179 research outputs found
Local Goup HVCs: Status of the Evidence
Get PDFThe evidence for locating the High Velocity Clouds in the Local Group is
summarized and evaluated. Recent measurements of the H surface
brightness and metallicity of a number of HVCs appear to be fatal to the
Galactic fountain as a significant contributor to the HVC phenomenon, but not
to the existence of the fountain itself. Observations of extragalactic
analogues to HVCs remain the {\it sine qua non} for deciding whether the Local
Group hypothesis is viable, but the constraints based on existing surveys
appear to be rather weak. MgII quasar absorption lines restrict how many HVC
analogues exist at intermediate redshift, depending on where these lines
originate. It is concluded that the evidence remains ambiguous, none of the
main hypotheses is fully consistent with all of the data, and the Local Group
hypothesis remains a viable explanation for the HVC phenomenon.Comment: 10 pages, no figures -- to appear in Gas and Galaxy Evolution --
Hibbard, Rupen & van Gorkom eds minor changes from original version in
metallicity sectio
GMC Formation in M33: the Role of Pressure
Full text linkWe present a complete map of the Giant Molecular Clouds in M33 and show that
they lie exclusively on filaments of HI. The GMCs are localized on the extended
HI filaments, and HI filaments extend well beyond the radius where the GMCs are
found, suggesting that the H_2 forms from the HI and not vice-versa. We propose
that the fraction of neutral gas that is molecular at a particular radius is
determined primarily by the ambient hydrostatic pressure at that radius. This
leads to a prediction that the transition radius, the radius where the mean
surface density of HI and H_2 are equal, occurs at a constant value of the
stellar surface density. We show that for a sample of 26 galaxies, the stellar
surface density at the transition radius is constant to 40%.Comment: 8 Pages, to appear in Star Formation in the Interstellar Medium
Workshop, ASP Conference Series (F. C. Adams, D. Johnstone, D. N. C. Lin, &
E. Ostriker, eds.
The Initial Mass Function of GMCs
Full text linkThe properties of GMCs in several Local Group galaxies are quantified and
compared. It is found that the mass spectrum of GMCs varies from galaxy to
galaxy. The variations are significant and do not appear to be the result of
systematic uncertainties. Nevertheless, it appears that all of the GMCs follow
the same size-linewidth and mass-linewidth relations with little scatter. The
power law indices of these relations imply that the GMCs are self-gravitating,
and that the mean surface density of Local Group GMCs is approximately
constant. This, in turn, implies that the mean internal pressure of GMCs is
also constant. If the IMF of stars is determined by a Jeans instability, this
constant internal pressure suggests that the distribution of stellar masses
does not vary significantly in galactic disks when averaged over suitably large
areas. Thus, although the distribution of GMC masses produced by various Local
Group galaxies is quite variable, the large-scale properties of the GMCs is
not.Comment: 9 pages, 5 figures, to appear the proceedings of "IMF@50: The Initial
Mass Function 50 Years Later" (eds. E. Corbelli, F. Palla, and H. Zinnecker
High Resolution BIMA Observations of CO, HCN, and 13CO in NGC 1068
Get PDFWe present high-resolution CO, HCN, and 13CO maps of the inner arcminute of
NGC 1068 made with the BIMA interferometer. Several features appear in the CO
map which have not previously been observed: (1) a firm detection of CO line
emission from a compact region centered on the nucleus of the galaxy; (2) the
detection of a triplet velocity structure characteristic of kinematically
independent regions shown on the spectrum of the unresolved nuclear emission ;
and (3) the detection of a molecular bar, the extent and position angle of
which are in good agreement with the 2 m stellar bar. The most intense CO
emission is nonnuclear; the structure and kinematics of this emission imply
that this gas is distributed along the inner spiral arms and not in a ring. The
bar's kinematic influence on the molecular gas in the spiral arms is modest,
with typical ordered noncircular motions of \la\ 30 \kms\ in the plane of the
galaxy. Interior to the spiral arms, the bar's influence is more dramatic, as
reflected by the twisted isovelocity contours in the CO and HCN velocity
fields. The surface density of molecular gas within the central 100 pc radius
of NGC 1068 is the same as that in the central 200 pc radius in the Milky Way
to within the uncertainties. There is evidence for an kinematic mode in
NGC 1068; we find the kinematic center of rotation to be displaced from the
radio continuum center by about 2.9", or 200 pc. The HCN image, in contrast to
the CO map, shows a strong concentration of emission centered on the nucleus.
The ratio of integrated intensities of the HCN emission to that of CO is about
0.6 and is the highest ratio measured in the central region of any galaxy.Comment: 35 pages of uuencoded, compressed postscript, 20 postscript figures
not included but available from thelfer@astro.berkeley.edu or from
ftp://astro.astro.umd.edu/pub/thelfer/n1068_figs.ps.Z To appear in The
Astrophysical Journal, V. 450, Sept. 199
Molecular Clouds (a review)
Full text linkWe review the progress made in understanding molecular cloud formation and
evolution toward star formation since the first Crete conference in 1990. It
now seems clear that molecular clouds in the Galaxy at least form from
compressed HI entering a spiral arm. The internal clumpy structure within
clouds probably results from the magnification of inhomogeneities in the
precursor HI. This structure is self-similar in nature independent of the star
forming nature of the cloud. However, at high densities and small scales, there
are departures from self-similarity that may be related to the mass scale of
stars. If confirmed, it may be that the mass of a star is related more to the
mass of the core from which it forms than by the details of the formation
process itself.Comment: Invited review for second Crete conference (The Physics of Star
Formation and Early Stellar Evolution) eds. Kylafis and Lada. 26 pages with 8
embedded figures. Nicer (and larger) color figs available at
http://www.tuc.nrao.edu/~jpwillia
Global Star Formation from z=5e-8 to z=20
Full text linkStarting with the assertion that the problem of isolated, single star
formation is essentially solved, this paper examines some of the missing steps
needed to go from there to understanding the star formation history of the
Universe. Along the way, some results on the formation of star clusters in the
Milky Way and the properties of GMCs in nearby galaxies are briefly examined.Comment: 11 pages, 8 figures (some compressed), to appear in the proceedings
of the Young Local Universe conference (Moriond 2004, eds. Chalabaev, Fukui
and Montmerle
Dark Matter in Dwarf Galaxies: The First Dark Galaxy?
Full text linkWe present new HI observations of the high-velocity cloud (HVC) that we
resolved near the Local Group dwarf galaxy LGS 3. The cloud is rotating, with
an implied mass that makes it dark matter-dominated no matter what its distance
from the Milky Way is. Our new, high-sensitivity Arecibo observations
demonstrate that the faint HI features that we previously described as tidal
tails are indeed real and do connect to the main body of the HVC. Thus, these
observations are consistent with our original hypothesis of a tidal interaction
between the HVC and LGS 3. We suggest that the HVC may be one of the missing
dark matter satellites in the Local Group that are seen in Cold Dark Matter
numerical simulations but have not yet been identified observationally.Comment: 4 pages, 2 color figures, uses newpasp.sty. To appear in the
proceedings of the conference "Satellites and Tidal Streams", La Palma,
Spain, 26-30 May 2003, eds. F. Prada, D. Martinez-Delgado, T. Mahone
Dense Gas in the Milky Way
Get PDFWe present a study of dense gas emission in the Milky Way in order to serve
as a basis for comparison with extragalactic results. This study combines new
observations of HCN, CS, and CO in individual GMCs and in the Milky Way plane
with published studies of emission from these molecules in the inner 500 pc of
the Milky Way. We find a strong trend in the fraction of emission from dense
gas tracers as a function of location in the Milky Way: in the bulge,
I_{HCN}/I_{CO} = 0.081 \pm 0.004, in the plane, I_{HCN}/I_{CO} = 0.026 \pm
0.008 on average, and over the full extent of nearby GMCs, I_{HCN}/I_{CO} =
0.014 \pm 0.020. Similar trends are seen in I_{CS}/I_{CO}. The low intensities
of the HCN and CS emission in the plane suggests that these lines are produced
by gas at moderate densities; they are thus not like the emission produced by
the dense, pc-scale star forming cores in nearby GMCs. The contrast between the
bulge and disk ratios in the Milky Way is likely to be caused by a combination
of higher kinetic temperatures as well as a higher dense gas fraction in the
bulge of the Milky Way.Comment: 34 pages LaTeX, AASTEX macros, includes 11 postscript figures. To
appear in ApJ 478, March 199
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