arXiv subjects

F. Combes

Publications and source records attributed to F. Combes.

At least 505 records · Page 28Linked to original sources

arXiv1999

Molecular gas and the dynamics of galaxies

In this review, I discuss some highlights of recent research on molecular gas in galaxies; large-scale CO maps of nearby galaxies are being made, which extend our knowledge on global properties, radial gradients, and spiral structure of the molecular ISM. Very high resolution are provided by the interferometers, that reveal high velocity gradients in galaxy nuclei, and formation of embedded structures, like bars within bars. Observation of the CO and other lines in starburst galaxies have questioned the H2-to-CO conversion factor. Surveys of dwarfs have shown how the conversion factor depends on metallicity. The molecular content is not deficient in galaxy clusters, as is the atomic gas. 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 are a privileged tool to follow evolution of galaxies and observe the ISM dynamics at high redshift: due to the high excitation of the molecular gas, the stronger high-$J$ CO lines are redshifted into the observable band, which facilitates the detection.

astro-ph↗

arXiv1999

CMB and Molecules at High Redshift

It becomes possible now to detect cold molecules at high redshift in the millimeter domain. Since the first discovery in 1992 by Brown and van den Bout of CO lines at z=2.28 in a gravitationally lensed starburst galaxy, nearly ten objects are now known to possess large quantities of molecular gas beyond z=1 and up to z = 5, through millimeter and sub-millimeter emission lines. The continuum dust emission is the most easily detected: in the mm domain, the emission is stronger for the more redshifted objects. For the CO lines, the situation is less favorable, and the reported detections are helped by gravitational amplification. The increase of the CMB temperature T_{bg} with redshift helps the rotational line excitation (especially at high z), but not its detection. Absorption in front of quasars is a more sensitive probe of cold gas at high redshift, able to detect individual clouds of a few solar masses (instead of 10^{10} Mo for emission). From the diffuse components, one can measure the cosmic black body temperature as a function of redshift. The high column densities component allow to observe important molecules not observable from the ground, like O2, H2O and LiH for example.

astro-ph↗

arXiv1999

Interferometer 12CO observations of the box-shaped bulge spiral NGC 4013

The nucleus of the box-shaped galaxy NGC4013 has been observed with the IRAM interferometer in the J=1-0 and J=2-1 lines of 12CO. Our maps show the existence of a fast--rotating(130 km/s) molecular gas disk of radius r=110pc. Several arguments support the existence of a bar potential in NGC4013. The figure-of-eight pattern of the major axis p-v plot, the ring-like distribution of gas, and the existence of gas emission at non-circular velocities are best accounted by a bar. We have also detected gas at high z distances from the plane (z=200-300pc). The latter component is related to a system of 4 Halpha filaments of diffuse ionized gas that come out from the nucleus. The galactic fountain model seems the best to account for the Halpha and CO filaments. Although the peanut distortion can be spontaneously formed by a stellar bar in the disk, gas at high z might have been ejected after a nuclear starburst. The Halpha filaments start in the plane of the disk at r=200pc, and reach several Kpc height at r=600pc, coinciding with the maximum peanut distortion. Although a link between the bar and the box-shaped bulge in NGC4013 is suggested we find noticeable differences between the results of previous numerical simulations and the present observations. The discrepancy concerns the parameters of the bar generating the peanut. We see in NGC4013 the existence of a strong ILR region. The inclusion of a dissipative component, which remains to be thoroughly studied, may change the evolution of the stellar peanut: although in simulations the peanut appears initially near a marginal ILR, the inflow of gas driven by the bar, can make two ILRs appear.

astro-ph↗

arXiv1998

Renormalization Group Flow and Fragmentation in the Self-Gravitating Thermal Gas

The self-gravitating thermal gas (non-relativistic particles of mass m at temperature T) is exactly equivalent to a field theory with a single scalar field phi(x) and exponential self-interaction. We build up perturbation theory around a space dependent stationary point phi_0(r) in a finite size domain delta \leq r \leq R ,(delta << R), which is relevant for astrophysical applica- tions (interstellar medium,galaxy distributions).We compute the correlations of the gravitational potential (phi) and of the density and find that they scale; the latter scales as 1/r^2. A rich structure emerges in the two-point correl- tors from the phi fluctuations around phi_0(r). The n-point correlators are explicitly computed to the one-loop level.The relevant effective coupling turns out to be lambda=4 pi G m^2 / (T R). The renormalization group equations (RGE) for the n-point correlator are derived and the RG flow for the effective coupling lambda(tau) [tau = ln(R/delta), explicitly obtained.A novel dependence on tau emerges here.lambda(tau) vanishes each time tau approaches discrete values tau=tau_n = 2 pi n/sqrt7-0, n=0,1,2, ...Such RG infrared stable behavior [lambda(tau) decreasing with increasing tau] is here connected with low density self-similar fractal structures fitting one into another.For scales smaller than the points tau_n, ultraviolet unstable behaviour appears which we connect to Jeans' unstable behaviour, growing density and fragmentation. Remarkably, we get a hierarchy of scales and Jeans lengths following the geometric progression R_n=R_0 e^{2 pi n /sqrt7} = R_0 [10.749087...]^n . A hierarchy of this type is expected for non-spherical geometries,with a rate different from e^{2 n/sqrt7}.

astro-ph↗

arXiv1998

Dust and Molecules at High Redshift

In the last years, progress has been very rapid in the domain of molecules at high redshift, and we know in better detail now the molecular and dust content in several systems beyond z=1 and up to z = 5. The first discovery in 1992 by Brown and van den Bout of CO lines at z=2.28 in a gravitationally lensed starburst galaxy, strongly stimulated searches of other systems, but these were harder than foreseen, and less than 10 other systems have been discovered in CO emission. Redshifts range between 2 and 5, the largest being BR1202-0725 at z=4.69. Most of these systems, if not all, are gravitationally amplified objects. Some have been discovered first through their dust emission, relatively easy to detect because of the negative K-correction effect. The detection of all these systems could give an answer about the debated question of the star-formation rate as a function of redshift. The maximum of star-formation rate, found around z=2 from optical studies, could shift to higher z if the most remote objects are hidden by dust. Absorption in front of quasars can also probe cold gas at high redshift, taking advantage of very high spatial (milli arcsec) and spectral (30m/s) resolutions. From the diffuse components, one can measure the cosmic black body temperature as a function of redshift. All these preliminary studies will be carried out at large scales with future millimeter instruments, and some perspectives are given.

astro-ph↗

arXiv1998

The Gas Dynamics of Shell Galaxies

It is widely accepted that shell galaxies form as a result of a merger between an elliptical and a small disk galaxy. Simulations of the stellar component have shown that the shells are created either by ``phase-wrapping'' of debris on nearly radial orbits (Quinn 1984), or by ``spatial-wrapping'' of matter in thin disks (Dupraz & Combes 1987; Hernquist & Quinn 1989; Quillen etal 1993). However, recent HI observations of several shell galaxies (Schiminovich etal 1994, 1995) revealed the presence of gaseous shells displaced from the stellar ones, which questioned the validity of the ``phase-wrapping'' mechanism. Our numerical simulations indicate that these new observations can actually be accommodated within the standard picture for the formation of shell galaxies.

astro-ph↗

arXiv1998

Single CO Peak in 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 & Buta 1998, 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↗

arXiv1998

Molecular Gas in The Cartwheel Galaxy

We present the first detection of molecular gas in the Cartwheel, the prototype of a collisional ring galaxy formed in the head-on encounter of two galaxies. Until now, only very little atomic gas and no CO had been detected in the centre, where gas is theoretically expected to pile up. Using the Swedish ESO Submm Telescope, we detected both 12CO(1-0) and (2-1) line emission towards the central position. The line ratio and the line widths suggest that the CO(2--1) emission is sub-thermal and that the CO(1-0) emission arises within the central 22'' (13 kpc); it is probably associated with the inner ring and nucleus. We infer a mass of molecular gas (H2) of 1.5 to 6 10^9 M_sol, which is significantly higher than the approx. 10^8 M_sol of atomic gas within that region. The low excitation of the gas, whether it is due to a low temperature or a low density, is consistent with the weak star-forming activity observed in the centre of the Cartwheel.

astro-ph↗

arXiv1998

Time-scale for accretion of matter

Mass accretion is the key factor for evolution of galaxies. It can occur through secular evolution, when gas in the outer parts is driven inwards by dynamical instabilities, such as spirals or bars. This secular evolution proceeds very slowly when spontaneous, and can be accelerated when triggered by companions. Accretion can also occur directly through merging of small companions, or more violent interaction and coalescence. We discuss the relative importance of both processes, their time-scale and frequency along a Hubble time. Signatures of both processes can be found in the Milky Way. It is however likely that our Galaxy had already gathered the bulk of its mass about 8-10 Gyr ago, as is expected in hierarchical galaxy formation scenarios.

astro-ph↗

arXiv1998

Fractal Structures and Scaling Laws in the Universe: Statistical Mechanics of the Self-Gravitating Gas

Fractal structures are observed in the universe in two very different ways. Firstly, in the gas forming the cold interstellar medium in scales from 10^{-4} pc till 100 pc. Secondly, the galaxy distribution has been observed to be fractal in scales up to hundreds of Mpc. We give here a short review of the statistical mechanical (and field theoretical) approach developed by us. We consider a non-relativistic self-gravitating gas in thermal equilibrium at temperature T inside a volume V. The statistical mechanics of such system has special features and, as is known, the thermodynamical limit does not exist in its customary form. Moreover, the treatments through microcanonical, canonical and grand canonical ensembles yield different results.We present here for the first time the equation of state for the self-gravitating gas in the canonical ensemble. We find that it has the form p = [N T/ V] f(eta), where p is the pressure, N is the number of particles and η\equiv {G m^2 N \over V^{1/3} T}. The N \to\infty and V \to\infty limit exists keeping ηfixed. We compute the function f(η) using Monte Carlo simulations and for small eta analytically. We compute the thermodynamic quantities of the system as free energy, entropy, chemical potential, specific heat, compressibility and speed of sound. We reproduce the well-known gravitational phase transition associated to the Jeans' instability. Namely, a gaseous phase for eta < eta_c and a condensed phase for eta > eta_c. Moreover, we derive the precise behaviour of the physical quantities near the transition. In particular, the pressure vanishes as p \sim(eta_c-eta)^B with B \sim 0.2 and eta_c \sim 1.6 and the energy fluctuations diverge as \sim(eta_c-eta)^{B-1}. The speed of sound decreases monotonically and approaches the value sqrt{T/6} at the transition.

astro-ph↗

arXiv1998

Search for LiH in the ISM towards B0218+357

We report a tentative detection with the IRAM 30m telescope of the LiH molecule in absorption in front of the lensed quasar B0218+357. We have searched for the J = 0 -- 1 rotational line of lithium hydride at 444 GHz (redshifted to 263 GHz). The line, if detected, is optically thin, very narrow, and corresponds to a column density of N(LiH) = 1.6 10$^{12}$ cm$^{-2}$ for an assumed excitation temperature of 15 K, or a relative abundance LiH/H$_2 \sim$ 3 10$^{-12}$. We discuss the implications of this result.

astro-ph↗

arXiv1998

The Role of Interactions and Mergers

Galaxy evolution depends strongly on the environment. Dynamical interactions and subsequent evolution make galaxies more concentrated, with higher surface densities, and also trigger star-formation, which consumes the available cold gas. Already at z=0, a large variety of galaxy types are observed, with different evolution stages, from the unevolved gas rich dwarf irregulars, or low surface brightness galaxies, to early-type concentrated galaxies, with no remaining gas. The dynamical processes of galaxy interactions, including internal evolution, are reviewed and evidence is shown of much larger interaction/merging rate at high redshift.

astro-ph↗

arXiv1998

NGC 5907 revisited: a stellar halo formed by cannibalism?

We report on further observations of the luminous halo of NGC 5907. New V, I and B deep photometry confirms the existence of an extended stellar halo redder than the disk. Our data are consistent with a faint halo, or very thick disk, composed of a metal-rich old stellar population. We propose that it could be the remnant of a merged small elliptical, and we support our hypothesis with N-body simulations.

astro-ph↗

arXiv1998

Molecular gas in the barred spiral M100 - II. 12 CO(1-0) interferometer observations and numerical simulations

Using the IRAM interferometer we have mapped at high resolution (2"x1") the 12CO(1-0) emission in the nucleus of the doubled barred SABbc spiral M100. Molecular gas is distributed in a two spiral arm structure starting from the end points of the nuclear bar (r=600 pc) and a central source. The kinematics of the gas indicates the existence of a steep rotation curve (180 km/s at 100 pc) and strong streaming motions characteristic of a trailing spiral wave inside corotation. Gas flow simulations analyse the gas response to a gravitational potential derived from the K-band plate, including the two nested bars. We develop two families of models: first, a single pattern speed solution shared by the outer bar+spiral and by the nuclear bar, and secondly, a two independent bars solution, where the nuclear bar is dynamically decoupled and rotates faster than the primary bar. We found the best fit solution consisting of a fast pattern (160 km/s/kpc) for the nuclear bar (with corotation at 1.2 kpc) decoupled from the slow pattern of the outer bar+spiral (23 km/s/kpc, with corotation at 8-9 kpc).

astro-ph↗

arXiv1998

Fractal Dimensions and Scaling Laws in the Interstellar Medium and Galaxy Distributions: a new Field Theory Approach

We develop a field theoretical approach to the cold interstellar medium (ISM) and large structure of the universe. We show that a non-relativistic self- gravitating gas in thermal equilibrium with variable number of atoms or fragments is exactly equivalent to a field theory of a scalar field phi(x) with exponential self-interaction. We analyze this field theory perturbatively and non-perturbatively through the renormalization group(RG).We show scaling behaviour (critical) for a continuous range of the physical parameters as the temperature. We derive in this framework the scaling relation M(R) \sim R^{d_H} for the mass on a region of size R, and Delta v \sim R^\frac12(d_H -1) for the velocity dispersion. For the density-density correlations we find a power-law behaviour for large distances \sim |r_1 - r_2|^{2D - 6}.The fractal dimension D turns to be related with the critical exponent νby D = 1/ ν. Mean field theory yields ν= 1/2, D = 2. Both the Ising and the mean field values are compatible with the present ISM observational data:1.4\leq D \leq 2. We develop a field theoretical approach to the galaxy distribution considering a gas of self-gravitating masses on the FRW background, in quasi-thermal equi- librium. We show that it exhibits scaling behaviour by RG methods. The galaxy correlations are computed without assuming homogeneity. We find <ρ({\vec r_0})ρ({\vec r_0} + {\vec r}) > \sim r^{D-3} $. The theory allows to compute the three and higher density correlators without any assumption.We find that the connected N-points density scales as r_1^{N(D-3)}, when $ r_1 >> r_i, 2\leq i \leq N $. There are no free parameters in this theory.

astro-ph↗

arXiv1998

Clumpuscule Formation at High Redshift

Over the last two decades, realistic studies have often concluded that the first bound objects to form can be very small, much smaller than a solar mass. After recombination, the Jeans mass drops rapidly to the order of a Giant Molecular Cloud (GMC) mass ($\sim 10^5$ M$_\odot$), and the H$_2$ cooling can make the collapse quasi-isothermal; this leads to recursive fragmentation, and formation of clumps so dense that 3-body reactions transform the gas almost entirely to the molecular phase. This could lead to star formation in some places, but since star formation is very inefficient, most of the molecular gas could consist of a fractal built on clumpuscules thermalized with the background radiation, and filling a tiny fraction of the volume. The bulk of the gas mass can therefore be trapped in this phase, well before the first stars re-heat and re-ionize the diffuse gaseous medium}. This results in a very contrasted multi-phase baryonic medium, that has partly remained until the present time.

astro-ph↗

arXiv1998

The fractal structure of the universe : a new field theory approach

While the universe becomes more and more homogeneous at large scales, statistical analysis of galaxy catalogs have revealed a fractal structure at small-scales (λ< 100 h^{-1} Mpc), with a fractal dimension D=1.5-2 (Sylos Labini et al 1996). We study the thermodynamics of a self-gravitating system with the theory of critical phenomena and finite-size scaling and show that gravity provides a dynamical mechanism to produce this fractal structure. We develop a field theoretical approach to compute the galaxy distribution, assuming them to be in quasi-isothermal equilibrium. Only a limited, (although large), range of scales is involved, between a short-distance cut-off below which other physics intervene, and a large-distance cut-off, where the thermo- dynamic equilibrium is not satisfied. The galaxy ensemble can be considered at critical conditions, with large density fluctuations developping at any scale. From the theory of critical phenomena, we derive the two independent critical exponents nu and eta and predict the fractal dimension D = 1/nu to be either 1.585 or 2, depending on whether the long-range behaviour is governed by the Ising or the mean field fixed points, respectively. Both set of values are compatible with present observations. In addition, we predict the scaling behaviour of the gravitational potential to be r^{-(1 + eta)/2}. That is, r^{-0.5} for mean field or r^{- 0.519} for the Ising fixed point. The theory allows to compute the three and higher density correlators without any assumption or Ansatz. We find that the N-points density scales as r_1^{(N-1)(D-3)}, when r_1 >> r_i, 2 leq i leq N . There are no free parameters in this theory.

astro-ph↗

arXiv1997

Identification of molecular complexes in M81

We report about high spatial resolution observations made with the IRAM interferometer of the CO emission from a 1.1x1.1 kpc plane-of-sky field on a spiral arm of Messier 81. With a beam of 5''(90 pc), we identify 6 giant molecular cloud complexes with virial masses of about 10^6 Msun, including one associated with a giant HII region. The deduced N(H2)/I(CO) ratios are about 3 times larger on average than those measured near the solar neighborhood, suggesting that the complexes are not self-gravitationally bound except, possibly, for the complex associated with the giant HII region; they could be the average of several clouds of mass a few 10^5 Msun and diameter < 100 pc. The linewidths are very narrow with respect to the measured sizes, so that the size-linewidth relation for M 81 clouds is very different from that in the Milky Way. The narrow linewidths imply smaller virial masses than for Galactic complexes of the same size, and this is consistent with the weaker CO emission from the GMCs in M 81. We conclude from these observations that the molecular medium in M 81 differs from that in the Milky Way.

astro-ph↗