168 research outputs found
Hi Spin Temperatures and Heating Requirements in Outer Regions of Disk Galaxies
ABSRACT:We show how to use 21-cm emission and absorption studies to estimate
the heat inputs to the neutral gas in low pressure environments, such as in
outer disks of spiral galaxies, in galactic halos or in intergalactic space.
For a range of model parameters we calculate the gas kinetic and spin
temperatures ( and ) and the relation between and the heat
input to the gas. We outline the conditions for a ``two phase medium'' to
exist. We find that although can be much smaller than , is
always K for column densities greater that
cm. This excludes the possibility that relevant HI masses at the
periphery of galaxies are invisible at 21-cm in emission. The outermost
interstellar gas in a disk galaxy is more directly affected by external
processes and in this paper we estimate the intensity of the extragalactic
background at energies close to 0.1 keV by comparing our theoretical results
with HI emission/absorption studies. We take into account the possibility that
some energy produced in the inner regions affects the energy balance in outer
regions. We find that in the absence of any other local heat source QSO
dominated background models are still compatible with the spin temperature
limits derived for the two best documented HI emission/absorption studies in
outer regions.Comment: 24 pages, 8 figures ARCETRI-PR-93-2
Star-forming dwarf galaxies in the Virgo cluster: the link between molecular gas, atomic gas, and dust
We present CO(1-0) and CO(2-1) observations of a sample of 20
star-forming dwarfs selected from the Herschel Virgo Cluster Survey, with
oxygen abundances ranging from 12 + log(O/H) ~ 8.1 to 8.8. CO emission is
observed in ten galaxies and marginally detected in another one. CO fluxes
correlate with the FIR 250 m emission, and the dwarfs follow the same
linear relation that holds for more massive spiral galaxies extended to a wider
dynamical range. We compare different methods to estimate H2 molecular masses,
namely a metallicity-dependent CO-to-H2 conversion factor and one dependent on
H-band luminosity. The molecular-to-stellar mass ratio remains nearly constant
at stellar masses <~ 10 M, contrary to the atomic hydrogen
fraction, M/M, which increases inversely with M. The flattening
of the M/M ratio at low stellar masses does not seem to be related
to the effects of the cluster environment because it occurs for both
HI-deficient and HI-normal dwarfs. The molecular-to-atomic ratio is more
tightly correlated with stellar surface density than metallicity, confirming
that the interstellar gas pressure plays a key role in determining the balance
between the two gaseous components of the interstellar medium. Virgo dwarfs
follow the same linear trend between molecular gas mass and star formation rate
as more massive spirals, but gas depletion timescales, , are not
constant and range between 100 Myr and 6 Gyr. The interaction with the Virgo
cluster environment is removing the atomic gas and dust components of the
dwarfs, but the molecular gas appears to be less affected at the current stage
of evolution within the cluster. However, the correlation between HI deficiency
and the molecular gas depletion time suggests that the lack of gas
replenishment from the outer regions of the disc is lowering the star formation
activity.Comment: 19 pages, 11 figures, accepted for publication in Astronomy &
Astrophysic
Very Luminous Carbon Stars in the Outer Disk of the Triangulum Spiral Galaxy
Stars with masses in the range from about 1.3 to 3.5 Mo pass through an
evolutionary stage where they become carbon stars. In this stage, which lasts a
few Myr, these stars are extremely luminous pulsating giants. They are so
luminous in the near-infrared that just a few of them can double the integrated
luminosity of intermediate-age (0.6 to 2 Gyr) Magellanic Cloud clusters at 2.2
microns. Astronomers routinely use such near-infrared observations to minimize
the effects of dust extinction, but it is precisely in this band that carbon
stars can contribute hugely. The actual contribution of carbon stars to the
outer disk light of evolving spiral galaxies has not previously been
morphologically investigated. Here we report new and very deep near-IR images
of the Triangulum spiral galaxy M33=NGC 598, delineating spectacular arcs of
carbon stars in its outer regions. It is these arcs which dominate the
near-infrared m=2 Fourier spectra of M33. We present near-infrared photometry
with the Hale 5-m reflector, and propose that the arcs are the signature of
accretion of low metallicity gas in the outer disk of M33.Comment: 4 pages, 4 figures. Revised version submitted to A&A Letter
Kinematic modelling of disk galaxies I. A new method to fit tilted rings to data cubes
This is the first of a series of papers in which the kinematics of disk
galaxies over a range of scales is scrutinised employing spectroscopy. A
fundamental aspect of these studies is presented here: the new publicly
available software tool TiRiFiC
(http://www.astro.uni-bonn.de/~gjozsa/tirific.html) enables a direct fit of a
``tilted-ring model'' to spectroscopic data cubes. The algorithm generates
model data cubes from the tilted-ring parametrisation of a rotating disk, which
are automatically adjusted to reach an optimum fit via a chi-squared
minimisation method to an observed data cube. The structure of the new
software, the shortcomings of the previously available programs to produce a
tilted-ring model, and the performance of TiRiFiC are discussed. Our method is
less affected by the well-known problem of beam smearing that occurs when
fitting to the velocity field. Since with our method we fit many data points in
a data cube simultaneously, TiRiFiC is sensitive to very faint structures and
can hence be used to derive tilted-ring models significantly extending in
radius beyond those derived from a velocity field. The software is able to
parametrise HI disks of galaxies that are intersected by the line-of-sight
twice or more, i.e. if the disks are heavily warped, and/or with a significant
shift of the projected centre of rotation, and/or if seen edge-on. Furthermore,
our method delivers the surface-brightness profile of the examined galaxy in
addition to the orientational parameters and the rotation curve. In order to
derive kinematic and morphological models of disk galaxies, especially reliable
rotation curves, a direct-fit method as implemented in our code should be the
tool of choice.Comment: 37 pages, 24 figures. Accepted for publication in Astronomy and
Astrophysic
Radial HI Profiles at the Periphery of Galactic Disks: The Role of Ionizing Background Radiation
Observations of neutral hydrogen in spiral galaxies reveal a sharp cutoff in
the radial density profile at some distance from the center. Using 22 galaxies
with known HI distributions as an example, we discuss the question of whether
this effect can be associated exclusively with external ionizing radiation, as
is commonly assumed. We show that before the surface density reaches
(the same for
galaxies of different types), it is hard to expect the gas to be fully ionized
by background radiation. For two of 13 galaxies with a sharp drop in the HI
profile, the "steepening" can actually be caused by ionization. At the same
time, for the remaining galaxies, the observed cutoff in the radial HI profile
is closer to the center than if it was a consequence of ionization by
background radiation and, therefore, it should be caused by other factors.Comment: 15 pages, 6 figure
Keck spectroscopy and Spitzer Space Telescope analysis of the outer disk of the Triangulum Spiral Galaxy M33
In an earlier study of the spiral galaxy M33, we photometrically identified
arcs or outer spiral arms of intermediate age (0.6 Gyr - 2 Gyr) carbon stars
precisely at the commencement of the HI-warp. Stars in the arcs were
unresolved, but were likely thermally-pulsing asymptotic giant branch carbon
stars. Here we present Keck I spectroscopy of seven intrinsically bright and
red target stars in the outer, northern arc in M33. The target stars have
estimated visual magnitudes as faint as V \sim 25 mag. Absorption bands of CN
are seen in all seven spectra reported here, confirming their carbon star
status. In addition, we present Keck II spectra of a small area 0.5 degree away
from the centre of M33; the target stars there are also identified as carbon
stars. We also study the non-stellar PAH dust morphology of M33 secured using
IRAC on board the Spitzer Space Telescope. The Spitzer 8 micron image attests
to a change of spiral phase at the start of the HI warp. The Keck spectra
confirm that carbon stars may safely be identified on the basis of their red
J-K_s colours in the outer, low metallicity disk of M33. We propose that the
enhanced number of carbon stars in the outer arms are an indicator of recent
star formation, fueled by gas accretion from the HI-warp reservoir.Comment: 9 pages, 5 figures, accepted in A&
The Herschel Virgo Cluster Survey: IV. Resolved dust analysis of spiral galaxies
We present a resolved dust analysis of three of the largest angular size
spiral galaxies, NGC 4501 and NGC 4567/8, in the Herschel Virgo Cluster Survey
(HeViCS) Science Demonstration field. Herschel has unprecedented spatial
resolution at far-infrared wavelengths and with the PACS and SPIRE instruments
samples both sides of the peak in the far infrared spectral energy distribution
(SED).We present maps of dust temperature, dust mass, and gas-to-dust ratio,
produced by fitting modified black bodies to the SED for each pixel. We find
that the distribution of dust temperature in both systems is in the range ~19 -
22 K and peaks away from the centres of the galaxies. The distribution of dust
mass in both systems is symmetrical and exhibits a single peak coincident with
the galaxy centres. This Letter provides a first insight into the future
analysis possible with a large sample of resolved galaxies to be observed by
Herschel.Comment: Letter accepted for publication in A&A (Herschel special issue
The far-infrared view of M87 as seen by the Herschel Space Observatory
The origin of the far-infrared emission from the nearby radio galaxy M87
remains a matter of debate. Some studies find evidence of a far-infrared excess
due to thermal dust emission, whereas others propose that the far-infrared
emission can be explained by synchrotron emission without the need for an
additional dust emission component. We observed M87 with PACS and SPIRE as part
of the Herschel Virgo Cluster Survey (HeViCS). We compare the new Herschel data
with a synchrotron model based on infrared, submm and radio data to investigate
the origin of the far-infrared emission. We find that both the integrated SED
and the Herschel surface brightness maps are adequately explained by
synchrotron emission. At odds with previous claims, we find no evidence of a
diffuse dust component in M87.Comment: 4 pages, 2 figures, proceedings IAU Symposium 275 (Jets at all
scales
A panchromatic spatially resolved analysis of nearby galaxies-II. The main sequence-gas relation at sub-kpc scale in grand-design spirals
In this work, we analyse the connection between gas availability and the position of a region with respect to the spatially resolved main-sequence (MS) relation. Following the procedure presented in Enia et al. (2020), for a sample of five face-on, grand design spiral galaxies located on the MS we obtain estimates of stellar mass and star formation rate surface densities (E∗ and ESFR) within cells of 500 pc size. Thanks to HI 21cm and 12CO(2-1) maps of comparable resolution, within the same cells we estimate the surface densities of the atomic (EHI) and molecular (EH2) gas and explore the correlations among all these quantities. E∗, ESFR, and EH2 define a 3D relation whose projections are the spatially resolved MS, the Kennicutt-Schmidt law and the molecular gas MS. We find that EH2 steadily increases along the MS relation and is almost constant perpendicular to it. EHI is nearly constant along the MS and increases in its upper envelope. As a result, ESFR can be expressed as a function of E∗ and E HI, following the relation log ESFR = 0.97log E∗ + 1.99log EH I-11.11. We show that the total gas fraction significantly increases towards the starburst regions, accompanied by a weak increase in star formation efficiency. Finally, we find that H2/HI varies strongly with the distance from the MS, dropping dramatically in regions of intense star formation, where the UV radiation from newly formed stars dissociates the H2 molecule, illustrating the self-regulating nature of the star formation process
Newly Identified Star Clusters in M33. III. Structural Parameters
We present the morphological properties of 161 star clusters in M33 using the
Advanced Camera For Surveys Wide Field Channel onboard the Hubble Space
Telescope using observations with the F606W and F814W filters. We obtain, for
the first time, ellipticities, position angles, and surface brightness profiles
for a significant number of clusters. On average, M33 clusters are more
flattened than those of the Milky Way and M31, and more similar to clusters in
the Small Magellanic Cloud. The ellipticities do not show any correlation with
age or mass, suggesting that rotation is not the main cause of elongation in
the M33 clusters. The position angles of the clusters show a bimodality with a
strong peak perpendicular to the position angle of the galaxy major axis. These
results support the notion that tidal forces are the reason for the cluster
flattening. We fit King and EFF models to the surface brightness profiles and
derive structural parameters including core radii, concentration, half-light
radii and central surface brightness for both filters. The surface brightness
profiles of a significant number of clusters show irregularities such as bumps
and dips. Young clusters (Log age < 8) are notably better fitted by models with
no radial truncation (EFF models), while older clusters show no significant
differences between King or EFF fits. M33 star clusters seem to have smaller
sizes, smaller concentrations, and smaller central surface brightness as
compared to clusters in the MW, M31, LMC and SMC. Analysis of the structural
parameters presents a age-radius relation also detected in other star cluster
systems. The overall analysis shows differences in the structural evolution
between the M33 cluster system and cluster systems in nearby galaxies. These
differences could have been caused by the strong differences in these various
environments.Comment: 30 pages, 12 figures, accepted for publication in MNRA
- …