation and the strength of the tidal tails are signatures of the last disk crossing, so that observed tidal tails can constrain strongly the cluster orbit and the galactic model (vertical scale of the disc).

astro-ph↗

arXiv1999

Single CO peak in the double bar galaxy NGC 5850

NGC 5850 is a prototype of double-barred galaxy (Friedli et al. 1996) classified as SBb(sr) I-II (Higdon et al. 1998; Prieto et al. 1997). This kind of system is primordial to understand the physical mechanism responsible for feeding galaxy nuclei and boost the star formation rate. The CO(1-0) emission has been mapped in NGC 5850, i) in the very center, using the IRAM Plateau de Bure interferometer, to reach a 2.4\arcsec$\times$1.5\arcsec\ (PA$=$$-165^\circ$) spatial resolution, and ii) in the primary bar with the IRAM-30m telescope, with a 22\arcsec beam. We have found CO emission in the center of NGC 5850, located in a single peak on the northern part of the nuclear ring. The high velocity dispersion of the molecular gas may prevent star formation in that region. Gas simulations performed with a single bar pattern and without the tidal influence of the companion NGC 5846 are unable to reproduce the features observed in NGC 5850 (Combes, Leon, Friedli, 1999, in preparation). The decoupling of a second bar appears necessary. The presence of the single molecular peak could be due to an $m=1$ mode excited by the massive companion.

astro-ph↗

arXiv1999

Time Delay of PKS1830-211 Using Molecular Absorption Lines

The use of molecular absorption lines in deriving the timde delay in PKS1830-211 is described, as well as results from a three year monitoring campaign. The time delay and the implied value for the Hubble constant are presented.

astro-ph↗

arXiv1999

Gas and Dust in Protogalaxies

The study of high-redshift objects is rapidly developing, allowing to build the star formation history of the Universe. Since most of the flux from starbursts comes out in the FIR region, the submm and mm are privileged domains for the exploration at high z. I review the recent work on galaxies at high redshift in this wavelength region, for the continuum as well as for the line detection (dust and molecular gas). Perspectives are discussed to detect early objects (maybe protogalaxies) with the future large millimeter instruments.

astro-ph↗

arXiv1999

Formation of Gaseous Shells

HI observations have revealed in several shell galaxies the presence of gaseous shells slightly displaced from the stellar shells radially, in the outward direction. We propose a mechanism to form this gaseous shells, based on the well-known phase-wrapping process of the companion matter in a merger, with nearly radial orbits. The mechanism relies on the existence of a clumpy interstellar matter, and on dynamical friction experienced by the companion core.

astro-ph↗

arXiv1999

Bar-driven Galaxy Evolution and Time-scales to Feed AGN

Recent progress in the understanding of the role of bars and gravitational instabilities in galaxy disks is reviewed. It has been proposed that bars can produce mass transfer towards the center, and progressively metamorphose late-type galaxies in early-types, along the Hubble sequence. Through this mass transfer, bars are self-destroyed, and can act only during a certain "duty-cycle" in the galaxy life. After sufficient gas infall, another bar-phase can spontaneously occur. This recurrent evolution is strongly dependent on environment. A scenario is proposed, based on N-body simulations time-scales of the bar-life events, to explain the observed bar frequency, gas mass fraction, bulge and possible black hole mass growth, in a typical spiral.

astro-ph↗

arXiv1999

Small Group Dynamics and Extended Gas

Interactions between galaxies have spectacular effects on gas dynamics, and small galaxy groups are a privileged place to investigate them. In particular, they could test the existence of cold H2 gas as dark matter in the outer parts of galaxies. HI observations have revealed that galaxies in small groups are deficient in atomic gas, like in richer galaxy clusters such as Virgo, although in a lesser extent. Galaxy interactions could be the cause of this deficiency, stripping the gas out of galaxies and enriching the inter-cluster medium (ICM) in hot gas, which in turn strips gas through ram pressure. Alternatively, the gas present at the formation of the group could have been heated to its virial temperature, and be observed now as X-rays. The dynamical processes related to this extended gas in small galaxy groups are reviewed.

astro-ph↗

arXiv1999

HI Spatial Distribution in the Galaxy NGC 3783

We have mapped the emission from atomic hydrogen at 21 cm from the galaxy NGC 3783 with the Australia Telescope Compact Array. Our main results are: a) the HI morphology is irregular and perturbed, gathered in three blobs apparently unrelated to the optical morphology; b) the observed HI velocity distribution indicates a normal disk in differential rotation with a constant velocity out to a radius of 160'' (30 kpc), c) the inclination of the disk is about 25 deg with the kinematic major axis at a position angle slightly different from that of the stellar bar, d) the HI mass inside a radius of 18'' is only 2.1 10^7 Msun, the total HI mass within 180'' is 1.1 10^9 Msun and the dynamical mass is 2 10^{11} Msun. The bulk of the gas in NGC 3783 is outside the diameter of the stellar bar; e) Numerical simulations of the gas flow in the barred potential derived from the red image indicate that the pattern speed is Omega_p = 38 km/s/kpc: the ring of Halpha emitting regions encircling the bar would then correspond to UHR, and the Halpha accumulation in the center to a nuclear ring. Various possibilities are discussed to account for the active nucleus fuelling.

astro-ph↗

arXiv1999

Astrophysical Fractals: Interstellar Medium and Galaxies

The interstellar medium is structured as a hierachy of gas clouds, that looks self-similar over 6 orders of magnitude in scales and 9 in masses. This is one of the more extended fractal in the Universe. At even larger scales, the ensemble of galaxies looks also self-similar over a certain ranges of scales, but more limited, may be over 3-4 orders of magnitude in scales. These two fractals appear to be characterized by similar Hausdorff dimensions, between 1.6 and 2. The various interpretations of these structures are discussed, in particular formation theories based on turbulence and self-gravity. In the latter, the fractal ensembles are considered in a critical state, as in second order phase transitions, when large density fluctuations are observed, that also obey scaling laws, and look self-similar over an extended range.

astro-ph↗

arXiv1999

Spiral Galaxies and Tracers of Mass Accretion

Can the present dynamics of spiral galaxies tell us something about the merging history, the formation and evolution of disks? Galaxy interactions thicken or destroy disks; the simultaneous presence of thick and thin disks is a tracer of past accretion, and also of recent disk re-formation. Observation of a large number of counter-rotating disks is also evidence of past mergers, as well as the frequency of polar-ring galaxies. Finally, the ubiquitous presence of warps in the outer parts of HI disks might also provide a clue of how frequently disks accrete mass with different angular momentum.

astro-ph↗

arXiv1999

Bulge Formation

The currently discussed theories of bulge formation are reviewed, including the primordial scenario, where bulges form rapidly and then accrete disks, the secular scenario, where bulges are formed by dynamical evolution of disks through bars and galaxy interactions, and some combinations of both, where formation of bulges and disks are more continuous and interleaved. The various scenarios make specific predictions about the relative masses, angular momenta, colours, metallicities of bulges relative to disks, and the bulge-to-disk ratio as a function of time. Dynamical processes relevant to the formation of bulges (bar instabilities, mergers) are described and tested against observed statistics. Current data suggest a dynamical feedback from gravitational instabilities in bulge and disk formation. It is very difficult to discriminate between the various scenarios from surveys at z=0 only, and observations at high redshift are presently the best hope for large progress.

astro-ph↗

arXiv1999

CO lines in high redshift galaxies: perspective for future mm instruments

Nearly 10 high redshift (z>2) starburst galaxies have recently been detected in the CO lines, revealing the early presence in the universe of objects with large amounts of already-enriched molecular gas. The latter has sufficient density to be excited in the high-level rotational CO lines, which yield more flux, making easier high-redshift detections; however the effect is not as strong as for the sub-millimeter and far-infrared dust continuum emission. With the help of simple galaxy models, based on these first detections, we estimate the flux in all CO lines expected for such starbursting objects at various redshifts. We discuss the detection perspectives with the future millimeter instruments.

astro-ph↗