36 research outputs found
Extended Molecular Gas in the Nearby Starburst Galaxy Maffei 2
We present a 9'x9' fully-sampled map of the CO J=1-0 emission in the nearby
starburst galaxy Maffei 2 obtained at the Five College Radio Astronomy
Observatory. The map reveals previously known strong CO emission in the central
starburst region as well as an extended asymmetric distribution with bright CO
lines at the ends of the bar and in a feature at the north-east edge of the
molecular disk. This northern feature, proposed previously to be an interacting
companion galaxy, could be a dwarf irregular galaxy, although the CO data are
also consistent with the feature being simply an extension of one of the spiral
arms. We estimate the total molecular gas mass of Maffei 2 to be (1.4-1.7)x10^9
Mo or ~3-4% of its dynamical mass. Adopting the recently determined lower value
for the CO-to-H2 conversion factor in the central region, our data lead to the
surprising result that the largest concentrations of molecular gas in Maffei 2
lie at the bar ends and in the putative dwarf companion rather than in the
central starburst. A gravitational stability analysis reveals that the extended
disk of Maffei 2 lies above the critical density for star formation; however,
whether the central region is also gravitationally unstable depends both on the
details of the rotation curve and the precise value of the CO-to-H2 conversion
factor in this region.Comment: accepted to ApJ (Sept 10 2004 issue
Globular Cluster Formation in M82
We present high resolution mid-infrared (mid-IR; 11.7 and 17.65 micron) maps
of the central 400 pc region of the starburst galaxy M82. Seven star forming
clusters are identified which together provide ~ 15% of the total mid-IR
luminosity of the galaxy. Combining the mid-IR data with thermal radio
measurements and near- and mid-IR line emission, we find that these young
stellar clusters have inferred masses and sizes comparable to globular
clusters. At least 20% of the star formation in M82 is found to occur in
super-star clusters.Comment: 12 pages including three color figures; accepted for publication in
Ap
Detailed Radio Spectra of Selected Compact Sources in the Nucleus of M82
We have determined detailed radio spectra for 26 compact sources in the
starburst nucleus of M82, between 74 and 1.3 cm. Seventeen show low-frequency
turnovers. One other has a thermal emission spectrum, and we identify it as an
HII region. The low frequency turnovers are due to absorption by the
interstellar gas in M82. New information on the AGN candidate 44.01+595, shows
it to have a non-thermal falling powerlaw spectrum at the highest frequencies,
and that it is strongly absorbed below 2 GHz. We derive large magnetic fields
in the supernova remnants, of order 1-2 milliGauss, hence large pressures in
the sources suggest that the brightest ones are either expanding or are
strongly confined by a dense interstellar medium. From the largest source in
our sample, we derive a supernova rate of 0.016 SN/yr.Comment: 19 pages, 7 tables, 29 figures, LaTeX, requires AAS macros v. 4.0. To
appear in ApJ July 20, 199
An X-ray Mini-survey of Nearby Edge-on Starburst Galaxies II. The Question of Metal Abundance
(abbreviated) We have undertaken an X-ray survey of a far-infrared flux
limited sample of seven nearby edge-on starburst galaxies. Here, we examine the
two X-ray-brightest sample members NGC 253 and M 82 in a self-consistent
manner, taking account of the spatial distribution of the X-ray emission in
choosing our spectral models. There is significant X-ray absorption in the disk
of NGC 253. When this is accounted for we find that multi-temperature thermal
plasma models with significant underlying soft X-ray absorption are more
consistent with the imaging data than single-temperature models with highly
subsolar abundances or models with minimal absorption and non-equilibrium
thermal ionization conditions. Our models do not require absolute abundances
that are inconsistent with solar values or unusually supersolar ratios of the
alpha-burning elements with respect to Fe (as claimed previously). We conclude
that with current data, the technique of measuring abundances in starburst
galaxies via X-ray spectral modeling is highly uncertain.
Based on the point-like nature of much of the X-ray emission in the PSPC
hard-band image of NGC 253, we suggest that a significant fraction of the
``extended'' X-ray emission in the 3-10 keV band seen along the disk of the
galaxy with ASCA and BeppoSAX (Cappi et al.) is comprised of discrete sources
in the disk, as opposed to purely diffuse, hot gas. This could explain the low
Fe abundances of ~1/4 solar derived for pure thermal models.Comment: (accepted for publication in the Astrophysical Journal
A Search for Extraplanar Dust in Nearby Edge-On Spirals
We present high resolution BV images of 12 edge-on spiral galaxies observed
with the WIYN 3.5-m telescope. These images were obtained to search for
extraplanar (|z| > 0.4 kpc) absorbing dust structures similar to those
previously found in NGC 891 (Howk & Savage 1997). Our imaged galaxies include a
sample of seven massive L_*-like spiral galaxies within D<25 Mpc that have
inclinations i > 87 deg from the plane of the sky. We find that five of these
seven systems show extraplanar dust, visible as highly-structured absorbing
clouds against the background stellar light of the galaxies. The more prominent
structures are estimated to have associated gas masses >10^5 M_sun; the implied
potential energies are > 10^(52) ergs. All of the galaxies in our sample that
show detectable halpha emission at large z also show extraplanar dust
structures. None of those galaxies for which extraplanar halpha searches were
negative show evidence for extensive high-z dust. The existence of extraplanar
dust is a common property of massive spiral galaxies. We discuss several
mechanisms for shaping the observed dust features, emphasizing the possibility
that these dusty clouds represent the dense phase of a multiphase medium at
high-z in spiral galaxies. The correlation between high-z dust and extraplanar
Halpha emission may simply suggest that both trace the high-z interstellar
medium in its various forms (or phases), the existence of which may ultimately
be driven by vigorous star formation in the underlying disk. (Abstract
abridged)Comment: 26 pages; 15 jpeg figures. To appear in The Astronomical Journal, May
1999. Gzipped tar files of high-resolution figures in postscript and jpeg
formats are available at
http://www.astro.wisc.edu/~howk/Papers/papers.html#surve
Star Formation in M51 Triggered by Galaxy Interaction
We have mapped the inner 360'' regions of M51 in the 158micron [CII] line at
55'' spatial resolution using the Far-infrared Imaging Fabry-Perot
Interferometer (FIFI) on the Kuiper Airborne Observatory (KAO). The emission is
peaked at the nucleus, but is detectable over the entire region mapped, which
covers much of the optical disk of the galaxy. There are also two strong
secondary peaks at ~43% to 70% of the nuclear value located roughly 120'' to
the north-east, and south-west of the nucleus. These secondary peaks are at the
same distance from the nucleus as the corotation radius of the density wave
pattern. The density wave also terminates at this location, and the outlying
spiral structure is attributed to material clumping due to the interaction
between M51 and NGC5195. This orbit crowding results in cloud-cloud collisions,
stimulating star formation, that we see as enhanced [CII] line emission. The
[CII] emission at the peaks originates mainly from photodissociation regions
(PDRs) formed on the surfaces of molecular clouds that are exposed to OB
starlight, so that these [CII] peaks trace star formation peaks in M51. The
total mass of [CII] emitting photodissociated gas is ~2.6x10^{8} M_{sun}, or
about 2% of the molecular gas as estimated from its CO(1-0) line emission. At
the peak [CII] positions, the PDR gas mass to total gas mass fraction is
somewhat higher, 3-17%, and at the secondary peaks the mass fraction of the
[CII] emitting photodissociated gas can be as high as 72% of the molecular
mass.... (continued)Comment: 14 pages, 6 figures, Accepted in ApJ (for higher resolution figures
contact the author
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
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
OH Satellite Line Masers and an AGN Candidate in M82
We report the detection of OH satellite line masers at 1720 MHz and 1612 MHz
from the nuclear region of the starburst galaxy M82. The observations were
aimed at detecting 1720 MHz maser emission from the known radio emitting SNR in
the nuclear region. At 1720 MHz we detect six features above the 5-sigma limit
set by noise, four in emission and two in absorption. Three of the emission
features appear closely associated with known discrete continuum radio sources,
and one of the absorption features is precisely coincident with the discrete
continuum source 44.01+59.6. The latter source also exhibits strong 1612 MHz
emission at the same velocity. No other 1612 MHz features were detected. All of
the 1720 MHz emission features are consistent with masers pumped by collisions
with molecular hydrogen at densities between 10^3 cm^{-3} and 10^5 cm^{-3}, and
T_k between 50 K and 250 K. The absorption and emission associated with the two
satellite lines in 44.01+59.6, together with other evidence, points to the
possibility that this source is the AGN in M82.Comment: accepted ApJ
A 180 Kpc Tidal Tail in the Luminous Infrared Merger Arp 299
We present VLA HI observations and UH88 deep optical B- and R-band
observations of the IR luminous merger Arp 299 (= NGC 3690 + IC 694). These
data reveal a gas-rich, optically faint tidal tail with a length of over 180
kpc. The size of this tidal feature necessitates an old interaction age for the
merger (~750 Myr since first periapse), which is currently experiencing a very
young star burst (~20 Myr). The observations reveal a most remarkable structure
within the tidal tail: it appears to be composed of two parallel filaments
separated by ~20 kpc. One of the filaments is gas rich with little if any
starlight, while the other is gas poor. We believe that this bifurcation
results from a warped disk in one of the progenitors. The quantities and
kinematics of the tidal HI suggest that Arp 299 results from the collision of a
retrograde Sab-Sb galaxy (IC 694) and a prograde Sbc-Sc galaxy (NGC 3690) that
occurred 750 Myr ago and which will merge into a single object in ~60 Myr. We
suggest that the present IR luminous phase in this system is due in part to the
retrograde spin of IC 694. Finally, we discuss the apparent lack of tidal dwarf
galaxies within the tail.Comment: LaTex, 14 pages, 11 figures, 4 tables, uses emulateapj.sty. Accepted
to AJ for July 1999. For version with full-resolution images see
http://www.cv.nrao.edu/~jhibbard/a299/HIpaper/a299HI.htm