Gas circulation and galaxy evolution

Galaxies must form and evolve via the acquisition of gas from the intergalactic environment, however the way this gas accretion takes place is still poorly understood. Star-forming galaxies are surrounded by multiphase halos that appear to be mostly produced by internal processes, e.g., galactic fountains. However, a small fraction of the halo gas shows features that point to an external origin. Estimates of the halo-gas accretion rate in the local Universe consistently give values much lower than what would be required to sustain star formation at the observed rate. Thus, most of the gas accretion must be "hidden" and not seen directly. I discuss possible mechanisms that can cause the intergalactic gas to cool and join the star-forming galactic disks. A possibility is that gas accretion is driven by the galactic-fountain process via turbulent mixing of the fountain gas with the coronal low-metallicity gas.Comment: 12 pages, 5 figures. Invited review at the conference "Hunting for the Dark: The Hidden Side of Galaxy Formation", Malta, 19-23 Oct. 2009. Eds. V.P. Debattista and C.C. Popescu, AIP Conf. Se

The Galactic fountain as an origin for the Smith Cloud

The recent discovery of an enriched metallicity for the Smith high-velocity HI cloud (SC) lends support to a Galactic origin for this system. We use a dynamical model of the galactic fountain to reproduce the observed properties of the SC. In our model, fountain clouds are ejected from the region of the disc spiral arms and move through the halo interacting with a pre-existing hot corona. We find that a simple model where cold gas outflows vertically from the Perseus spiral arm reproduces the kinematics and the distance of the SC, but is in disagreement with the cloud's cometary morphology, if this is produced by ram-pressure stripping by the ambient gas. To explain the cloud morphology we explore two scenarios: a) the outflow is inclined with respect to the vertical direction; b) the cloud is entrained by a fast wind that escapes an underlying superbubble. Solutions in agreement with all observational constraints can be found for both cases, the former requires outflow angles >40 deg while the latter requires >1000 km/s winds. All scenarios predict that the SC is in the ascending phase of its trajectory and have large - but not implausible - energy requirements.Comment: Submitted to MNRAS letters, revised after referee's comments. Comments are welcom

Angular momentum, accretion and radial flows in chemodynamical models of spiral galaxies

Gas accretion and radial flows are key ingredients of the chemical evolution of spiral galaxies. They are also tightly linked to each other (accretion drives radial flows, due to angular momentum conservation) and should therefore be modelled simultaneously. We summarise an algorithm that can be used to consistently compute accretion profiles, radial flows and abundance gradients under quite general conditions and we describe illustrative applications to the Milky Way. We find that gas-phase abundance gradients strongly depend on the angular momentum of the accreting material and, in the outer regions, they are significantly affected by the choice of boundary conditions.Comment: 4 pages, 2 figures. Proceedings of the 592 WE-Heraeus Seminar. To appear in Astronomische Nachricthen, special issue "Reconstructing the Milky Way's history: spectroscopic surveys, asteroseismology and chemodynamical models", Guest Editors C. Chiappini, J. Montalban and M. Steffe

Dynamics of Starbursting Dwarf Galaxies. III. A HI study of 18 nearby objects

We investigate the dynamics of starbursting dwarf galaxies, using both new and archival HI observations. We consider 18 nearby galaxies that have been resolved into single stars by HST observations, providing their star formation history and total stellar mass. We find that 9 objects have a regularly-rotating HI disk, 7 have a kinematically disturbed HI disk, and 2 show unsettled HI distributions. Two galaxies (NGC 5253 and UGC 6456) show a velocity gradient along the minor axis of the HI disk, that we interpret as strong radial motions. For galaxies with a regularly rotating disk we derive rotation curves, while for galaxies with a kinematically disturbed disk we estimate the rotation velocities in their outer parts. We derive baryonic fractions within about 3 optical scale lengths and find that, on average, baryons constitute at least 30$\%$ of the total mass. Despite the star formation having injected $\sim$10$^{56}$ ergs in the ISM in the last $\sim$500 Myr, these starbursting dwarfs have both baryonic and gas fractions similar to those of typical dwarf irregulars, suggesting that they did not eject a large amount of gas out of their potential wells.Comment: Published on A&A (23 pages, 9 tables, 12 figures, plus an optical-HI atlas). Typos fixe

The triggering of starbursts in low-mass galaxies

Strong bursts of star formation in galaxies may be triggered either by internal or external mechanisms. We study the distribution and kinematics of the HI gas in the outer regions of 18 nearby starburst dwarf galaxies, that have accurate star-formation histories from HST observations of resolved stellar populations. We find that starburst dwarfs show a variety of HI morphologies, ranging from heavily disturbed HI distributions with major asymmetries, long filaments, and/or HI-stellar offsets, to lopsided HI distributions with minor asymmetries. We quantify the outer HI asymmetry for both our sample and a control sample of typical dwarf irregulars. Starburst dwarfs have more asymmetric outer HI morphologies than typical irregulars, suggesting that some external mechanism triggered the starburst. Moreover, galaxies hosting an old burst (>100 Myr) have more symmetric HI morphologies than galaxies hosting a young one (<100 Myr), indicating that the former ones probably had enough time to regularize their outer HI distribution since the onset of the burst. We also investigate the nearby environment of these starburst dwarfs and find that most of them ($\sim$80$\%$) have at least one potential perturber at a projected distance <200 kpc. Our results suggest that the starburst is triggered either by past interactions/mergers between gas-rich dwarfs or by direct gas infall from the IGM.Comment: 21 pages, 8 figures, 6 tables, accepted for publication in MNRA

S0 galaxies are faded spirals: clues from their angular momentum content

The distribution of galaxies in the stellar specific angular momentum versus stellar mass plane ($j_{\star}$-$M_{\star}$) provides key insights into their formation mechanisms. In this paper, we determine the location in this plane of a sample of ten field/group unbarred lenticular (S0) galaxies from the CALIFA survey. We performed a bulge-disc decomposition both photometrically and kinematically to study the stellar specific angular momentum of the disc components alone and understand the evolutionary links between S0s and other Hubble types. We found that eight of our S0 discs have a distribution in the $j_{\star}$-$M_{\star}$ plane that is fully compatible with that of spiral discs, while only two have values of $j_{\star}$ lower than the spirals. These two outliers show signs of recent merging. Our results suggest that merger and interaction processes are not the dominant mechanisms in S0 formation in low-density environments. Instead, S0s appear to be the result of secular processes and the fading of spiral galaxies after the shutdown of star formation.Comment: 35 pages, 22 figures. Accepted for publication in MNRA

A novel 3D technique to study the kinematics of lensed galaxies

We present a 3D Bayesian method to model the kinematics of strongly lensed galaxies from spatially-resolved emission-line observations. This technique enables us to simultaneously recover the lens-mass distribution and the source kinematics directly from the 3D data cube. We have tested this new method with simulated OSIRIS observations for nine star-forming lensed galaxies with different kinematic properties. The simulated rotation curves span a range of shapes which are prototypes of different morphological galaxy types, from dwarf to massive spiral galaxies. We have found that the median relative accuracy on the inferred lens and kinematic parameters are at the level of 1 and 2 per cent, respectively. We have also tested the robustness of the technique against different inclination angles, signal-to-noise ratios, the presence of warps or non-circular motions and we have found that the accuracy stays within a few per cent in most cases. This technique represents a significant step forward with respect to the methods used until now, as the lens parameters and the kinematics of the source are derived from the same 3D data. This enables us to study the possible degeneracies between the two and estimate the uncertainties on all model parameters consistently.Comment: Accepted for publication in MNRA

High Velocity Gas in the Halos of Spiral Galaxies

Recent, high sensitivity, HI observations of nearby spiral galaxies show that their thin `cold' disks are surrounded by thick layers (halos) of neutral gas with anomalous kinematics. We present results for three galaxies viewed at different inclination angles: NGC891 (edge-on), NGC2403 (i=60 deg), and NGC6946 (almost face-on). These studies show the presence of halo gas up to distances of 10-15 kpc from the plane. Such gas has a mean rotation 25-50 km/s lower than that of the gas in the plane, and some complexes are detected at very high velocities, up to 200-300 km/s. The nature and origin of this halo gas are poorly understood. It can either be the result of a galactic fountain or of accretion from the intergalactic medium. It is probably the analogous of some of the High Velocity Clouds (HVCs) of the Milky Way.Comment: 6 pages, 4 figures. To appear in the ASP proceedings of IAU Symposium 217, "Recycling intergalactic and interstellar matter", eds. Pierre-Alain Duc, Jonathan Braine, Elias Brink