133 research outputs found
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
Mass models of disk galaxies from gas dynamics
I review methods and techniques to build mass models of disk galaxies from
gas dynamics. I focus on two key steps: (1) the derivation of rotation curves
using 3D emission-line datacubes from HI, CO, and/or H-alpha observations, and
(2) the calculation of the gravitational field from near-infrared images and
emission-line maps, tracing the stellar and gas mass distributions,
respectively. Mass models of nearby galaxies led to the establishment of the
radial acceleration relation (RAR): the observed centripetal acceleration from
rotation curves closely correlates with that predicted from the baryonic
distribution at each galaxy radius, even when dark matter supposedly dominates
the gravitational field. I conclude by discussing the (uncertain) location of
Local Group dwarf spheroidal galaxies on the RAR defined by more massive disk
galaxies.Comment: Invited Review for the IAU Symposium 379 "Dynamical Masses of Local
Group Galaxies" (10 pages, 2 figures
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
Uncorrelated velocity and size residuals across galaxy rotation curves
The mass--velocity--size relation of late-type galaxies decouples into
independent correlations between mass and velocity (the Tully-Fisher relation),
and between mass and size. This behaviour is different to early-type galaxies
which lie on a Fundamental Plane. We study the coupling of the Tully-Fisher and
mass-size relations in observations (the SPARC sample) and in empirical galaxy
formation models based on halo abundance matching, and rotation curve fits with
a hydrodynamically motivated halo profile. We systematically investigate the
correlation coefficient between the Tully-Fisher residuals and
mass-size residuals as a function of the radius at which the
velocity is measured, and thus present the relation
across rotation curves. We find no significant correlation in either the data
or models for any , aside from where baryonic mass
dominates. We show that this implies an anticorrelation between galaxy size and
halo concentration (or halo mass) at fixed baryonic mass, and provides evidence
against the hypothesis that galaxy and halo specific angular momentum are
proportional. Finally, we study the relations produced by
the baryons and dark matter separately by fitting halo profiles to the rotation
curves. The balance between these components illustrates the "disk-halo
conspiracy" required for no overall correlation.Comment: 7 pages, 4 figures; revised to match MNRAS published versio
The Tight Empirical Relation between Dark Matter Halo Mass and Flat Rotation Velocity for Late-Type Galaxies
We present a new empirical relation between galaxy dark matter halo mass
() and the velocity along the flat portion of the rotation
curve (), derived from 120 late-type galaxies from the SPARC
database. The orthogonal scatter in this relation is comparable to the observed
scatter in the baryonic Tully-Fisher relation (BTFR), indicating a tight
coupling between total halo mass and galaxy kinematics at .
The small vertical scatter in the relation makes it an extremely competitive
estimator of total halo mass. We demonstrate that this conclusion holds true
for different priors on that give a tight BTFR, but requires
that the halo density profile follows DC14 rather than NFW. We provide
additional relations between and other velocity definitions at
smaller galactic radii (i.e. , , and ) which can be useful for estimating halo masses from kinematic
surveys, providing an alternative to abundance matching. Furthermore, we
constrain the dark matter analog of the Radial Acceleration Relation and also
find its scatter to be small, demonstrating the fine balance between baryons
and dark matter in their contribution to galaxy kinematics.Comment: 6 pages, 4 figures, Accepted to MNRAS Letter
Fitting the radial acceleration relation to individual SPARC galaxies
Galaxies follow a tight radial acceleration relation (RAR): the acceleration
observed at every radius correlates with that expected from the distribution of
baryons. We use the Markov Chain Monte Carlo method to fit the mean RAR to 175
individual galaxies in the SPARC database, marginalizing over stellar
mass-to-light ratio (), galaxy distance, and disk
inclination. Acceptable fits with astrophysically reasonable parameters are
found for the vast majority of galaxies. The residuals around these fits have
an rms scatter of only 0.057 dex (13). This is in agreement with the
predictions of modified Newtonian dynamics (MOND). We further consider a
generalized version of the RAR that, unlike MOND, permits galaxy-to-galaxy
variation in the critical acceleration scale. The fits are not improved with
this additional freedom: there is no credible indication of variation in the
critical acceleration scale. The data are consistent with the action of a
single effective force law. The apparent universality of the acceleration scale
and the small residual scatter are key to understanding galaxies.Comment: 12 pages, 7 figures, 2 tables. Accepted for publication in A&A. The
same as the first version with typos corrected. A set of 175 figures is
available at http://astroweb.cwru.edu/SPARC
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