188 research outputs found
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
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 (-) 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
- plane that is fully compatible with that of spiral
discs, while only two have values of 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
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 10 ergs in the ISM in the last 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 (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
Angular Momentum Accretion onto Disc Galaxies
Throughout the Hubble time, gas makes its way from the intergalactic medium into galaxies fuelling their star formation and promoting their growth. One of the key properties of the accreting gas is its angular momentum, which has profound implications for the evolution of, in particular, disc galaxies. Here, we discuss how to infer the angular momentum of the accreting gas using observations of present-day galaxy discs. We first summarize evidence for ongoing inside-out growth of star forming discs. We then focus on the chemistry of the discs and show how the observed metallicity gradients can be explained if gas accretes onto a disc rotating with a velocity 20 - 30% lower than the local circular speed. We also show that these gradients are incompatible with accretion occurring at the edge of the discs and flowing radially inward. Finally, we investigate gas accretion from a hot corona with a cosmological angular momentum distribution and describe how simple models of rotating coronae guarantee the inside-out growth of disc galaxies
A Dynamical Model for the Extra-planar Gas in Spiral Galaxies
Recent HI observations reveal that the discs of spiral galaxies are
surrounded by extended gaseous haloes. This extra-planar gas reaches large
distances (several kpc) from the disc and shows peculiar kinematics (low
rotation and inflow). We have modelled the extra-planar gas as a continuous
flow of material from the disc of a spiral galaxy into its halo region. The
output of our models are pseudo-data cubes that can be directly compared to the
HI data. We have applied these models to two spiral galaxies (NGC891 and
NGC2403) known to have a substantial amount of extra-planar gas. Our models are
able to reproduce accurately the vertical distribution of extra-planar gas for
an energy input corresponding to a small fraction (<4%) of the energy released
by supernovae. However they fail in two important aspects: 1) they do not
reproduce the right gradient in rotation velocity; 2) they predict a general
outflow of the extra-planar gas, contrary to what is observed. We show that
neither of these difficulties can be removed if clouds are ionized and
invisible at 21cm as they leave the disc but become visible at some point on
their orbits. We speculate that these failures indicate the need for accreted
material from the IGM that could provide the low angular momentum and inflow
required.Comment: 19 pages, 20 figures, accepted for publication in MNRAS. High
resolution version available at
http://www-thphys.physics.ox.ac.uk/users/FilippoFraternali/interests.htm
The angular momentum-mass relation: a fundamental law from dwarf irregulars to massive spirals
In a CDM Universe, the specific stellar angular momentum ()
and stellar mass () of a galaxy are correlated as a consequence of the
scaling existing for dark matter haloes ().
The shape of this law is crucial to test galaxy formation models, which are
currently discrepant especially at the lowest masses, allowing to constrain
fundamental parameters, e.g. the retained fraction of angular momentum. In this
study, we accurately determine the empirical relation (Fall
relation) for 92 nearby spiral galaxies (from S0 to Irr) selected from the
Spitzer Photometry and Accurate Rotation Curves (SPARC) sample in the
unprecedented mass range . We
significantly improve all previous estimates of the Fall relation by
determining profiles homogeneously for all galaxies, using extended HI
rotation curves, and selecting only galaxies for which a robust could
be measured (converged radial profile). We find the relation to be
well described by a single, unbroken power-law
over the entire mass range, with and orthogonal intrinsic
scatter of dex. We finally discuss some implications for galaxy
formation models of this fundamental scaling law and, in particular, the fact
that it excludes models in which discs of all masses retain the same fraction
of the halo angular momentum.Comment: A&A Letters, accepte
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