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The High-Density Ionized Gas in the Central Parsec of the Galaxy
We report a study of the line emission at 1.3 mm from the region around Sgr A* made with the Submillimeter Array at a resolution of 2'' over a field of 60'' (2 pc) and a velocity range of . This field encompasses most of the Galactic center's "minispiral." With an isothermal homogeneous H II model, we determined the physical conditions of the ionized gas at specific locations in the Northern and Eastern Arms from the line data along with Very Large Array data from the line at 3.6 cm and from the radio continuum emission at 1.3 cm. The typical electron density and kinetic temperature in the minispiral arms are and 5000-13,000 K, respectively. The and line profiles are broadened due to the large velocity shear within and along the beam produced by dynamical motions in the strong gravitational field near Sgr A*. We constructed a three-dimensional model of the minispiral using the orbital parameters derived under the assumptions that the gas flows are in Keplerian motion. The gas in the Eastern Arm appears to collide with the Northern Arm flow in the "Bar" region, which is located 0.1-0.2 pc south of and behind Sgr A*. Finally, a total Lyman continuum flux of photons is inferred from the assumption that the gas is photoionized and the ionizing photons for the high-density gas in the minispiral arms are from external sources, which is equivalent to ~250 O9-type zero-age-main-sequence stars.Astronom
Gas Kinematics on GMC scales in M51 with PAWS: cloud stabilization through dynamical pressure
We use the high spatial and spectral resolution of the PAWS CO(1-0) survey of
the inner 9 kpc of the iconic spiral galaxy M51 to examine the effect of gas
streaming motions on the star-forming properties of individual GMCs. We compare
our view of gas flows in M51 -- which arise due to departures from axi-symmetry
in the gravitational potential (i.e. the nuclear bar and spiral arms) -- with
the global pattern of star formation as traced by Halpha and 24\mu m emission.
We find that the dynamical environment of GMCs strongly affects their ability
to form stars, in the sense that GMCs situated in regions with large streaming
motions can be stabilized, while similarly massive GMCs in regions without
streaming go on to efficiently form stars. We argue that this is the result of
reduced surface pressure felt by clouds embedded in an ambient medium
undergoing large streaming motions, which prevents collapse. Indeed, the
variation in gas depletion time expected based on the observed streaming
motions throughout the disk of M51 quantitatively agrees with the variation in
observed gas depletion time scale. The example of M51 shows that streaming
motions, triggered by gravitational instabilities in the form of bars and
spiral arms, can alter the star formation law; this can explain the variation
in gas depletion time among galaxies with different masses and morphologies. In
particular, we can explain the long gas depletion times in spiral galaxies
compared to dwarf galaxies and starbursts. We suggest that adding a dynamical
pressure term to the canonical free-fall time produces a single star formation
law that can be applied to all star-forming regions and galaxies, across cosmic
time.Comment: 28 pages, 14 figures, accepted for publication in Ap
The PdBI Arcsecond Whirlpool Survey (PAWS): Multi-phase cold gas kinematic of M51
The kinematic complexity and the favorable position of M51 on the sky make
this galaxy an ideal target to test different theories of spiral arm dynamics.
Taking advantage of the new high resolution PdBI Arcsecond Whirlpool Survey
(PAWS) data, we undertake a detailed kinematic study of M51 to characterize and
quantify the origin and nature of the non-circular motions. Using a tilted-ring
analysis supported by several other archival datasets we update the estimation
of M51's position angle (PA=(173 +/- 3) deg) and inclination (i=(22 +/- 5)
deg). Harmonic decomposition of the high resolution (40 pc) CO velocity field
shows the first kinematic evidence of an m=3 wave in the inner disk of M51 with
a corotation at R(CR,m=3)=1.1 +/- 0.1 kpc and a pattern speed of Omega_p(m=3) =
140 km/(s kpc). This mode seems to be excited by the nuclear bar, while the
beat frequencies generated by the coupling between the m=3 mode and the main
spiral structure confirm its density-wave nature. We observe also a signature
of an m=1 mode that is likely responsible for the lopsidedness of M51 at small
and large radii. We provide a simple method to estimate the radial variation of
the amplitude of the spiral perturbation (Vsp) attributed to the different
modes. The main spiral arm structure has =50-70 km/s, while the streaming
velocity associated with the m=1 and m=3 modes is, in general, 2 times lower.
Our joint analysis of HI and CO velocity fields at low and high spatial
resolution reveals that the atomic and molecular gas phases respond differently
to the spiral perturbation due to their different vertical distribution and
emission morphology.Comment: 42 pages, 12 figures, accepted for publication in Ap
The PdBI Arcsecond Whirlpool Survey (PAWS): Environmental Dependence of Giant Molecular Cloud Properties in M51
Using data from the PdBI Arcsecond Whirlpool Survey (PAWS), we have generated
the largest extragalactic Giant Molecular Cloud (GMC) catalog to date,
containing 1,507 individual objects. GMCs in the inner M51 disk account for
only 54% of the total 12CO(1-0) luminosity of the survey, but on average they
exhibit physical properties similar to Galactic GMCs. We do not find a strong
correlation between the GMC size and velocity dispersion, and a simple virial
analysis suggests that 30% of GMCs in M51 are unbound. We have analyzed the GMC
properties within seven dynamically-motivated galactic environments, finding
that GMCs in the spiral arms and in the central region are brighter and have
higher velocity dispersions than inter-arm clouds. Globally, the GMC mass
distribution does not follow a simple power law shape. Instead, we find that
the shape of the mass distribution varies with galactic environment: the
distribution is steeper in inter-arm region than in the spiral arms, and
exhibits a sharp truncation at high masses for the nuclear bar region. We
propose that the observed environmental variations in the GMC properties and
mass distributions are a consequence of the combined action of large-scale
dynamical processes and feedback from high mass star formation. We describe
some challenges of using existing GMC identification techniques for decomposing
the 12CO(1-0) emission in molecule-rich environments, such as M51's inner disk.Comment: 73 pages, 18 figures, 14 tables, accepted for publication in Ap
A Comparative Study of Giant Molecular Clouds in M51, M33 and the Large Magellanic Cloud
We compare the properties of giant molecular clouds (GMCs) in M51 identified
by the Plateau de Bure Interferometer Whirlpool Arcsecond Survey (PAWS) with
GMCs identified in wide-field, high resolution surveys of CO emission in M33
and the Large Magellanic Cloud (LMC). We find that GMCs in M51 are larger,
brighter and have higher velocity dispersions relative to their size than
equivalent structures in M33 and the LMC. These differences imply that there
are genuine variations in the average mass surface density of the different GMC
populations. To explain this, we propose that the pressure in the interstellar
medium surrounding the GMCs plays a role in regulating their density and
velocity dispersion. We find no evidence for a correlation between size and
linewidth in any of M51, M33 or the LMC when the CO emission is decomposed into
GMCs, although moderately robust correlations are apparent when regions of
contiguous CO emission (with no size limitation) are used. Our work
demonstrates that observational bias remains an important obstacle to the
identification and study of extragalactic GMC populations using CO emission,
especially in molecule-rich galactic environments.Comment: 25 pages, 11 figures, accepted for publication in ApJ. Uses
emulateapj LaTeX macros. For more information on PAWS, further papers and
data, see http://www.mpia.de/PAWS
The PdBI Arcsecond Whirlpool Survey (PAWS). I. A Cloud-Scale/Multi-Wavelength View of the Interstellar Medium in a Grand-Design Spiral Galaxy
The PdBI (Plateau de Bure Interferometer) Arcsecond Whirlpool Survey (PAWS)
has mapped the molecular gas in the central ~9kpc of M51 in its 12CO(1-0) line
emission at cloud-scale resolution of ~40pc using both IRAM telescopes. We
utilize this dataset to quantitatively characterize the relation of molecular
gas (or CO emission) to other tracers of the interstellar medium (ISM), star
formation and stellar populations of varying ages. Using 2-dimensional maps, a
polar cross-correlation technique and pixel-by-pixel diagrams, we find: (a)
that (as expected) the distribution of the molecular gas can be linked to
different components of the gravitational potential, (b) evidence for a
physical link between CO line emission and radio continuum that seems not to be
caused by massive stars, but rather depend on the gas density, (c) a close
spatial relation between the PAH and molecular gas emission, but no predictive
power of PAH emission for the molecular gas mass,(d) that the I-H color map is
an excellent predictor of the distribution (and to a lesser degree the
brightness) of CO emission, and (e) that the impact of massive (UV-intense)
young star-forming regions on the bulk of the molecular gas in central ~9kpc
can not be significant due to a complex spatial relation between molecular gas
and star-forming regions that ranges from co-spatial to spatially offset to
absent. The last point, in particular, highlights the importance of galactic
environment -- and thus the underlying gravitational potential -- for the
distribution of molecular gas and star formation.Comment: 52 pages, 14 figures, 2 tables, (several minor typos corrected)
accepted by ApJ, high resolution version available, see
http://www.mpia.de/PAWS/pub/paws_schinnerer.pdf ; for more information on
PAWS, further papers and the data, see http://www.mpia.de/PAW
Short GMC lifetimes: an observational estimate with the PdBI Arcsecond Whirlpool Survey (PAWS)
We describe and execute a novel approach to observationally estimate the
lifetimes of giant molecular clouds (GMCs). We focus on the cloud population
between the two main spiral arms in M51 (the inter-arm region) where cloud
destruction via shear and star formation feedback dominates over formation
processes. By monitoring the change in GMC number densities and properties from
one side of the inter-arm to the other, we estimate the lifetime as a fraction
of the inter-arm travel time. We find that GMC lifetimes in M51's inter-arm are
finite and short, 20 to 30 Myr. Such short lifetimes suggest that cloud
evolution is influenced by environment, in which processes can disrupt GMCs
after a few free-fall times. Over most of the region under investigation shear
appears to regulate the lifetime. As the shear timescale increases with
galactocentric radius, we expect cloud destruction to switch primarily to star
formation feedback at larger radii. We identify a transition from shear- to
feedback-dominated disruption through a change in the behavior of the GMC
number density. The signature suggests that shear is more efficient at
completely dispersing clouds, whereas feedback transforms the population, e.g.
by fragmenting high mass clouds into lower mass pieces. Compared to the
characteristic timescale for molecular hydrogen in M51, our short lifetimes
suggest that gas can remain molecular while clouds disperse and reassemble. We
propose that galaxy dynamics regulates the cycling of molecular material from
diffuse to bound (and ultimately star-forming) objects, contributing to long
observed molecular depletion times in normal disk galaxies. We also speculate
that, in more extreme environments such as elliptical galaxies and concentrated
galaxy centers, star formation can be suppressed when the shear timescale
becomes so short that some clouds can not survive to collapse and form stars.Comment: 16 pages, 8 figures, accepted for publication in Ap
The High-Density Ionized Gas in the Central Parsecs of the Galaxy
We report the results from observations of H30 line emission in Sgr A
West with the Submillimeter Array at a resolution of 2\arcsec and a field of
view of about 40\arcsec. The H30 line is sensitive to the high-density
ionized gas in the minispiral structure. We compare the velocity field obtained
from H30 line emission to a Keplerian model, and our results suggest
that the supermassive black hole at Sgr A* dominates the dynamics of the
ionized gas. However, we also detect significant deviations from the Keplerian
motion, which show that the impact of strong stellar winds from the massive
stars along the ionized flows and the interaction between Northern and Eastern
arms play significant roles in the local gas dynamics.Comment: 4 pages, 2 figure
Probability Distribution Functions OF 12CO(J = 1-0) Brightness and Integrated Intensity in M51: The PAWS View
We analyse the distribution of CO brightness temperature and integrated
intensity in M51 at ~40 pc resolution using new CO data from the Plateau de
Bure Arcsecond Whirlpool Survey (PAWS). We present probability distribution
functions (PDFs) of the CO emission within the PAWS field, which covers the
inner 11 x 7 kpc of M51. We find variations in the shape of CO PDFs within
different M51 environments, and between M51 and M33 and the Large Magellanic
Cloud (LMC). Globally, the PDFs for the inner disk of M51 can be represented by
narrow lognormal functions that cover 1 to 2 orders of magnitude in CO
brightness and integrated intensity. The PDFs for M33 and the LMC are narrower
and peak at lower CO intensities. However, the CO PDFs for different dynamical
environments within the PAWS field depart from the shape of the global
distribution. The PDFs for the interarm region are approximately lognormal, but
in the spiral arms and central region of M51, they exhibit diverse shapes with
a significant excess of bright CO emission. The observed environmental
dependence of the shape of the CO PDFs is qualitatively consistent with changes
that would be expected if molecular gas in the spiral arms has a larger range
of average densities, gas temperatures and velocity fluctuations, though
further work is required to disentangle the importance of large-scale dynamical
effects versus star formation feedback in regulating these properties. We show
that the shape of the CO PDFs for different M51 environments is only weakly
related to global properties of the CO emission, but is strongly correlated
with some properties of the local giant molecular cloud (GMC) and young stellar
cluster populations. For galaxies with strong spiral structure such as M51, our
results indicate that galactic-scale dynamical processes play a significant
role in the formation and evolution of GMCs and stellar clusters.(abridged)Comment: 30 pages, 18 figures, accepted to ApJ PAWS special issu
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