336 research outputs found
Planetary Nebulae in Face-On Spiral Galaxies. III. Planetary Nebula Kinematics and Disk Mass
Much of our understanding of dark matter halos comes from the assumption that
the mass-to-light ratio (M/L) of spiral disks is constant. The best way to test
this hypothesis is to measure the disk surface mass density directly via the
kinematics of old disk stars. To this end, we have used planetary nebulae (PNe)
as test particles and have measured the vertical velocity dispersion (sigma_z)
throughout the disks of five nearby, low-inclination spiral galaxies: IC 342,
M74 (NGC 628), M83 (NGC 5236), M94 (NGC 4736), and M101 (NGC 5457). By using HI
to map galactic rotation and the epicyclic approximation to extract sigma_z
from the line-of-sight dispersion, we find that, with the lone exception of
M101, our disks do have a constant M/L out to ~3 optical scale lengths.
However, once outside this radius, sigma_z stops declining and becomes flat
with radius. Possible explanations for this behavior include an increase in the
disk mass-to-light ratio, an increase in the importance of the thick disk, and
heating of the thin disk by halo substructure. We also find that the disks of
early type spirals have higher values of M/L and are closer to maximal than the
disks of later-type spirals, and that the unseen inner halos of these systems
are better fit by pseudo-isothermal laws than by NFW models.Comment: 18 pages, 15 figures, 5 tables; accepted to Ap
Molecular gas in the galaxy M83. I - The molecular gas distribution
We present CO(1-0) and CO(2-1) Swedish-ESO Submillimetre Telescope (SEST)
observations of the barred spiral galaxy M83 (NGC5236). The maps cover the
entire optical disk. The CO emission is strongly peaked toward the nucleus,
which breaks up into two separate components in the CO(2-1) data due to the
higher spatial resolution. Emission from the bar is strong, in particular on
the leading edges of the bar. The molecular gas arms are clearly resolved and
can be traced for more than 360\degr . Emission in the inter-arm regions is
detected. The average CO CO(2-1)/CO(1-0) line ratio is 0.77. The ratio is lower
than this on the spiral arms and higher in the inter-arm regions. The arms show
regularly spaced concentrations of molecular gas, Giant Molecular Associations
(GMA's), whose masses are of the order 10^7 Msun. The total molecular gas mass
is estimated to be 3.9*10^9 Msun. This mass is comparable to the total HI mass,
but H_2 dominates in the optical disk. In the disk, H_2 and HI show very
similar distributions, including small scale clumping. We compare the molecular
gas distribution with those of other star formation tracers, such as B and
H_alpha images.Comment: 20 pages, 15 figures, A&A accepted. A higher resolution version
available at http://www.astro.su.se/~andreas/publications
Molecular Gas in Spiral Galaxies
In this review, I highlight a number of recent surveys of molecular gas in
nearby spiral galaxies. Through such surveys, more complete observations of the
distribution and kinematics of molecular gas have become available for galaxies
with a wider range of properties (e.g., brightness, Hubble type, strength of
spiral or bar structure). These studies show the promise of both
interferometers and single-dish telescopes in advancing our general
understanding of molecular gas in spiral galaxies. In particular, I highlight
the contributions of the recent BIMA Survey of Nearby Galaxies (SONG).Comment: 8 pages, 1 figure. To appear in the proceedings of the 4th
Cologne-Bonn-Zermatt-Symposium, "The Dense Interstellar Medium in Galaxies",
which was held in Zermatt, Switzerland in September 200
Low, Milky-Way like, Molecular Gas Excitation of Massive Disk Galaxies at z~1.5
We present evidence for Milky-Way-like, low-excitation molecular gas
reservoirs in near-IR selected massive galaxies at z~1.5, based on IRAM Plateau
de Bure Interferometer CO[3-2] and NRAO Very Large Array CO[1-0] line
observations for two galaxies that had been previously detected in CO[2-1]
emission. The CO[3-2] flux of BzK-21000 at z=1.522 is comparable within the
errors to its CO[2-1] flux, implying that the CO[3-2] transition is
significantly sub-thermally excited. The combined CO[1-0] observations of the
two sources result in a detection at the 3 sigma level that is consistent with
a higher CO[1-0] luminosity than that of CO[2-1]. Contrary to what is observed
in submillimeter galaxies and QSOs, in which the CO transitions are thermally
excited up to J>=3, these galaxies have low-excitation molecular gas, similar
to that in the Milky Way and local spirals. This is the first time that such
conditions have been observed at high redshift. A Large Velocity Gradient
analysis suggests that molecular clouds with density and kinetic temperature
comparable to local spirals can reproduce our observations. The similarity in
the CO excitation properties suggests that a high, Milky-Way-like, CO to H_2
conversion factor could be appropriate for these systems. If such
low-excitation properties are representative of ordinary galaxies at high
redshift, centimeter telescopes such as the Expanded Very Large Array and the
longest wavelength Atacama Large Millimeter Array bands will be the best tools
for studying the molecular gas content in these systems through the
observations of CO emission lines.Comment: 5 pages, 4 figures. ApJ Letters in pres
CO(1-0), CO(2-1) and Neutral Gas in NGC 6946: Molecular Gas in a Late-Type, Gas Rich, Spiral Galaxy
We present "On The Fly" maps of the CO(1-0) and CO(2-1) emission covering a
10' X 10' region of the NGC 6946. Using our CO maps and archival VLA HI
observations we create a total gas surface density map, Sigma_gas, for NGC
6946. The predominantly molecular inner gas disk transitions smoothly into an
atomic outer gas disk, with equivalent atomic and molecular gas surface
densities at R = 3.5' (6 kpc). We estimate that the total H2 mass is 3 X 10^9
Mo, roughly 1/3 of the interstellar hydrogen gas mass, and about 2% of the
dynamical mass of the galaxy at our assumed distance of 6 Mpc. The value of the
CO(2-1)/CO(1-0) line ratio ranges from 0.35 to 2; 50% of the map is covered by
very high ratio, >1, gas. The very high ratios are predominantly from interarm
regions and appear to indicate the presence of wide-spread optically thin gas.
Star formation tracers are better correlated with the total neutral gas disk
than with the molecular gas by itself implying SFR is proportional to
Sigma_gas. Using the 100 FIR and 21 cm continuum from NGC 6946 as star
formation tracers, we arrive at a gas consumption timescale of 2.8 Gyr, which
is relatively uniform across the disk. The high star formation rate at the
nucleus appears to be due to a large accumulation of molecular gas rather than
a large increase in the star formation efficiency. The mid-plane gas pressure
in the outer (R > 10 kpc) HI arms of NGC 6946 is close to the value at the
radial limit (10 kpc) of our observed CO disk. If the mid-plane gas pressure is
a factor for the formation of molecular clouds, these outer HI gas arms should
contain molecular gas which we do not see because they are beyond our detection
limit
Nuclear Bar Catalyzed Star Formation: 13^CO, C18^O and Molecular Gas Properties in the Nucleus of Maffei 2
(Abridged) We present resolution maps of CO, its isotopologues, and HCN from
in the center of Maffei 2. The J=1-0 rotational lines of 12^CO, 13^CO, C18^O
and HCN, and the J=2-1 lines of 13^CO and C18^O were observed with the OVRO and
BIMA arrays. The 2-1/1-0 line ratios of the isotopologues constrain the bulk of
the molecular gas to originate in low excitation, subthermal gas. From LVG
modeling, we infer that the central GMCs have n(H_2) ~10^2.75 cm^-3 and T_k ~
30 K. Continuum emission at 3.4 mm, 2.7 mm and 1.4 mm was mapped to determine
the distribution and amount of HII regions and dust. Column densities derived
from C18^O and 1.4 mm dust continuum fluxes indicate the CO conversion factor
in the center of Maffei 2 is lower than Galactic by factors of ~2-4. Gas
morphology and the clear ``parallelogram'' in the Position-Velocity diagram
shows that molecular gas orbits within the potential of a nuclear (~220 pc)
bar. The nuclear bar is distinct from the bar that governs the large scale
morphology of Maffei 2. Giant molecular clouds in the nucleus are nonspherical
and have large linewidths. Dense gas and star formation are concentrated at the
sites of the x_1-x_2 orbit intersections of the nuclear bar, suggesting that
the starburst is dynamically triggered.Comment: 50 pages, 14 figures, accepted for publication in Ap
'A new kind of rays': gothic fears, cultural anxieties and the discovery of X-rays in the 1890s
In 1895, the world of modern physics was effectively ushered in with the discovery of X-rays by the German physicist, Wilhelm Conrad Röntgen. X-rays rapidly changed the ways in which the human body was perceived, and their discovery was documented and fiercely debated in scientific articles, newspaper reports, literary writings, cartoons and films. This article examines a range of these responses, both 'scientific' and 'popular', and considers the particular significance of their repeated recourse to the Gothic and the uncanny as a means of expressing both excitement and disquiet at what the new X-ray phenomenon might mean
Evidence for Environmentally Dependent Cluster Disruption in M83
Using multi-wavelength imaging from the Wide Field Camera 3 on the Hubble
Space Telescope we study the stellar cluster populations of two adjacent fields
in the nearby face-on spiral galaxy, M83. The observations cover the galactic
centre and reach out to ~6 kpc, thereby spanning a large range of environmental
conditions, ideal for testing empirical laws of cluster disruption. The
clusters are selected by visual inspection to be centrally concentrated,
symmetric, and resolved on the images. We find that a large fraction of objects
detected by automated algorithms (e.g. SExtractor or Daofind) are not clusters,
but rather are associations. These are likely to disperse into the field on
timescales of tens of Myr due to their lower stellar densities and not due to
gas expulsion (i.e. they were never gravitationally bound). We split the sample
into two discrete fields (inner and outer regions of the galaxy) and search for
evidence of environmentally dependent cluster disruption. Colour-colour
diagrams of the clusters, when compared to simple stellar population models,
already indicate that a much larger fraction of the clusters in the outer field
are older by tens of Myr than in the inner field. This impression is quantified
by estimating each cluster's properties (age, mass, and extinction) and
comparing the age/mass distributions between the two fields. Our results are
inconsistent with "universal" age and mass distributions of clusters, and
instead show that the ambient environment strongly affects the observed
populations.Comment: 6 pages, 3 figures, MNRAS in pres
The Long-Term Variability of the X-Ray Sources in NGC 6946 and NGC 4485/4490
We analyze data from five Chandra observations of the spiral galaxy NGC 6946
and from three Chandra observations of the irregular/spiral interacting galaxy
pair NGC 4485/4490, with an emphasis on investigating the long-term variability
exhibited by the source populations. We detect 90 point sources coincident with
NGC 6946 down to luminosities of a few times 10^36 erg/s, and 38 sources
coincident with NGC 4485/90 down to a luminosity of ~1x10^37 erg/s. Twenty-five
(15) sources in NGC 6946 (NGC 4485/90) exhibit long-term (i.e., weeks to years)
variability in luminosity; 11 (4) are transient candidates. The single
ultraluminous X-ray source (ULX) in NGC 6946 and all but one of the eight ULXs
in NGC 4485/90 exhibit long-term flux variability. Two of the ULXs in NGC
4485/90 have not been identified before as ultraluminous sources. The
widespread variability in both systems is indicative of the populations being
dominated by X-ray binaries, and this is supported by the X-ray colors of the
sources. The distribution of colors among the sources indicates a large
fraction of high-mass X-ray binaries in both systems. The shapes of the X-ray
luminosity functions of the galaxies do not change significantly between
observations and can be described by power laws with cumulative slopes ~0.6-0.7
(NGC 6946) and ~0.4 (NGC 4485/90).Comment: 26 pages, 9 figures, 15 tables - to appear in the August 2008 issue
of ApJS - new version corrects a few typo
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