351 research outputs found
Dense gas in nearby galaxies: XV. Hot ammonia in NGC253, Maffei2 and IC342
The detection of NH3 inversion lines up to the (J,K)=(6,6) level is reported
toward the central regions of the nearby galaxies NGC253, Maffei2, and IC342.
The observed lines are up to 406K (for (J,K)=(6,6)) and 848K (for the (9,9)
transition) above the ground state and reveal a warm (T_kin= 100 - 140 K)
molecular component toward all galaxies studied. The tentatively detected
(J,K)=(9,9) line is evidence for an even warmer (>400K) component toward IC342.
Toward NGC253, IC342 and Maffei2 the global beam averaged NH3 abundances are
1-2 10^-8, while the abundance relative to warm H2 is around 10^-7. The
temperatures and NH3 abundances are similar to values found for the Galactic
central region. C-shocks produced in cloud-cloud collisions can explain kinetic
temperatures and chemical abundances. In the central region of M82, however,
the NH3 emitting gas component is comparatively cool (~ 30K). It must be dense
(to provide sufficient NH3 excitation) and well shielded from dissociating
photons and comprises only a small fraction of the molecular gas mass in M82.
An important molecular component, which is warm and tenuous and characterized
by a low ammonia abundance, can be seen mainly in CO. Photon dominated regions
(PDRs) can explain both the high fraction of warm H_2 in M82 and the observed
chemical abundances.Comment: 11 pages, 3 Figures, 5 Table
Methanol detection in M82
We present a multilevel study of the emission of methanol, detected for the
first time in this galaxy, and discuss the origin of its emission. The high
observed methanol abundance of a few 10^-9 can only be explained if injection
of methanol from dust grains is taken into account. While the overall
[CH3OH]/[NH3] ratio is much larger than observed towards other starbursts, the
dense high excitation component shows a similar value to that found in NGC 253
and Maffei 2. Our observations suggest the molecular material in M 82 to be
formed by dense warm cores, shielded from the UV radiation and similar to the
molecular clouds in other starbursts, surrounded by a less dense
photodissociated halo. The dense warm cores are likely the location of recent
and future star formation within M 82.Comment: Accepted for publication in A&A Letter
Ammonia in the hot core W51-IRS2: 12 new maser lines and a maser component with a velocity drift
With the 100-m telescope at Effelsberg, 19 ammonia (NH3) maser lines have
been detected toward the prominent massive star forming region W51-IRS2. Eleven
of these inversion lines, the (J,K) = (6,2), (5,3), (7,4), (8,5), (7,6), (7,7),
(9,7), (10,7), (9,9), (10,9), and (12,12) transitions, are classified as masers
for the first time in outer space. All detected masers are related to highly
excited inversion doublets. The (5,4) maser originates from an inversion
doublet 340 K above the ground state, while the (12,12) transition, at 1450 K,
is the most highly excited NH3 maser line so far known. Strong variability is
seen not only in ortho- but also in para-NH3 transitions. Bright narrow
emission features are observed, for the first time, in (mostly) ortho-ammonia
transitions, at V ~ 45 km/s, well separated from the quasi-thermal emission
near 60 km/s. These features were absent 25 years ago and show a velocity drift
of about +0.2 km/s/yr. The component is likely related to the SiO maser source
in W51-IRS2 and a possible scenario explaining the velocity drift is outlined.
The 57 km/s component of the (9,6) maser line is found to be strongly linearly
polarized. Maser emission in the (J,K) to (J+1,K) inversion doublets is
strictly forbidden by selection rules for electric dipole transitions in the
ground vibrational state. However, such pairs (and even triplets with (J+2,K))
are common toward W51-IRS2. Similarities in line widths and velocities indicate
that such groups of maser lines arise from the same regions, which can be
explained by pumping through vibrational excitation. The large number of NH3
maser lines in W51-IRS2 is most likely related to the exceptionally high
kinetic temperature and NH3 column density of this young massive star forming
region.Comment: Accepted for publication in Astronomy & Astrophysics, 11 pages, 12
postscript figures, 1 tabl
Tracing shocks and photodissociation in the Galactic center region
We present a systematic study of the HNCO, C18O, 13CS, and C34S emission
towards 13 selected molecular clouds in the Galactic center region. The
molecular emission in these positions are used as templates of the different
physical and chemical processes claimed to be dominant in the circumnuclear
molecular gas of galaxies. The relative abundance of HNCO shows a variation of
more than a factor of 20 amo ng the observed sources. The HNCO/13CS abundance
ratio is highly contrasted (up to a factor of 30) between the shielded
molecular clouds mostly affected by shocks, where HNCO is released to gas-phase
from grain mantles, and those pervaded by an intense UV radiation field, where
HNCO is photo-dissociated and CS production favored via ion reactions. We
propose the relative HNCO to CS abundance ratio as a highly contrasted
diagnostic tool to distinguish between the influence of shocks and/or the
radiation field in the nuclear regions of galaxies and their relation to the
evolutionary state of their nuclear star formation bursts.Comment: 25 pages, 5 figures, Accepted for publication in Ap
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