2,244 research outputs found
Towards the Secondary Bar: Gas Morphology and Dynamics in NGC 4303
The bulk of the molecular line emission in the double barred galaxy NGC4303
as observed in its CO(1-0) line with the OVRO mm-interferometer comes from two
straight gas lanes which run north-south along the leading sides of the
large-scale primary bar. Inside a radius of ~ 400 pc the molecular gas forms a
spiral pattern which, for the northern arm, can be traced to the nucleus.
Comparison of the OVRO and archival HST data with dynamical models of gas flow
in the inner kiloparsec of single- and double-barred galaxies shows that the
observed global properties of the molecular gas are in agreement with models
for the gas flow in a strong, large-scale bar, and the two-arm spiral structure
seen in CO in the inner kiloparsec can already be explained by a density wave
initiated by the potential of that bar. Only a weak correlation between the
molecular gas distribution and the extinction seen in the HST V-H map is found
in the inner 400 pc of NGC4303: The innermost part of one arm of the nuclear CO
spiral correlates with a weak dust filament in the color map, while the overall
dust distribution follows a ring or single-arm spiral pattern well correlated
with the UV continuum. This complicated nuclear geometry of the stellar and
gaseous components allows for two scenarios: (A) A self-gravitating m=1 mode is
present forming the spiral structure seen in the UV continuum. In this case the
gas kinematics would be unaffected by the small (~ 4'') inner bar. (B) The UV
continuum traces a complete ring which is heavily extincted north of the
nucleus. Such a ring forms in hydrodynamic models of double bars, but the
models cannot account for the UV emission observed on the leading side of the
inner bar. (abridged)Comment: 47 pages, 14 figures, accepted for publication in Ap
HCN Survey of Normal Spiral, IR-luminous and Ultraluminous Galaxies
We report systematic HCN J=1-0 (and CO) observations of a sample of 53
infrared (IR) and/or CO-bright and/or luminous galaxies, including seven
ultraluminous infrared galaxies, nearly 20 luminous infrared galaxies, and more
than a dozen of the nearest normal spiral galaxies. This is the largest and
most sensitive HCN survey of galaxies to date. All galaxies observed so far
follow the tight correlation between the IR luminosity and the HCN
luminosity initially proposed by Solomon, Downes, & Radford,
which is detailed in a companion paper. We also address here the issue of HCN
excitation. There is no particularly strong correlation between
and the 12m luminosity; in fact, of all the four \IRAS bands, the 12m
luminosity has the weakest correlation with the HCN luminosity. There is also
no evidence of stronger HCN emission or a higher ratio of HCN and CO
luminosities for galaxies with excess 12m
emission. This result implies that mid-IR radiative pumping, or populating, of
the J=1 level of HCN by a mid-IR vibrational transition is not important
compared with the collisional excitation by dense molecular hydrogen.
Furthermore, large velocity gradient calculations justify the use of HCN J=1-0
emission as a tracer of high-density molecular gas (\approxgt 3\times
10^4/\tau cm) and give an estimate of the mass of dense molecular gas
from HCN observations. Therefore, may be used as a measure of the
total mass of dense molecular gas, and the luminosity ratio may indicate the fraction of molecular gas that is dense.Comment: ApJS, May issue (final version, 12 pages + 5 figures + 4 tables).
Fig. 2a,b,c and Fig. 3a,b are in GIF format due to the space limitation of
astro-ph. Added an error-bar in both Fig. 4 and Fig. 5a. Please see the
companion paper by Gao & Solomon (Paper II, astro-ph/0310339) for the
detailed analysis and implications of this HCN surve
Stellar Evolutionary Effects on the Abundances of PAH and SN-Condensed Dust in Galaxies
Spectral and photometric observations of nearby galaxies show a correlation
between the strength of their mid-IR aromatic features, attributed to PAH
molecules, and their metal abundance, leading to a deficiency of these features
in low-metallicity galaxies. In this paper, we suggest that the observed
correlation represents a trend of PAH abundance with galactic age, reflecting
the delayed injection of carbon dust into the ISM by AGB stars in the final
post-AGB phase of their evolution. AGB stars are the primary sources of PAHs
and carbon dust in galaxies, and recycle their ejecta back to the interstellar
medium only after a few hundred million years of evolution on the main
sequence. In contrast, more massive stars that explode as Type II supernovae
inject their metals and dust almost instantaneously after their formation. We
first determined the PAH abundance in galaxies by constructing detailed models
of UV-to-radio SED of galaxies that estimate the contribution of dust in
PAH-free HII regions, and PAHs and dust from photodissociation regions, to the
IR emission. All model components: the galaxies' stellar content, properties of
their HII regions, and their ionizing and non-ionizing radiation fields and
dust abundances, are constrained by their observed multiwavelength spectrum.
After determining the PAH and dust abundances in 35 nearby galaxies using our
SED model, we use a chemical evolution model to show that the delayed injection
of carbon dust by AGB stars provides a natural explanation to the dependence of
the PAH content in galaxies with metallicity. We also show that larger dust
particles giving rise to the far-IR emission follow a distinct evolutionary
trend closely related to the injection of dust by massive stars into the ISM.Comment: ApJ, 69 pages, 46 figures, Accepte
Molecular Gas in M82: Resolving the Outflow and Streamers
We present a high-resolution (3.6'', 70pc) CO(1-0) mosaic of the molecular
gas in M 82 covering an area of 2.5' x 3.5' (2.8kpc x 3.9kpc) obtained with the
OVRO millimeter interferometer. The observations reveal the presence of huge
amounts of molecular gas (> 70% of the total molecular mass, M_tot=1.3 x 10^9
M_sun) outside the central 1 kpc disk. Molecular streamers are detected in and
below M82's disk out to distances from the center of 1.7 kpc. Some of these
streamers are well correlated with optical absorption features; they form the
basis of some of the prominent tidal HI features around M 82. This provides
evidence that the molecular gas within M 82's optical disk is disrupted by the
interaction with M 81. Molecular gas is found in M 82's outflow/halo, reaching
distances up to 1.2 kpc below the plane; CO line-splitting has been detected
for the first time in the outflow. The maximum outflow velocity is 230 km/s; we
derive an opening angle of 55 deg for the molecular outflow cone. The total
amount of gas in the outflow is >3 x 10^8 M_sun and its kinetic energy is of
order 10^55 erg, about one percent of the estimated total mechanical energy
input of M 82's starburst. Our study implies that extreme starburst
environments can move significant amounts of molecular gas in to a galaxy's
halo (and even to the intergalactic medium).Comment: accepted for publication in the ApJ Letters full PS file @
http://www.aoc.nrao.edu/~fwalter/walter_m82.p
Status of cosmic-ray antideuteron searches
The precise measurement of cosmic-ray antiparticles serves as important means
for identifying the nature of dark matter. Recent years showed that identifying
the nature of dark matter with cosmic-ray positrons and higher energy
antiprotons is difficult, and has lead to a significantly increased interest in
cosmic-ray antideuteron searches. Antideuterons may also be generated in dark
matter annihilations or decays, offering a potential breakthrough in unexplored
phase space for dark matter. Low-energy antideuterons are an important approach
because the flux from dark matter interactions exceeds the background flux by
more than two orders of magnitude in the low-energy range for a wide variety of
models. This review is based on the "dbar14 - dedicated cosmic-ray antideuteron
workshop", which brought together theorists and experimentalists in the field
to discuss the current status, perspectives, and challenges for cosmic-ray
antideuteron searches and discusses the motivation for antideuteron searches,
the theoretical and experimental uncertainties of antideuteron production and
propagation in our Galaxy, as well as give an experimental cosmic-ray
antideuteron search status update. This report is a condensed summary of the
article "Review of the theoretical and experimental status of dark matter
identification with cosmic-ray antideuteron" (arXiv:1505.07785).Comment: 9 pages, 4 figures, ICRC 2015 proceeding
The Nature of Starburst Activity in M82
We present new evolutionary synthesis models of M82 based mainly on
observations consisting of near-infrared integral field spectroscopy and
mid-infrared spectroscopy. The models incorporate stellar evolution, spectral
synthesis, and photoionization modeling, and are optimized for 1-45 micron
observations of starburst galaxies. The data allow us to model the starburst
regions on scales as small as 25 pc. We investigate the initial mass function
(IMF) of the stars and constrain quantitatively the spatial and temporal
evolution of starburst activity in M82. We find a typical decay timescale for
individual burst sites of a few million years. The data are consistent with the
formation of very massive stars (> 50-100 Msun) and require a flattening of the
starburst IMF below a few solar masses assuming a Salpeter slope at higher
masses. Our results are well matched by a scenario in which the global
starburst activity in M82 occurred in two successive episodes each lasting a
few million years, peaking about 10 and 5 Myr ago. The first episode took place
throughout the central regions of M82 and was particularly intense at the
nucleus while the second episode occurred predominantly in a circumnuclear ring
and along the stellar bar. We interpret this sequence as resulting from the
gravitational interaction M82 and its neighbour M81, and subsequent bar-driven
evolution. The short burst duration on all spatial scales indicates strong
negative feedback effects of starburst activity, both locally and globally.
Simple energetics considerations suggest the collective mechanical energy
released by massive stars was able to rapidly inhibit star formation after the
onset of each episode.Comment: 48 pages, incl. 16 Postscript figures; accepted for publication in
the Astrophysical Journa
Formation of a Massive Black Hole at the Center of the Superbubble in M82
We performed 12CO(1-0), 13CO(1-0), and HCN(1-0) interferometric observations
of the central region (about 450 pc in radius) of M82 with the Nobeyama
Millimeter Array, and have successfully imaged a molecular superbubble and
spurs. The center of the superbubble is clearly shifted from the nucleus by 140
pc. This position is close to that of the massive black hole (BH) of >460 Mo
and the 2.2 micron secondary peak (a luminous supergiant dominated cluster),
which strongly suggests that these objects may be related to the formation of
the superbubble. Consideration of star formation in the cluster based on the
infrared data indicates that (1) energy release from supernovae can account for
the kinetic energy of the superbubble, (2) the total mass of stellar-mass BHs
available for building-up the massive BH may be much higher than 460 Mo, and
(3) it is possible to form the middle-mass BH of 100-1000 Mo within the
timescale of the superbubble. We suggest that the massive BH was produced and
is growing in the intense starburst region.Comment: 9 pages, 3 figures, to appear in ApJ Lette
Scanning near-field optical microscopy with aperture probes: Fundamentals and applications
In this review we describe fundamentals of scanning near-field optical microscopy with aperture probes. After the discussion of instrumentation and probe fabrication, aspects of light propagation in metal-coated, tapered optical fibers are considered. This includes transmission properties and field distributions in the vicinity of subwavelength apertures. Furthermore, the near-field optical image formation mechanism is analyzed with special emphasis on potential sources of artifacts. To underline the prospects of the technique, selected applications including amplitude and phase contrast imaging, fluorescence imaging, and Raman spectroscopy, as well as near-field optical desorption, are presented. These examples demonstrate that scanning near-field optical microscopy is no longer an exotic method but has matured into a valuable tool. (C) 2000 American Institute of Physics. [S0021-9606(00)70316-3]
The Star Formation Rate and Dense Molecular Gas in Galaxies
(abridged) HCN luminosity Lhcn is a tracer of DENSE molecular gas, n(H_2) >~
3x10^4 cm^{-3}, associated with star-forming giant molecular cloud (GMC) cores.
We present the results and analysis of our survey of HCN emission from 65
infrared galaxies including 9 ultraluminous infrared galaxies (ULIGs,
Lir>10^{12}Lsun), 22 luminous infrared galaxies (LIGs,
10^{11}<Lir<~10^{12}Lsun) and 34 normal spiral galaxies with lower IR
luminosity (most are large spiral galaxies). We have measured the global HCN
line luminosity and the observations are reported in Gao and Solomon (2003,
Paper I). This paper analyzes the relationships between the total far-IR
luminosity a tracer of the star formation rate, the global HCN line luminosity
a measure of the total DENSE molecular gas content, and the CO luminosity a
measure of the total molecular content. We find a tight linear correlation
between the IR and HCN luminosities Lir and Lhcn (in the log-log plot) with a
correlation coefficient R=0.94. The IR--HCN linear correlation is valid over 3
orders of magnitude including ULIGs. The direct consequence of the linear
IR--HCN correlation is that the star formation law in terms of DENSE molecular
gas content has a power law index of 1.0. The global star formation rate is
linearly proportional to the mass of dense molecular gas in normal spiral
galaxies, LIGs, and ULIGs. This is strong evidence in favor of star formation
as the power source in ultraluminous galaxies since the star formation in these
galaxies appears to be normal and expected given their high mass of dense
star-forming molecular gas.Comment: ApJ, May 1 issue (final version, 20 pages + 9 figures + 3 tables +
appendix/refs.). Please see Gao & Solomon (Paper I, astro-ph/0310341) for the
presentation and discussion of the HCN surve
A multi-model study of the hemispheric transport and deposition of oxidised nitrogen.
Fifteen chemistry-transport models are used to quantify, for the first time, the export of oxidised nitrogen (NOy) to and from four regions (Europe, North America, South Asia, and East Asia), and to estimate the uncertainty in the results. Between 12 and 24% of the NOx emitted is exported from each region annually. The strongest impact of each source region on a foreign region is: Europe on East Asia, North America on Europe, South Asia on East Asia, and East Asia on North America. Europe exports the most NOy, and East Asia the least. East Asia receives the most NOy from the other regions. Between 8 and 15% of NOx emitted in each region is transported over distances larger than 1000 km, with 3–10% ultimately deposited over the foreign regions
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