131 research outputs found
High Resolution Molecular Gas Maps of M33
New observations of CO (J=1->0) line emission from M33, using the 25 element
BEARS focal plane array at the Nobeyama Radio Observatory 45-m telescope, in
conjunction with existing maps from the BIMA interferometer and the FCRAO 14-m
telescope, give the highest resolution (13'') and most sensitive (RMS ~ 60 mK)
maps to date of the distribution of molecular gas in the central 5.5 kpc of the
galaxy. A new catalog of giant molecular clouds (GMCs) has a completeness limit
of 1.3 X 10^5 M_sun. The fraction of molecular gas found in GMCs is a strong
function of radius in the galaxy, declining from 60% in the center to 20% at
galactocentric radius R_gal ~ 4 kpc. Beyond that radius, GMCs are nearly
absent, although molecular gas exists. Most (90%) of the emission from low mass
clouds is found within 100 pc projected separation of a GMC. In an annulus 2.1<
R_gal <4.1 kpc, GMC masses follow a power law distribution with index -2.1.
Inside that radius, the mass distribution is truncated, and clouds more massive
than 8 X 10^5 M_sun are absent. The cloud mass distribution shows no
significant difference in the grand design spiral arms versus the interarm
region. The CO surface brightness ratio for the arm to interarm regions is 1.5,
typical of other flocculent galaxies.Comment: 14 pages, 14 figures, accepted in ApJ. Some tables poorly typeset in
emulateapj; see source files for raw dat
Giant Molecular Clouds in M33 - I. BIMA All Disk Survey
We present the first interferometric CO(J=1->0) map of the entire H-alpha
disk of M33. The 13" diameter synthesized beam corresponds to a linear
resolution of 50 pc, sufficient to distinguish individual giant molecular
clouds (GMCs). From these data we generated a catalog of 148 GMCs with an
expectation that no more than 15 of the sources are spurious. The catalog is
complete down to GMC masses of 1.5 X 10^5 M_sun and contains a total mass of
2.3 X 10^7 M_sun. Single dish observations of CO in selected fields imply that
our survey detects ~50% of the CO flux, hence that the total molecular mass of
M33 is 4.5 X 10^7 M_sun, approximately 2% of the HI mass. The GMCs in our
catalog are confined largely to the central region (R < 4 kpc). They show a
remarkable spatial and kinematic correlation with overdense HI filaments; the
geometry suggests that the formation of GMCs follows that of the filaments. The
GMCs exhibit a mass spectrum dN/dM ~ M^(-2.6 +/- 0.3), considerably steeper
than that found in the Milky Way and in the LMC. Combined with the total mass,
this steep function implies that the GMCs in M33 form with a characteristic
mass of 7 X 10^4 M_sun. More than 2/3 of the GMCs have associated HII regions,
implying that the GMCs have a short quiescent period. Our results suggest the
rapid assembly of molecular clouds from atomic gas, with prompt onset of
massive star formation.Comment: 19 pages, Accepted for Publication in the Astrophysical Journal
Supplemen
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
ESO Imaging Survey VII. Distant Cluster Candidates over 12 square degrees
In this paper the list of candidate clusters identified from the I-band data
of the ESO Imaging Survey (EIS) is completed using the images obtained over a
total area of about 12 square degrees. Together with the data reported earlier
the total I-band coverage of EIS is 17 square degrees, which has yielded a
sample of 252 cluster candidates in the redshift range 0.2 \lsim z \lsim 1.3.
This is the largest optically-selected sample currently available in the
Southern Hemisphere. It is also well distributed in the sky thus providing
targets for a variety of VLT programs nearly year round.Comment: 5 pages, 3 figures, submitted to Astronomy & Astrophysic
Infrared Excess and Molecular Gas in the Galactic Worm GW46.4+5.5
We have carried out high-resolution (~3') HI and CO line observations along
one-dimensional cuts through the Galactic worm GW46.4+5.5. By comparing the HI
data with IRAS data, we have derived the distributions of I_100 excess and
tau_100 excess, which are respectively the 100 mum intensity and 100 mum
optical depth in excess of what would be expected from HI emission. In two
observed regions, we were able to make a detailed comparison of the infrared
excess and the CO emission. We have found that tau_100 excess has a very good
correlation with the integrated intensity of CO emission, W_CO, but I_100
excess does not. There are two reasons for the poor correlation between I_100
excess and W_CO: firstly, there are regions with enhanced infrared emissivity
without CO, and secondly, dust grains associated with molecular gas have a low
infrared emissivity. In one region, these two factors completely hide the
presence of molecular gas in the infrared. In the second region, we could
identify the area with molecular gas, but I_100 excess significantly
underestimates the column density of molecular hydrogen because of the second
factor mentioned above. We therefore conclude that tau_100 excess, rather than
I_100 excess, is an accurate indicator of molecular content along the line of
sight. We derive tau_100/N(H)=(1.00+-0.02)*10^-5~(10^20 cm^-2)^-1, and
X=N(H_2)/W_CO=~0.7*10^20 cm^-2 (K km s^-1)^-1. Our results suggest that I_100
excess could still be used to estimate the molecular content if the result is
multiplied by a correction factor xi_c=_HI/_H_2 (~2 in
the second region), which accounts for the different infrared emissivities of
atomic and molecular gas. We also discuss some limitations of this work.Comment: 10 pages, 9 postscript figures, uses aas2pp4.sty to be published in
Astrophyslcal Journa
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