331 research outputs found
HI clouds in the proximity of M33
Neutral hydrogen clouds are found in the Milky Way and Andromeda halo both as
large complexes and smaller isolated clouds. Here we present a search for Hi
clouds in the halo of M33, the third spiral galaxy of the Local Group. We have
used two complementary data sets: a 3^o x 3^o map of the area provided by the
Arecibo Legacy Fast ALFA (ALFALFA) survey and deeper pointed observations
carried out with the Arecibo telescope in two fields that permit sampling of
the north eastern and south-western edges of the HI disc. The total amount of
Hi around M33 detected by our survey is M. At least 50%
of this mass is made of HI clouds that are related both in space and velocity
to the galaxy. We discuss several scenarios for the origin of these clouds
focusing on the two most interesting ones: dark-matter dominated gaseous
satellites, debris from filaments flowing into M33 from the intergalactic
medium or generated by a previous interaction with M31. Both scenarios seem to
fit with the observed cloud properties. Some structures are found at anomalous
velocities, particularly an extended HI complex previously detected by Thilker
et al. (2002). Even though the ALFALFA observations seem to indicate that this
cloud is possibly connected to M33 by a faint gas bridge, we cannot firmly
establish its extragalactic nature or its relation to M33. Taking into account
that the clouds associated with M33 are likely to be highly ionised by the
extragalactic UV radiation, we predict that the total gas mass associated with
them is > 5 x 10^7 M. If the gas is steadily falling towards the M33
disc it can provide the fuel needed to sustain a current star formation rate of
0.5 M yr.Comment: 16 pages, 19 figures. Accepted for publication in A&
Stellar structures in the outer regions of M33
We present Subaru/Suprime-Cam deep V and I imaging of seven fields in the
outer regions of M33. Our aim is to search for stellar structures corresponding
to extended HI clouds found in a recent 21-cm survey of the galaxy. Three
fields probe a large HI complex to the southeastern (SE) side of the galaxy. An
additional three fields cover the northwestern (NW) side of the galaxy along
the HI warp. A final target field was chosen further north, at a projected
distance of approximately 25 kpc, to study part of the large stellar plume
recently discovered around M33. We analyse the stellar population at R > 10 kpc
by means of V, I colour magnitude diagrams reaching the red clump. Evolved
stellar populations are found in all fields out to 120' (~ 30 kpc), while a
diffuse population of young stars (~ 200 Myr) is detected out to a
galactocentric radius of 15 kpc. The mean metallicity in the southern fields
remains approximately constant at [M/H] = -0.7 beyond the edge of the optical
disc, from 40' out to 80'. Along the northern fields probing the outer \hi
disc, we also find a metallicity of [M/H] = -0.7 between 35' and 70' from the
centre, which decreases to [M/H] = -1.0 at larger angular radii out to 120'. In
the northernmost field, outside the disc extent, the stellar population of the
large stellar feature possibly related to a M33-M31 interaction is on average
more metal-poor ([M/H] = -1.3) and older (> 6 Gyr). An exponential disc with a
large scale-length (~ 7 kpc) fits well the average distribution of stars
detected in both the SE and NW regions from a galactocentric distance of 11 kpc
out to 30 kpc. The stellar distribution at large radii is disturbed and,
although there is no clear correlation between the stellar substructures and
the location of the HI clouds, this gives evidence for tidal interaction or
accretion events.Comment: 13 pages, 13 figures. Accepted for publications in Astronomy and
Astrophysics; minor revisions of the tex
Bar imprints on the inner gas kinematics of M33
We present measurements of the stellar and gaseous velocities in the central
5' of the Local Group spiral M33. The data were obtained with the ARC 3.5m
telescope. Blue and red spectra with resolutions from 2 to 4\AA covering the
principal gaseous emission and stellar absorption lines were obtained along the
major and minor axes and six other position angles. The observed radial
velocities of the ionized gas along the photometric major axis of M33 remain
flat at ~22 km s^{-1} all the way into the center, while the stellar velocities
show a gradual rise from zero to 22 km s^{-1} over that same region. The
central star cluster is at or very close to the dynamical center, with a
velocity that is in accordance with M33's systemic velocity to within our
uncertainties. Velocities on the minor axis are non-zero out to about 1' from
the center in both the stars and gas. Together with the major axis velocities,
they point at significant deviations from circular rotation. The most likely
explanation for the bulk of the velocity patterns are streaming motions along a
weak inner bar with a PA close to that of the minor axis, as suggested by
previously published IR photometric images. The presence of bar imprints in M33
implies that all major Local Group galaxies are barred. The non-circular
motions over the inner 200 pc make it difficult to constrain the shape of M33's
inner dark matter halo profile. If the non-circular motions we find in this
nearby Sc galaxy are present in other more distant late-type galaxies, they
might be difficult to recognize.Comment: 20 pages, 12 figures, ApJ in pres
The HI distribution in the outskirts of M33 with the ALFALFA survey
Spiral galaxies appear to be dynamical systems whose disks are still forming
at the current epoch and which continue to accrete mass. The presence of
extraplanar gas in spirals indicates that galactic halos can contain at least
part of the material needed to fuel the star formation activity in their disks.
Here we present the analysis of the ALFALFA survey data in the region of M33
aimed at searching high velocity clouds around this Local Group galaxy. We find
a varied population of HI clouds with masses ranging between 4 x 10^4 and few
times 10^6 M_sun. We also detect an extended HI complex at anomalous
velocities, whose extragalactic nature cannot be firmly established. We
estimate that the total amount of neutral hydrogen mass associated to these
clouds is around 10^7 M_sun.Comment: 3 pages, 2 figures, to appear in the Conference Proceedings "The
Evolution of Galaxies through the Neutral Hydrogen Window",eds. R. Minchin &
E. Momjia
Simulations of the flocculent spiral M33: what drives the spiral structure?
We perform simulations of isolated galaxies in order to investigate the
likely origin of the spiral structure in M33. In our models, we find that
gravitational instabilities in the stars and gas are able to reproduce the
observed spiral pattern and velocity field of M33, as seen in HI, and no
interaction is required. We also find that the optimum models have high levels
of stellar feedback which create large holes similar to those observed in M33,
whilst lower levels of feedback tend to produce a large amount of small scale
structure, and undisturbed long filaments of high surface density gas, hardly
detected in the M33 disc. The gas component appears to have a significant role
in producing the structure, so if there is little feedback, both the gas and
stars organise into clear spiral arms, likely due to a lower combined
(using gas and stars), and the ready ability of cold gas to undergo spiral
shocks. By contrast models with higher feedback have weaker spiral structure,
especially in the stellar component, compared to grand design galaxies. We did
not see a large difference in the behaviour of with most of these
models, however, because stayed relatively constant unless the disc
was more strongly unstable. Our models suggest that although the stars produce
some underlying spiral structure, this is relatively weak, and the gas physics
has a considerable role in producing the large scale structure of the ISM in
flocculent spirals.Comment: 17 pages, 17 figures, accepted for publication in MNRA
The population of Young Stellar Clusters throughout the disk of M33
The properties of young stellar clusters (YSCs) in M33, identified from the
center out to about twice the size of the bright star-forming disk,are
investigated. We find 915 discrete MIR sources as far as the extent of the
warped HI disk, i.e. 16 kpc from the galaxy center. Their surface density has a
steep radial decline beyond 4.5 kpc, and flattens out beyond the optical radius
at 8.5 kpc. We are able to identify YSCs out to 12 kpc. At large galactocentric
radii, the paucity of very luminous clusters and the relevance of hot dust
emission become evident from the analysis of the bolometric and MIR luminosity
functions. The YSC mass and size are correlated with a log-log slope of 2.09,
similar to that measured for giant molecular clouds in M33 and the Milky Way,
which represent the protocluster environment. Most of the YSCs in our sample
have low extinction and ages between 3 and 10 Myr. In the inner regions of M33
the clusters span a wide range of mass (10^2<M<3 10^5 msun) and luminosity
10^38<L{bol}<3 10^{41}erg/s, while at galactocentric radii larger than 4 kpc we
find a deficiency of massive clusters. Beyond 7 kpc, where the Halpha surface
brightness drops significantly, the dominant YSC population has M<10^3 msun and
a slightly older age (10 Myrs). This implies the occurrence of star formation
events about 10 Myr ago as far as 10-12 kpc from the center of M33. The cluster
L{FUV}--L{Halpha} relation is non-linear for L{FUV}<10^{39}erg/s, in agreement
with randomly sampled models of the IMF which, furthermore, shows no
appreciable variation throughout the M33 disk.Comment: Accepted for publication in A&A, 16 pages, 14 figure
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
Star formation in M33: Spitzer photometry of discrete sources
Combining the relative vicinity of the Local Group spiral galaxy M33 with the
Spitzer images, we investigate the properties of infrared (IR) emission sites
and assess the reliability of the IR emission as a star formation tracer. The
mid- and far-IR emission of M33 was obtained from IRAC and MIPS images from the
Spitzer archive. We compared the photometric results for several samples of
three known types of discrete sources (HII regions, supernovae remnants and
planetary nebulae) with theoretical diagnostic diagrams, and derived the
spectral energy distribution (from 3.6 to 24 micron) of each type of object.
Moreover, we generated a catalogue of 24 micron sources and inferred their
nature from the observed and theoretical colours of the known type sources. We
estimated the star formation rate in M33 both globally and locally, from the IR
emission and from the Halpha emission line. The colours of the typical IR
emissions of HII regions, supernovae remnants and planetary nebulae are
continuous among the different samples, with overlapping regions in the
diagnostic diagrams. The comparison between the model results and the colours
of HII regions indicates a dusty envelope at relatively high temperatures ~600
K, and moderate extinction Av < 10. The 24 micron sources IR colours follow the
regions observationally defined by the three classes of known objects but the
majority of them represent HII regions. The derived total IR luminosity
function is in fact very similar to the HII luminosity function observed in the
Milky Way and in other late type spirals. Even though our completeness limit is
5x10^37 ergs s-1, in low density regions we are able to detect sources five
times fainter than this, corresponding to the faintest possible HII region.
[abridged]Comment: 19 pages, 18 figures (low resolution), accepted for publication by
A&A; corrected typo
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
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