276 research outputs found
Distribution of HNCO 5 in Massive Star-forming Regions
The goal of this paper is to study the spatial distribution of HNCO in
massive star-forming regions, and investigate its spatial association with
infrared sources, as well as physical conditions in region of HNCO emission. We
have mapped nine massive star-forming regions in HNCO 5 with the
Purple Mountain Observatory 13.7m telescope. The C18O maps of these sources
were obtained simultaneously. The HNCO emission shows compact distribution,
with emission peak centred on water masers. Nearly all the HNCO clumps show
signs of embedded mid-infrared or far-infrared sources. The FWHM sizes of HNCO
clumps are significantly smaller than C18O clumps but rather similar to HC3N
clumps. We also found good correlation between the integrated intensities,
linewidths and LSR velocities of HNCO and HC3N emission, implying similar
excitation mechanism of these two species. As such, collisional excitation is
likely to be the dominant excitation mechanism for HNCO 5
emission in galactic massive star-forming regions.Comment: 9 pages, 4 figures, accepted by A&
Carbon monoxide in an extremely metal-poor galaxy
Extremely metal-poor galaxies with metallicity below 10% of the solar value
in the local universe are the best analogues to investigating the interstellar
medium at a quasi-primitive environment in the early universe. In spite of the
ongoing formation of stars in these galaxies, the presence of molecular gas
(which is known to provide the material reservoir for star formation in
galaxies, such as our Milky Way) remains unclear. Here, we report the detection
of carbon monoxide (CO), the primary tracer of molecular gas, in a galaxy with
7% solar metallicity, with additional detections in two galaxies at higher
metallicities. Such detections offer direct evidence for the existence of
molecular gas in these galaxies that contain few metals. Using archived
infrared data, it is shown that the molecular gas mass per CO luminosity at
extremely low metallicity is approximately one-thousand times the Milky Way
value.Comment: 12 pages, 3 figures, 1 table. Supplementary data at
http://www.nature.com/article-assets/npg/ncomms/2016/161209/ncomms13789/extref/ncomms13789-s1.pd
Inefficient Star Formation In Extremely Metal Poor Galaxies
The first galaxies contain stars born out of gas with little or no metals.
The lack of metals is expected to inhibit efficient gas cooling and star
formation but this effect has yet to be observed in galaxies with oxygen
abundance relative to hydrogen below a tenth of that of the Sun. Extremely
metal poor nearby galaxies may be our best local laboratories for studying in
detail the conditions that prevailed in low metallicity galaxies at early
epochs. Carbon Monoxide (CO) emission is unreliable as tracers of gas at low
metallicities, and while dust has been used to trace gas in low-metallicity
galaxies, low-spatial resolution in the far-infrared has typically led to large
uncertainties. Here we report spatially-resolved infrared observations of two
galaxies with oxygen abundances below 10 per cent solar, and show that stars
form very inefficiently in seven star-forming clumps of these galaxies. The
star formation efficiencies are more than ten times lower than found in normal,
metal rich galaxies today, suggesting that star formation may have been very
inefficient in the early Universe.Comment: Author's version (10 pages, 4 figures). Published in Natur
The Weak Carbon Monoxide Emission In An Extremely Metal Poor Galaxy, Sextans A
Carbon monoxide (CO) is one of the primary coolants of gas and an easily
accessible tracer of molecular gas in spiral galaxies but it is unclear if CO
plays a similar role in metal poor dwarfs. We carried out a deep observation
with IRAM 30 m to search for CO emission by targeting the brightest far-IR peak
in a nearby extremely metal poor galaxy, Sextans A, with 7% Solar metallicity.
A weak CO J=1-0 emission is seen, which is already faint enough to place a
strong constraint on the conversion factor (a_CO) from the CO luminosity to the
molecular gas mass that is derived from the spatially resolved dust mass map.
The a_CO is at least seven hundred times the Milky Way value. This indicates
that CO emission is exceedingly weak in extremely metal poor galaxies,
challenging its role as a coolant in these galaxies.Comment: 4 pages, 1 table, 4 figures. ApJL in pres
Detection of a Compact Nuclear Radio Source in the Local Group Elliptical Galaxy M32
The Local Group compact elliptical galaxy M32 hosts one of the nearest
candidate super-massive black holes (SMBHs), which has a previously suggested
X-ray counterpart. Based on sensitive observations taken with the {\it Karl G.
Jansky} Very Large Array (VLA), we detect for the first time a compact radio
source coincident with the nucleus of M32, which exhibits an integrated flux
density of Jy at 6.6 GHz. We discuss several
possibilities for the nature of this source, favoring an origin of the
long-sought radio emission from the central SMBH, for which we also revisit the
X-ray properties based on recently acquired {\sl Chandra} and {\sl XMM-Newton}
data. Our VLA observations also discover radio emission from three previously
known optical planetary nebulae in the inner region of M32.Comment: 13 pages, 2 figures, accepted by ApJ Letter
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