2,133 research outputs found
A Universal Neutral Gas Profile for Nearby Disk Galaxies
Based on sensitive CO measurements from HERACLES and HI data from THINGS, we
show that the azimuthally averaged radial distribution of the neutral gas
surface density (Sigma_HI + Sigma_H2) in 33 nearby spiral galaxies exhibits a
well-constrained universal exponential distribution beyond 0.2*r25 (inside of
which the scatter is large) with less than a factor of two scatter out to two
optical radii r25. Scaling the radius to r25 and the total gas surface density
to the surface density at the transition radius, i.e., where Sigma_HI and
Sigma_H2 are equal, as well as removing galaxies that are interacting with
their environment, yields a tightly constrained exponential fit with average
scale length 0.61+-0.06 r25. In this case, the scatter reduces to less than 40%
across the optical disks (and remains below a factor of two at larger radii).
We show that the tight exponential distribution of neutral gas implies that the
total neutral gas mass of nearby disk galaxies depends primarily on the size of
the stellar disk (influenced to some degree by the great variability of
Sigma_H2 inside 0.2*r25). The derived prescription predicts the total gas mass
in our sub-sample of 17 non-interacting disk galaxies to within a factor of
two. Given the short timescale over which star formation depletes the H2
content of these galaxies and the large range of r25 in our sample, there
appears to be some mechanism leading to these largely self-similar radial gas
distributions in nearby disk galaxies.Comment: 7 pages, 4 figures, accepted for publication in the Astrophysical
Journa
1420 MHz Continuum Absorption Towards Extragalactic Sources in the Galactic Plane
We present a 21-cm emission-absorption study towards extragalactic sources in
the Canadian Galactic Plane Survey (CGPS). We have analyzed HI spectra towards
437 sources with S > 150 mJy, giving us a source density of 0.6 sources per
square degree at arcminute resolution. We present the results of a first
analysis of the HI temperatures, densities, and feature statistics. Particular
emphasis is placed on 5 features with observed spin temperatures below 40 K. We
find most spin temperatures in the range from 40 K to 300 K. A simple HI
two-component model constrains the bulk of the cold component to temperatures
(T_c) between 40 K and 100 K. T_c peaks in the Perseus arm region and clearly
drops off with Galactocentric radius, R, beyond that. The HI density follows
this trend, ranging from a local value of 0.4 cm^{-3} to less than 0.1 cm^{-3}
at R = 20 kpc. We find that HI emission alone on average traces about 75% of
the total HI column density, as compared to the total inferred by the emission
and absorption. Comparing the neutral hydrogen absorption to CO emission no
correlation is found in general, but all strong CO emission is accompanied by a
visible HI spectral feature. Finally, the number of spectral HI absorption
features per kpc drop off exponentially with increasing R.Comment: 13 pages, 13 figures, Accepted for March 2004 Ap
Exploring cloudy gas accretion as a source of interstellar turbulence in the outskirts of disks
High--resolution 2D--MHD numerical simulations have been carried out to
investigate the effects of continuing infall of clumpy gas in extended HI
galactic disks. Given a certain accretion rate, the response of the disk
depends on its surface gas density and temperature. For Galactic conditions at
a galactocentric distance of ~20 kpc, and for mass accretion rates consistent
with current empirical and theoretical determinations in the Milky Way, the
rain of compact high velocity clouds onto the disk can maintain transonic
turbulent motions in the warm phase (~2500 K) of HI. Hence, the HI line width
is expected to be ~6.5 km/s for a gas layer at 2500 K, if infall were the only
mechanism of driving turbulence. Some statistical properties of the resulting
forcing flow are shown in this Letter. The radial dependence of the gas
velocity dispersion is also discussed.Comment: 13 pages, 3 figures, accepted for publication in ApJ Letter
Near-UV photolysis cross sections of CH_3OOH and HOCH_2OOH determined via action spectroscopy
Knowledge of molecular photolysis cross sections is important for determining atmospheric lifetimes and fates of many species. A method and laser apparatus for measurement of these cross sections in the near-ultraviolet (UV) region is described. The technique is based on action spectroscopy, where the yield of a photodissociation product (in this case OH) is measured as a function of excitation energy. For compounds yielding OH, this method can be used to measure near-UV photodissociation cross section as low as 10â23 cm2 moleculeâ1. The method is applied to determine the photodissociation cross sections for methyl hydroperoxide (CH3OOH; MHP) and hydroxymethyl hydroperoxide (HOCH2OOH; HMHP) in the 305â365 nm wavelength range. The measured cross sections are in good agreement with previous measurements of absorption cross sections
Generation of galactic disc warps due to intergalactic accretion flows onto the disc
A new method is developed to calculate the amplitude of the galactic warps
generated by a torque due to external forces. This takes into account that the
warp is produced as a reorientation of the different rings which constitute the
disc in order to compensate the differential precession generated by the
external force, yielding a uniform asymptotic precession for all rings.
Application of this method to gravitational tidal forces in the Milky Way due
to the Magellanic Clouds leads to a very low amplitude of the warp. If the
force were due to an extragalactic magnetic field, its intensity would have to
be very high, to generate the observed warps. An alternative hypothesis is
explored: the accretion of the intergalactic medium over the disk. A cup-shaped
distortion is expected, due to the transmission of the linear momentum; but,
this effect is small and the predominant effect turns out to be the
transmission of angular momentum, i.e. a torque giving an integral-sign shape
warp. The torque produced by a flow of velocity ~100 km/s and baryon density
\~10^{-25} kg/m^3 is enough to generate the observed warps and this mechanism
offers quite a plausible explanation. First, because this order of accretion
rate is inferred from other processes observed in the Galaxy, notably its
chemical evolution. The inferred rate of infall of matter, ~1 solar-mass/yr, to
the Galactic disc that this theory predicts agrees with the quantitative
predictions of this chemical evolution resolving key issues, notably the
G-dwarf problem. Second, because the required density of the intergalactic
medium is within the range of values compatible with observation. By this
mechanism, we can explain the warp phenomenon in terms of intergalactic
accretion flows onto the disk of the galaxy.Comment: 18 pages, 11 figures, accepted to be published in A&
The Effect of the Random Magnetic Field Component on the Parker Instability
The Parker instability is considered to play important roles in the evolution
of the interstellar medium. Most studies on the development of the instability
so far have been based on an initial equilibrium system with a uniform magnetic
field. However, the Galactic magnetic field possesses a random component in
addition to the mean uniform component, with comparable strength of the two
components. Parker and Jokipii have recently suggested that the random
component can suppress the growth of small wavelength perturbations. Here, we
extend their analysis by including gas pressure which was ignored in their
work, and study the stabilizing effect of the random component in the
interstellar gas with finite pressure. Following Parker and Jokipii, the
magnetic field is modeled as a mean azimuthal component, , plus a random
radial component, , where is a random function
of height from the equatorial plane. We show that for the observationally
suggested values of , the tension due to the random
component becomes important, so that the growth of the instability is either
significantly reduced or completely suppressed. When the instability still
works, the radial wavenumber of the most unstable mode is found to be zero.
That is, the instability is reduced to be effectively two-dimensional. We
discuss briefly the implications of our finding.Comment: 10 pages including 2 figures, to appear in The Astrophysical Journal
Letter
Gravitational scattering of stars and clusters and the heating of the Galactic disk
Could the velocity spread, increasing with time, in the Galactic disk be
explained as a result of gravitational interactions of stars with giant
molecular clouds (GMCs) and spiral arms? Do the old open clusters high above
the Galactic plane provide clues to this question? We explore the effects on
stellar orbits of scattering by inhomogeneities in the Galactic potential due
to GMCs, spiral arms and the Galactic bar, and whether high-altitude clusters
could have formed in orbits closer to the Galactic plane and later been
scattered.
Simulations of test-particle motions are performed in a realistic Galactic
potential. The effects of the internal structure of GMCs are explored. The
destruction of clusters in GMC collisions is treated in detail with N-body
simulations of the clusters.
The observed velocity dispersions of stars as a function of time are well
reproduced. The GMC structure is found to be significant, but adequate models
produce considerable scattering effects. The fraction of simulated massive old
open clusters, scattered into orbits with |z| > 400 pc, is typically 0:5%, in
agreement with the observed number of high-altitude clusters and consistent
with the present formation rate of massive open clusters.
The heating of the thin Galactic disk is well explained by gravitational
scattering by GMCs and spiral arms, if the local correlation between the GMC
mass and the corresponding voids in the gas is not very strong. Our results
suggest that the high-altitude metal-rich clusters were formed in orbits close
to the Galactic plane and later scattered to higher orbits. It is possible,
though not very probable, that the Sun formed in such a cluster before
scattering occurred.Comment: 19 pages, 15 figure
Milky Way Kinematics: Measurements at the Subcentral Point of the Fourth Quadrant
We use atomic hydrogen (HI) data from the Southern Galactic Plane Survey to
study the kinematics of the fourth quadrant of the Milky Way. By measuring the
terminal velocity as a function of longitude throughout the fourth Galactic
quadrant we have derived the most densely sampled rotation curve available for
the Milky Way between 3 < R < 8 kpc. We determine a new joint rotation curve
fit for the first and fourth quadrants, which can be used for kinematic
distances interior to the Solar circle. From our data we place new limits on
the peak to peak variation of streaming motions in the fourth quadrant to be
~10 km/s. We show that the shape of the average HI profile beyond the terminal
velocity is consistent with gas of three velocity dispersions, a cold component
with km/s, a warmer component with km/s and a
fast component with km/s. Examining the widths with Galactic
radius we find that the narrowest two components show little variation with
radius and their small scale fluctuations track each other very well,
suggesting that they share the same cloud-to-cloud motions. The width of the
widest component is constant until R<4 kpc, where it increases sharply.Comment: 36 pages, 10 figures, accepted to ApJ. Full electronic version of
table 1 available at
ftp://ftp.atnf.csiro.au/pub/people/nmcclure/papers/velocity_tab1.te
Astrophysical Fractals: Interstellar Medium and Galaxies
The interstellar medium is structured as a hierachy of gas clouds, that looks
self-similar over 6 orders of magnitude in scales and 9 in masses. This is one
of the more extended fractal in the Universe. At even larger scales, the
ensemble of galaxies looks also self-similar over a certain ranges of scales,
but more limited, may be over 3-4 orders of magnitude in scales. These two
fractals appear to be characterized by similar Hausdorff dimensions, between
1.6 and 2. The various interpretations of these structures are discussed, in
particular formation theories based on turbulence and self-gravity. In the
latter, the fractal ensembles are considered in a critical state, as in second
order phase transitions, when large density fluctuations are observed, that
also obey scaling laws, and look self-similar over an extended range.Comment: 30 pages, 6 figures, Proceedings of "The Chaotic Universe", Roma
colloquium, 1-5 Feb 99, World Scientific Advanced Series in Astrophysics and
Cosmology, ed. V. Gurzadyan, Li-Zhi Fang and Remo Ruffin
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