arXiv subjects

D. Pfenniger

Publications and source records attributed to D. Pfenniger.

At least 19 recordsLinked to original sources

arXiv2014

CO map and steep Kennicutt-Schmidt relation in the extended UV disk of M63

Results from the UV satellite GALEX revealed large extensions of disks in some nearby spiral galaxies, extending out to 3 to 4 times the isophotal radius, r25. M63 is a remarkable example of a spiral galaxy with one of the most extended UV disks, so it offers the opportunity to search for the molecular gas and characterize the star formation in outer disk regions as revealed by the UV emission. We obtained deep CO(1-0) and CO(2-1) observations on the IRAM 30 m telescope along the major axis of the M63 disk from the center out to the galactocentric radius rgal = 1.6 r25 and over a bright UV region at rgal = 1.36 r25. CO(1-0) is detected all along the M63 major axis out to r25, and CO(2-1) is confined to rgal = 0.68 r25, which may betray lower excitation temperatures in the outer disk. CO(1-0) is also detected in the external bright UV region of M63. The radial profiles of the CO emission and of the Halpha, 24 micron, NUV and FUV star formation tracers and HI taken from the literature show a severe drop with the galactocentric radius, such that beyond r25 they are all absent with the exception of a faint UV emission and HI. The CO emission detection in the external UV region, where the UV flux is higher than the UV flux observed beyond r25, highlights a tight correlation between the CO and UV fluxes, namely the amount of molecular gas and the intensity of star formation. This external UV region is dominated by the atomic gas, suggesting that HI is more likely the precursor of H2 rather than the product of UV photodissociation. A broken power law needs to be invoked to describe the Kennicutt-Schmidt (K-S) relation of M63 from the center of the galaxy out to rgal = 1.36 r25. While all along the major axis out to r25 the K-S relation is almost linear, in the external UV region the SFR regime is highly nonlinear and characterized by a steep K-S relation and very low star formation efficiency.

astro-ph.GA↗

arXiv2013

On the detection of point sources in CMB maps

We use the Wilkinson Microwave Anisotropy Probe 7-year data (WMAP7) to further probe point source detection technique in the sky maps of the cosmic microwave background (CMB) radiation. The method by Tegmark et al. for foreground reduced maps and the Kolmogorov parameter as the descriptor are adopted for the analysis of WMAP satellite CMB temperature data. Part of the detected points coincide with point sources already revealed by other methods. However, we have also found 2 source candidates for which still no counterparts are known, and identified 7 point sources listed in Planck Early Release Compact Source Catalogue as high reliability sources.

astro-ph.CO↗

arXiv2009

Simulations of galactic disks including an additional dark baryonic component

The near proportionality between HI and dark matter in outer galactic disks prompted us to run N-body simulations of galactic disks in which the observed gas content is supplemented by a dark gas component representing between zero and five times the visible gas content. While adding baryons in the disk of galaxies may solve some issues, it poses the problem of disk stability. We show that the global stability is ensured if the ISM is multiphased, composed of two partially coupled phases, a visible warm gas phase and a weakly collisionless cold dark phase corresponding to a fraction of the unseen baryons. The phases are subject to stellar and UV background heating and gas cooling, and their transformation into each other is studied as a function of the coupling strength. This new model, which still possesses a dark matter halo, fits the rotation curves as well as the classical CDM halos, but is the only one to explain the existence of an open and contrasting spiral structure, as observed in the outer HI disks

astro-ph.GA↗

arXiv2008

Simulations of galactic disks including a dark baryonic component

$Λ$CDM numerical simulations predict that the "missing baryons" reside in a Warm-Hot gas phase in the over-dense cosmic filaments. However, there are now several theoretical and observational arguments that support the fact that galactic disks may be more massive than usually thought, containing a substantial fraction of the "missing baryons". Hereafter, we present new N-body simulations of galactic disks, where the gas content has been multiplied by a factor 5. The stability of the disk is ensured by assuming that the ISM is composed out of two partially coupled phases, a warm phase, corresponding the observed CO and HI gas and a cold collisionless phase corresponding to the unseen baryons.

astro-ph↗

arXiv2007

Molecular gas in high-velocity clouds: revisited scenario

We report a new search for 12CO(1-0) emission in high-velocity clouds (HVCs) performed with the IRAM 30 m telescope. This search was motivated by the recent detection of cold dust emission in the HVCs of Complex C. Despite a spatial resolution which is three times better and sensitivity twice as good compared to previous studies, no CO emission is detected in the HVCs of Complex C down to a best 5 sigma limit of 0.16 K km/s at a 22'' resolution. The CO emission non-detection does not provide any evidence in favor of large amounts of molecular gas in these HVCs and hence in favor of the infrared findings. We discuss different configurations which, however, allow us to reconcile the negative CO result with the presence of molecular gas and cold dust emission. H2 column densities higher than our detection limit, N(H2) = 3x10^{19} cm^{-2}, are expected to be confined in very small and dense clumps with 20 times smaller sizes than the 0.5 pc clumps resolved in our observations according to the results obtained in cirrus clouds, and might thus still be highly diluted. As a consequence, the inter-clump gas at the 1 pc scale has a volume density lower than 20 cm^{-3} and already appears as too diffuse to excite the CO molecules. The observed physical conditions in the HVCs of Complex C also play an important role against CO emission detection. It has been shown that the CO-to-H2 conversion factor in low metallicity media is 60 times higher than at the solar metallicity, leading for a given H2 column density to a 60 times weaker integrated CO intensity. And the very low dust temperature estimated in these HVCs implies the possible presence of gas cold enough (< 20 K) to cause CO condensation onto dust grains under interstellar medium pressure conditions and thus CO depletion in gas-phase observations.

astro-ph↗

arXiv2004

Bending instabilities at the origin of persistent warps

Based on N-body simulations, we show that realistic galactic disks are subject to bending instabilities of fire-hose type when the disks are substantially self-gravitating, that is, if they contain dark matter distributed in the disk. Depending on the degree of instability, S and U-shaped, as well as asymmetric warps are generated. In some cases, the warp may last several galactic rotations, particularly when the instability is marginal. Since the bending instability is very sensitive to the disk flattening, the fractions of dark matter distributed in the disk and in the dark halo are constrained. For a Milky Way like galaxy the extended dark halo can not exceed 30-40% of the total mass within 35 kpc if the Milky Way warp results from a bending instability. This mode of warping provides a unified picture of spiral galaxies, where bars, spiral arms and warps result all from disk gravitational instabilities, radial or transverse, which are constantly regenerated by the dissipative gas component.

astro-ph↗

arXiv2004

The Baryonic Tully-Fisher relation revisited

The Baryonic Tully-Fisher relation (BTF) can be substantially improved when considering that the galactic baryonic mass is likely to consist not only from the detected baryons, stars and gas, but also from a dark baryonic component proportional to the HI gas. The BTF relation is optimally improved when the HI mass is multiplied by a factor of about 3, but larger factors up to 11-16 still improve the fit over the original one using only the detected baryons. The strength of this improved relation is quantified with up-to-date statistical tests such as the Akaike Information Criterion or the Bayesian Information Criterion. In particular they allow to show that supposing a variable $M_\star/L$ ratio instead is much less significant. This result reinforces the suggestion made in several recent works that mass within galactic disks must be a multiple of the HI mass, and that galactic disks are substantially, but not necessarily fully, self-gravitating.

astro-ph↗

arXiv2002

Possible flakes of molecular hydrogen in the early Universe

The thermochemistry of H2 and HD in non-collapsed, non-reionized primordial gas up to the end of the dark age is investigated with recent radiation-matter and chemical reaction rates taking into account the efficient coolant HD, and the possibility of a gas-solid phase transition of H2. In the standard big-bang model we find that these molecules can freeze out and lead to the growth of flakes of solid molecular hydrogen at redshifts z ~ 6-12 in the unperturbed medium and under-dense regions. While this freezing caused by the mere adiabatic cooling of the expanding matter is less likely to occur in collapsed regions due to their higher than radiation background temperature, on the other hand the super-adiabatic expansion in voids strongly favors it. Later reionization (at z ~ 5-6) eventually destroys all these H2 flakes. The possible occurrence of H2 flakes is important for the degree of coupling between matter and radiation, as well as for the existence of a gas-grain chemistry at the end of the dark age.

astro-ph↗

arXiv2001

Periodic orbits in warped disk

It is often assumed that a warped galaxy can be modeled by a set of rings. This paper verifies numerically the validity of this assumption by the study of periodic orbits populating a heavy self-gravitating warped disk. The phase space structure of a warped model reveals that the circular periodic orbits of a flat disk are transformed in quasi annular periodic orbits which conserve their stability. This lets us also explore the problem of the persistence of a large outer warp. In particular, the consistency of its orbits with the density distribution is checked as a function of the pattern speed.

astro-ph↗

arXiv1999

Formation of massive counter-rotating disks: an alternative scenario

In this contribution, we investigate the scenario of major mergers between galaxies of comparable mass to the formation of massive counter-rotating disks. Such a scenario can be ruled out in general because they produce rather ellipticals. Yet for a narrow range of initial conditions, major mergers can nevertheless produce anyway rare galaxies such as NGC 4550, and so they are viable alternatives for the formation of these counter-rotating systems. The scenario is successful in producing a remarkably axisymmetric disk possessing strongly counter-rotating populations.

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↗

arXiv1997

Making Spirals with Counter-Rotating Disks, the case of NGC 4550

A single merger scenario for making galaxies such as NGC 4550 possessing equal coplanar counter-rotating stellar disks is investigated by collisionless N-body technique. The scenario is successful in producing an axisymmetric disk made of two almost equal counter-rotating populations. The final disk shows a clear bimodal line profile in the outer part, which demonstrates that disk-disk mergers do not always produce ellipticals.

astro-ph↗

arXiv1997

ISOCAM images at 6.75 and 15 microns: the circumnuclear region of NGC 4321 (M 100)

We present ISOCAM images (1.5"/px) in the LW2 (6.75 microns) and LW3 (15 microns) broad band filters of the circumnuclear starburst of NGC 4321 (M 100). A detailed comparison with the emission at other wavelengths (U, B, Halpha, and CO) is also presented. The nature and intensity of the mid-infrared emission is shown to be remarkably different from one star forming region to the next. For instance, star forming regions at the ends of the nuclear bar may contain enshrouded Wolf-Rayet stars.

astro-ph↗

arXiv1997

First Results from ISOCAM Images of Circumnuclear Starbursts in Barred Galaxies

For a sample of ten nearby barred galaxies, images at 6.75\um and 15\um of circumnuclear starbursts are being taken by ISOCAM on board of the ISO satellite. We first recall the main goals of our project, give its current status, and finally, as an example, preliminary results for NGC 4321 (M100) are presented.

astro-ph↗

arXiv1997

Perspectives for detecting cold H2 in outer galactic disks

We review here the main direct or indirect ways to detect the possible presence of large amounts of cold molecular hydrogen in the outer parts of disk galaxies, an hypothesis that we have recently developed. Direct ways range from H2 absorption in the UV domain to detection of the radio hyperfine structure: the ortho-H2 molecule has an hyperfine, or ultrafine, structure in its fundamental state, due to the coupling between the rotation-induced magnetic moment, and the nuclear spin. This gives rise to 2 magnetic dipole transitions, at the wavelengths of 0.5 and 5.5 km. Indirect ways are essentially the detection of the HD and LiH transitions, and in some environments like clusters of galaxies, more heavy trace molecules such as CO. We discuss from this point of view the recent discovery by COBE/FIRAS of a very cold Galactic dust component (4-7 K) which could correspond to a dominating gas mass component of the ISM, if interpreted as standard dust emission. Some of the proposed means could be applied to the well-known molecular clouds, to bring some new light to the problem of the H2/CO conversion ratio.

astro-ph↗

arXiv1996

Secular Evolution of Galaxy Morphologies

Today we have numerous evidences that spirals evolve dynamically through various secular or episodic processes, such as bar formation and destruction, bulge growth and mergers, sometimes over much shorter periods than the standard galaxy age of 10-15 Gyr. This, coupled to the known properties of the Hubble sequence, leads to a unique sense of evolution: from Sm to Sa. Linking this to the known mass components provides new indications on the nature of dark matter in galaxies. The existence of large amounts of yet undetected dark gas appears as the most natural option. Bounds on the amount of dark stars can be given since their formation is mostly irreversible and requires obviously a same amount of gas.

astro-ph↗

arXiv1996

Fundamental Aspects of the ISM Fractality

The ubiquitous clumpy state of the ISM raises a fundamental and open problem of physics, which is the correct statistical treatment of systems dominated by long range interactions. A simple solvable hierarchical model is presented which explains why systems dominated by gravity prefer to adopt a fractal dimension around 2 or less, like the cold ISM and large scale structures. This has direct relation with the general transparency, or blackness, of the Universe.

astro-ph↗

arXiv1996

Very Cold Gas and Dark Matter

We have recently proposed a new candidate for baryonic dark matter: very cold molecular gas, in near-isothermal equilibrium with the cosmic background radiation at 2.73 K. The cold gas, of quasi-primordial abundances, is condensed in a fractal structure, resembling the hierarchical structure of the detected interstellar medium. We present some perspectives of detecting this very cold gas, either directly or indirectly. The H$_2$ molecule has an "ultrafine" structure, due to the interaction between the rotation-induced magnetic moment and the nuclear spins. But the lines fall in the km domain, and are very weak. The best opportunity might be the UV absorption of H$_2$ in front of quasars. The unexpected cold dust component, revealed by the COBE/FIRAS submillimetric results, could also be due to this very cold H$_2$ gas, through collision-induced radiation, or solid H$_2$ grains or snowflakes. The $γ$-ray distribution, much more radially extended than the supernovae at the origin of cosmic rays acceleration, also points towards and extended gas distribution.

astro-ph↗