123 research outputs found
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
Broadband 300-GHz Power Amplifier MMICs in InGaAs mHEMT Technology
In this article, we report on compact solid-state power amplifier (SSPA) millimeter-wave monolithic integrated circuits (MMICs) covering the 280–330-GHz frequency range. The technology used is a 35-nm gate-length InGaAs metamorphic highelectron- mobility transistor (mHEMT) technology. Two power amplifier MMICs are reported, based on a compact unit amplifier cell, which is parallelized two times using two different Wilkinson power combiners. The Wilkinson combiners are designed using elevated coplanar waveguide and air-bridge thin-film transmission lines in order to implement low-loss 70-Ω lines in the back-endof-line of this InGaAs mHEMT technology. The five-stage SSPA MMICs achieve a measured small-signal gain around 20 dB over the 280–335-GHz frequency band. State-of-the-art output power performance is reported, achieving at least 13 dBm over the 286–310-GHz frequency band, with a peak output power of 13.7 dBm (23.4 mW) at 300 GHz. The PA MMICs are designed for a reduced chip width while maximizing the total gate width of 512 μm in the output stage, using a compact topology based on cascode and common-source devices, improving the output power per required chip width significantly
Mapping the cold dust in edge-on galaxies at 1.2 mm wavelength
Using the IRAM 30-m telescope, we have mapped the 1.2mm continuum emission in
NGC 891, NGC 5907 and NGC 4565. In particular this latter galaxy shows a weak,
extended plateau that is correlated with HI in addition to the central peak and
ring structure common to all these spirals. Moreover, the outer part of this
dust emission of NGC 4565 is clearly warped. The average dust temperature in
the outer parts is 15 K and the derived dust absorption cross section is very
close to that predicted for the local diffuse clouds.Comment: 4 pages LaTeX plus gzipped tar-file, including style and 2
ps-figures; to be published in the proceeedings of the "Dust-Morphology"
Conference, Johannesburg, 22-26 January, 1996, D. Block (ed.), (Kluwer
Dordrecht
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
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
Mesoscale modelling of the CO2 interactions between the surface and the atmosphere applied to the April 2007 CERES field experiment
This paper describes a numerical interpretation of the April 2007, CarboEurope Regional Experiment Strategy (CERES) campaign, devoted to the study of the CO2 cycle at the regional scale. Four consecutive clear sky days with intensive observations of CO2 concentration, fluxes at the surface and in the boundary layer have been simulated with the Meso-NH mesoscale model, coupled to ISBA-A-gs land surface model. The main result of this paper is to show how aircraft observations of CO2 concentration have been used to identify surface model errors and to calibrate the CO2 driving component of the surface model. In fact, the comparisons between modelled and observed CO2 concentrations within the Atmospheric Boundary Layer (ABL) allow to calibrate and correct not only the parameterization of respired CO2 fluxes by the ecosystem but also the Leaf Area Index (LAI) of the dominating land cover. After this calibration, the paper describes systematic comparisons of the model outputs with numerous data collected during the CERES campaign, in April 2007. For instance, the originality of this paper is the spatial integration of the comparisons. In fact, the aircraft observations of CO2 concentration and fluxes and energy fluxes are used for the model validation from the local to the regional scale. As a conclusion, the CO2 budgeting approach from the mesoscale model shows that the winter croplands are assimilating more CO2 than the pine forest, at this stage of the year and this case study
SiO chimneys and supershells in M82
In this Letter we present the first images of the emission of SiO and H13CO+
in the nucleus of the starburst galaxy M82. Contrary to other molecular species
that mainly trace the distribution of the star-forming molecular gas within the
disk, the SiO emission extends noticeably out of the galaxy plane. The bulk of
the SiO emission is restricted to two major features. The first feature,
referred to as the SiO supershell, is an open shell of 150 pc diameter, located
120 pc west from the galaxy center.
The SiO supershell represents the inner front of a molecular shell expanding
at 40 km/s, produced by mass ejection around a supercluster of young stars
containing supernova remnant SNR 41.95+57.5. The second feature is a vertical
filament, referred to as the SiO chimney, emanating from the disk at 200 pc
east from the galaxy center. The SiO chimney reaches a 500 pc vertical height,
and it is associated with the most prominent chimney identified in radio
continuum maps. The kinematics, morphology, and fractional abundances of the
SiO gas features in M82 can be explained in the framework of shocked chemistry
driven by local episodes of gas ejection from the starburst disk.Comment: 10 pages, 2 figures, published in ApJLetters, 200
Locating the Youngest HII Regions in M82 with 7 mm Continuum Maps
We present 7mm Very Large Array continuum images of the starburst galaxy M82.
On arcsecond scales, two-thirds of the 7mm continuum consists of free-free
emission from HII regions. In the subarcsecond resolution map, we identify 14
compact sources, including 9 bright HII regions with N_Lyc > 10^51 sec^-1. Four
of the HII regions have rising spectra, implying emission measures > 10^8 cm^-6
pc. Except for one compact source with peculiar features, all other compact
radio sources are found in dust lanes and do not have optical or near-infrared
continuum counterparts. Four regions of extended, high brightness (EM > 10^7
cm-6 pc) radio emission are found in our high resolution map, including some as
large as ~2", or 30 pc, representing either associations of small HII regions,
or sheetlike structures of denser gas. The good correlation between 7 mm
emission and Spitzer IRAC 8 micron continuum-removed PAH feature suggests that
PAH emission may track the recently formed OB stars. We find an excellent
correlation between molecular gas and star formation, particularly dense gas
traced by HCN, down to the ~ 45 pc scale in M82. We also find star formation
efficiencies (SFEs) of 1-10% on the same scale, based on CO maps. The highest
SFE are found in regions with the highest dense gas fractions.Comment: 18 pages, 10 figures. Accepted for publication in A
Mapping the submillimeter spiral wave in NGC 6946
We have analysed SCUBA 850\mum images of the (near) face-on spiral galaxy NGC
6946, and found a tight correlation between dust thermal emission and molecular
gas. The map of visual optical depth relates well to the distribution of
neutral gas (HI+H2) and implies a global gas-to-dust ratio of 90. There is no
significant radial variation of this ratio: this can be understood, since the
gas content is dominated by far by the molecular gas. The latter is estimated
through the CO emission tracer, which is itself dependent on metallicity,
similarly to dust emission. By comparing the radial profile of our visual
optical depth map with that of the SCUBA image, we infer an emissivity (dust
absorption coefficient) at 850\mum that is 3 times lower than the value
measured by COBE in the Milky Way, and 9 times lower than in NGC 891. A
decomposition of the spiral structure half way out along the disk of NGC 6946
suggests an interarm optical depth of between 1 and 2. These surprisingly high
values represent 40-80% of the visual opacity that we measure for the arm
region (abridged).Comment: 12 pages, 9 figures, accepted in A&
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
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