arXiv subjects

Françoise Combes

Publications and source records attributed to Françoise Combes.

66 records · Page 4Linked to original sources

arXiv2018

Radial migration in a stellar galactic disc with thick components

We study how migration affects stars of a galaxy with a thin stellar disc and thicker stellar components. The simulated galaxy has a strong bar and lasting spiral arms. We find that the amplitude of the churning (change in angular momentum) is similar for thin and thick components, and of limited amplitude, and that stars of all components can be trapped at the corotation of the bar. At the exception of those stars trapped at the corotation, we find that stars that are far from their initial guiding radius are more likely so due to blurring rather than churning effects. We compare the simulation to orbits integration with a fixed gravitational potential rotating at a constant speed. In the latter case, stars trapped at corotation are churned periodically outside and inside the corotation radius, with a zero net average. However, as the bar speed of the simulated galaxy decreases and its corotation radius increases, stars trapped at corotation for several Gyrs can be churned outwards on average. We study the location of extreme migrators (stars experimenting the largest churning) and find that extreme migrators come from regions on the leading side of the effective potential local maxima.

astro-ph.GA↗

arXiv2017

Inefficient jet-induced star formation in Centaurus A: High resolution ALMA observations of the northern filaments

NGC 5128 is one of the best targets to study AGN-feedback in the local Universe. At 13.5 kpc from the galaxy, optical filaments with recent star formation lie along the radio-jet direction. It is a testbed region for positive feedback (jet-induced star formation). APEX revealed strong CO emission in star-forming regions but also in regions with no detected tracers of star formation. When observed, star formation appears to be inefficient compared to the Kennicutt-Schmidt relation. We used ALMA to map the 12CO(1-0) emission all along the filaments at 1.3"~ 23.8 pc resolution. The CO emission is clumpy and distributed in two main structures: (i) the Horseshoe complex, outside the HI cloud, where gas is mostly excited by shocks and no star formation is observed; (ii) the Vertical filament, at the edge of the HI shell, which is a region of moderate star formation. We identified 140 molecular clouds. A statistical study reveals that they have very similar physical properties that in the inner Milky Way. However, the range of radius available with the present observations does not enable to investigate whether the clouds follow the Larson relation or not. The large virial parameter of the clouds suggests that gravity is not dominant. Finally, the total energy injection in the filaments is of the same order as in the inner part of the Milky Way. The strong CO emission detected in the filaments is an indication that the energy injected by the jet acts positively in the formation of dense molecular gas. The relatively high virial parameter of the molecular clouds suggests that the injected kinetic energy is too strong for star formation to be efficient. This is particularly the case in the Horseshoe complex where the virial parameter is the largest and where strong CO is detected with no associated star formation. This is the first evidence of inefficient AGN positive feedback.

astro-ph.GA↗

arXiv2017

From molecules to Young Stellar Clusters: the star formation cycle across the M33 disk

To shed light on the time evolution of local star formation episodes in M33, we study the association between 566 Giant Molecular Clouds (GMCs), identified through the CO (J=2-1) IRAM-all-disk survey, and 630 Young Stellar Cluster Candidates (YSCCs), selected via Spitzer-24~$μ$m emission. The spatial correlation between YSCCs and GMCs is extremely strong, with a typical separation of 17~pc, less than half the CO(2--1) beamsize, illustrating the remarkable physical link between the two populations. GMCs and YSCCs follow the HI filaments, except in the outermost regions where the survey finds fewer GMCs than YSCCs, likely due to undetected, low CO-luminosity clouds. The GMCs have masses between 2$\times 10^4$ and 2$\times 10^6$ M$_\odot$ and are classified according to different cloud evolutionary stages: inactive clouds are 32$\%$ of the total, classified clouds with embedded and exposed star formation are 16$\%$ and 52$\%$ of the total respectively. Across the regular southern spiral arm, inactive clouds are preferentially located in the inner part of the arm, possibly suggesting a triggering of star formation as the cloud crosses the arm. Some YSCCs are embedded star-forming sites while the majority have GALEX-UV and H$α$ counterparts with estimated cluster masses and ages. The distribution of the non-embedded YSCC ages peaks around 5~Myrs with only a few being as old as 8--10~Myrs. These age estimates together with the number of GMCs in the various evolutionary stages lead us to conclude that 14~Myrs is a typical lifetime of a GMC in M33, prior to cloud dispersal. The inactive and embedded phases are short, lasting about 4 and 2~Myrs respectively. This underlines that embedded YSCCs rapidly break out from the clouds and become partially visible in H$α$ or UV long before cloud dispersal.

astro-ph.GA↗

arXiv2016

PHIBSS: Exploring the Dependence of the CO-H$_2$ Conversion Factor on Total Mass Surface Density at ${\it z} < 1.5$

We present an analysis of the relationship between the CO-H$_{2}$ conversion factor ($α_{\rm CO}$) and total mass surface density ($Σ_{\rm tot}$) in star-forming galaxies at $z < 1.5$. Our sample, which is drawn from the IRAM Plateau de Bure HIgh-$z$ Blue Sequence Survey (PHIBSS) and the CO Legacy Database for GASS (COLD GASS), includes 'normal,' massive star-forming galaxies that dominate the evolution of the cosmic star formation rate (SFR) at this epoch and probe the $Σ_{\rm tot}$ regime where the strongest variation in $α_{\rm CO}$ is observed. We constrain $α_{\rm CO}$ via existing CO observations, measurements of the star formation rate, and an assumed molecular gas depletion time ($t_{\rm dep}$=$M_{\rm gas}$/SFR) --- the latter two of which establish the total molecular gas mass independent of the observed CO luminosity. For a broad range of adopted depletion times, we find that $α_{\rm CO}$ is independent of total mass surface density, with little deviation from the canonical Milky Way value. This runs contrary to a scenario in which $α_{\rm CO}$ decreases as surface density increases within the extended clouds of molecular gas that potentially fuel clumps of star formation in $z\sim1$ galaxies, similar to those observed in local ULIRGs. Instead, our results suggest that molecular gas, both at $z\sim0$ and $z\sim1$, is primarily in the form of self-gravitating molecular clouds. While CO observations suggest a factor of $\sim3$ reduction in the average molecular gas depletion time between $z \sim 0$ and $z\sim1$, we find that, for typical galaxies, the structure of molecular gas and the process of star formation at $z \sim 1$ is otherwise remarkably similar to that observed in local star-forming systems.

astro-ph.GA↗

arXiv2016

Star formation and gas flows in the centre of the NUGA galaxy NGC 1808 observed with SINFONI

NGC 1808 is a nearby barred spiral galaxy which hosts young stellar clusters in a patchy circumnuclear ring with a radius of $\sim 240\,\mathrm{pc}$. In order to study the gaseous and stellar kinematics and the star formation properties of the clusters, we perform seeing-limited $H+K$-band near-infrared integral-field spectroscopy with SINFONI of the inner $\sim 600\,\mathrm{pc}$. From the $M_\mathrm{BH}-σ_*$ relation, we find a black hole mass of a few $10^7\,M_\odot$. We estimate the age of the young stellar clusters in the circumnuclear ring to be $\lesssim 10\,\mathrm{Myr}$. No age gradient along the ring is visible. However, the starburst age is comparable to the travel time along the ring, indicating that the clusters almost completed a full orbit along the ring during their life time. In the central $\sim 600\,\mathrm{pc}$, we find a hot molecular gas mass of $\sim 730\,M_\odot$ which, with standard conversion factors, corresponds to a large cold molecular gas reservoir of several $10^8\,M_\odot$, in accordance with CO measurements from the literature. The gaseous and stellar kinematics show several deviations from pure disc motion, including a circumnuclear disc and signs of a nuclear bar potential. In addition, we confirm streaming motions on $\sim 200\,\mathrm{pc}$ scale that have recently been detected in CO(1-0) emission. Due to our enhanced angular resolution of $<1\,\mathrm{arcsec}$, we find further streaming motion within the inner arcsecond, that have not been detected until now. Despite the flow of gas towards the centre, no signs for significant AGN activity are found. This raises the questions what determines whether the infalling gas will fuel an AGN or star formation.

astro-ph.GA↗

arXiv2016

Atomic-to-molecular gas phase transition triggered by the radio jet in Centaurus A

NGC 5128 (Centaurus A) is one of the best example to study AGN-feedback in the local Universe. At 13.5 kpc from the galaxy, optical filaments with recent star formation are lying along the radio-jet direction. We used the Atacama Pathfinder EXperiment (APEX) to map the CO(2-1) emission all along the filaments structure. Molecular gas mass of 8.2x10^7 Msun was found over the 4.2 kpc-structure which represents about 3% of the total gas mass of the NGC 5128 cold gas content. Two dusty mostly molecular structures are identified, following the optical filaments. The region corresponds to the crossing of the radio jet with the northern HI shell, coming from a past galaxy merger. One filament is located at the border of the HI shell, while the other is entirely molecular, and devoid of HI gas. The molecular mass is comparable to the HI mass in the shell, suggesting a scenario where the atomic gas was shocked and transformed in molecular clouds by the radio jet. Comparison with combined FIR Herschel and UV GALEX estimation of star formation rates in the same regions leads to depletion times of more than 10 Gyr. The filaments are thus less efficient than discs in converting molecular gas into stars. Kinetic energy injection triggered by shocks all along the jet/gas interface is a possible process that appears to be consistent with MUSE line ratio diagnostics derived in a smaller region of the northern filaments. Whether the AGN is the sole origin of this energy input and what is the dominant (mechanical vs radiative) mode for this process is however still to be investigated.

astro-ph.GA↗

arXiv2016

Cold, clumpy accretion onto an active supermassive black hole

Supermassive black holes in galaxy centres can grow by the accretion of gas, liberating energy that might regulate star formation on galaxy-wide scales. The nature of the gaseous fuel reservoirs that power black hole growth is nevertheless largely unconstrained by observations, and is instead routinely simplified as a smooth, spherical inflow of very hot gas. Recent theory and simulations instead predict that accretion can be dominated by a stochastic, clumpy distribution of very cold molecular clouds - a departure from the "hot mode" accretion model - although unambiguous observational support for this prediction remains elusive. Here we report observations that reveal a cold, clumpy accretion flow towards a supermassive black hole fuel reservoir in the nucleus of the Abell 2597 Brightest Cluster Galaxy (BCG), a nearby (redshift z=0.0821) giant elliptical galaxy surrounded by a dense halo of hot plasma. Under the right conditions, thermal instabilities can precipitate from this hot gas, producing a rain of cold clouds that fall toward the galaxy's centre, sustaining star formation amid a kiloparsec-scale molecular nebula that inhabits its core. The observations show that these cold clouds also fuel black hole accretion, revealing "shadows" cast by the molecular clouds as they move inward at about 300 kilometres per second towards the active supermassive black hole in the galaxy centre, which serves as a bright backlight. Corroborating evidence from prior observations of warmer atomic gas at extremely high spatial resolution, along with simple arguments based on geometry and probability, indicate that these clouds are within the innermost hundred parsecs of the black hole, and falling closer towards it.

astro-ph.GA↗

arXiv2015

Star formation efficiency along the radio jet in Centaurus A

Centaurus A is the most nearby powerful AGN, widely studied at all wavelengths. Molecular gas has been found in the halo at a distance of ~20 kpc from the galaxy centre, associated with HI shells. The molecular gas lies inside some IR and UV bright star-forming filaments that have recently been observed in the direction of the radio jets. These archival data show that there is dust and very weak star formation on scales of hundreds of parsecs. On top of analysing combined archival data, we have performed searches of HCN(1-0) and HCO+(1-0) emission with ATCA at the interaction of the northern filaments and the HI shell of Cen A. Measuring the dense gas is another indicator of star formation efficiency inside the filaments. However, we only derived upper limits of 1.6x10^3 K.km/s.pc^2 at 3 sigma in the synthesised beam of 3.1". We also compared the CO masses with the SFR estimates in order to measure a star formation efficiency. Using a standard conversion factor leads to long depletion times (7 Gyr). We then corrected the mass estimates from metallicity effect by using gas-to-dust mass ratio as a proxy. From MUSE data, we estimated the metallicity spread (0.4-0.8 Zsun) in the filament, corresponding to gas-to-dust ratios of ~200-400. The CO/H2 conversion ratio is corrected for low metallicity by a factor between 1.4 and 3.2. Such a low-metallicity correction leads to even more massive clouds with higher depletion times (16 Gyr). We finally present ALMA observations that detect 3 unresolved CO(2-1) clumps of size <37x21 pc and masses around 10^4 Msun. The velocity width of the CO emission line is ~10 km/s, leading to a rather high virial parameter. This is a hint of a turbulent gas probably powered by kinetic energy injection from the AGN jet/wind and leading to molecular gas reservoir not forming star efficiently.

astro-ph.GA↗

arXiv2015

Ram Pressure Stripping in the Virgo Cluster

Gas can be violently stripped from their galaxy disks in rich clusters, and be dispersed over 100kpc-scale tails or plumes. Young stars have been observed in these tails, suggesting they are formed in situ. This will contribute to the intracluster light, in addition to tidal stripping of old stars. We want to quantify the efficiency of intracluster star formation. We present CO(1--0) and CO(2--1) observations, made with the IRAM-30m telescope, towards the ram-pressure stripped tail northeast of NGC4388 in Virgo. HII regions found all along the tails, together with dust patches have been targeted. We detect molecular gas in 4 positions along the tail, with masses between 7x10$^5$ to 2x10$^6$ M$_\odot$. Given the large distance from the NGC 4388 galaxy, the molecular clouds must have formed in situ, from the HI gas plume. We compute the relation between surface densities of star formation and molecular gas in these regions, and find that the star formation has very low efficiency. The corresponding depletion time of the molecular gas can be up to 500 Gyr and more. Since this value exceeds a by far Hubble time, this gas will not be converted into stars, and will stay in a gaseous phase to join the intracluster medium.

astro-ph.GA↗

arXiv2015

Far Ultraviolet Morphology of Star Forming Filaments in Cool Core Brightest Cluster Galaxies

We present a multiwavelength morphological analysis of star forming clouds and filaments in the central ($< 50$ kpc) regions of 16 low redshift ($z<0.3$) cool core brightest cluster galaxies (BCGs). New Hubble Space Telescope (HST) imaging of far ultraviolet continuum emission from young ($\sim 10$ Myr), massive ($> 5$ \Msol) stars reveals filamentary and clumpy morphologies, which we quantify by means of structural indices. The FUV data are compared with X-ray, Ly$α$, narrowband H$α$, broadband optical/IR, and radio maps, providing a high spatial resolution atlas of star formation locales relative to the ambient hot ($\sim10^{7-8}$ K) and warm ionised ($\sim 10^4$ K) gas phases, as well as the old stellar population and radio-bright AGN outflows. Nearly half of the sample possesses kpc-scale filaments that, in projection, extend toward and around radio lobes and/or X-ray cavities. These filaments may have been uplifted by the propagating jet or buoyant X-ray bubble, or may have formed {\it in situ} by cloud collapse at the interface of a radio lobe or rapid cooling in a cavity's compressed shell. The morphological diversity of nearly the entire FUV sample is reproduced by recent hydrodynamical simulations in which the AGN powers a self-regulating rain of thermally unstable star forming clouds that precipitate from the hot atmosphere. In this model, precipitation triggers where the cooling-to- freefall time ratio is $t_{\mathrm{cool}}/t_{\mathrm{ff}}\sim 10$. This condition is roughly met at the maxmial projected FUV radius for more than half of our sample, and clustering about this ratio is stronger for sources with higher star formation rates.

astro-ph.GA↗

arXiv2014

The interplay between a galactic bar and a supermassive black hole: nuclear fueling in a sub-parsec resolution galaxy simulation

We study the connection between the large-scale dynamics and the gas fueling toward a central black hole via the analysis of a Milky Way-like simulation at sub-parsec resolution. This allows us to follow a set of processes at various scales (e.g., the triggering of inward gas motion towards inner resonances via the large-scale bar, the connection to the central black hole via mini spirals) in a self-consistent manner. This simulation provides further insights on the role of shear for the inhibition of star formation within the bar in regions with significant amount of gas. We also witness the decoupling of the central gas and nuclear cluster from the large-scale disc, via interactions with the black hole. This break of symmetry in the mass distribution triggers the formation of gas clumps organised in a time-varying 250 pc ring-like structure, the black hole being offset by about 70 pc from its centre. Some clumps form stars, while most get disrupted or merge. Supernovae feedback further creates bubbles and filaments, some of the gas being expelled to 100 pc or higher above the galaxy plane. This helps remove angular momentum from the gas, which gets closer to the central dark mass. Part of the gas raining down is being accreted, forming a 10~pc polar disc-like structure around the black hole, leading to an episode of star formation. This gives rise to multiple stellar populations with significantly different angular momentum vectors, and may lead to a natural intermittence in the fueling of the black hole.

astro-ph.GA↗

arXiv2014

Local stability of a gravitating filament: a dispersion relation

Filamentary structures are ubiquitous in astrophysics and are observed at various scales. On a cosmological scale, matter is usually distributed along filaments, and filaments are also typical features of the interstellar medium. Within a cosmic filament, matter can contract and form galaxies, whereas an interstellar gas filament can clump into a series of bead-like structures which can then turn into stars. To investigate the growth of such instabilities, we derive a local dispersion relation for an idealized self-gravitating filament, and study some of its properties. Our idealized picture consists of an infinite self-gravitating and rotating cylinder with pressure and density related by a polytropic equation of state. We assume no specific density distribution, treat matter as a fluid, and use hydrodynamics to derive the linearized equations that govern the local perturbations. We obtain a dispersion relation for axisymmetric perturbations and study its properties in the (k_R, k_z) phase space, where k_R and k_z are respectively the radial and longitudinal wavenumbers. While the boundary between the stable and unstable regimes is symmetrical in k_R and k_z and analogous to the Jeans criterion, the most unstable mode displays an asymmetry that could constrain the shape of the structures that form within the filament. Here the results are applied to a fiducial interstellar filament, but could be extended for more astrophysical systems such as cosmological filaments and tidal tails.

astro-ph.CO↗