1,825 research outputs found
The molecular gas content of the advanced S+E merger NGC 4441 - Evidence for an extended decoupled nuclear disc?
Mergers between a spiral and an elliptical (S+E mergers) are poorly studied
so far despite the importance for galaxy evolution. NGC4441 is a nearby
candidate for an advanced remnant of such a merger, showing typical tidal
structures like an optical tail and two shells as well as two HI tails. The
study of the molecular gas content gives clues on the impact of the recent
merger event on the star formation. Simulations of S+E mergers predict
contradictory scenarios concerning the strength and the extent of an induced
starburst. Thus, observations of the amount and the distribution of the
molecular gas, the raw material of star formation, are needed to understand the
influence of the merger on the star formation history. 12CO and 13CO (1-0) and
(2-1) observations were obtained using the Onsala Space Observatory 20m and
IRAM 30m telescope as well as the Plateau de Bure interferometer. These data
allow us to carry out a basic analysis of the molecular gas properties such as
estimates of the molecular gas mass, its temperature and density and the star
formation efficiency. The CO observations reveal an extended molecular gas
reservoir out to ~4kpc, with a total molecular gas mass of ~5x10^8 M_sun.
Furthermore, high resolution imaging shows a central molecular gas feature,
most likely a rotating disc hosting most of the molecular gas ~4x10^8 M_sun.
This nuclear disc shows a different sense of rotation than the large-scale HI
structure, indicating a kinematically decoupled core. (abbreviated)Comment: 11 pages, accepted by A&
Luminous HC3N line emission in NGC4418 - buried AGN or nascent starburst?
IRAM 30m observations reveal that the deeply obscured IR-luminous galaxy
NGC4418 has a rich molecular chemistry - including unusually luminous HC3N line
emission. We furthermore detect: ortho-H2CO 2-1, 3-2; CN 1-0, 2-1; HCO+, 1-0.
3-2, HCN 3-2, HNC 1-0, 3-2 (and tentatively OCS 12-11). The HCN, HCO+, H2CO and
CN line emission can be fitted to densities of n=5 x 10E4 - 10E5 cm-3 and gas
temperatures Tk=80-150 K. Both HNC and HC3N are, however, significantly more
excited than the other species which requires higher gas densities - or
radiative excitation through e.g. mid-IR pumping. The HCN line intensity is
fainter than that of HCO+ and HNC for the 3-2 transition, in contrast to
previous findings for the 1-0 lines where the HCN emission is the most
luminous. We tentatively suggest that the observed molecular line emission is
consistent with a young starburst, where the emission can be understood as
emerging from dense, warm gas with an additional PDR component. We find that
X-ray chemistry is not required to explain the observed mm line emission,
including the HCN/HCO+ 1-0 and 3-2 line ratios. The luminous HC3N line emission
is an expected signature of dense, starforming gas. A deeply buried AGN can not
be excluded, but its impact on the surrounding molecular medium is then
suggested to be limited. However, detailed modelling of HC3N abundances in
X-ray dominated regions (XDRs) should be carried out. The possibility of
radiative excitation should also be further investigatedComment: 7 pages, one eps figure, uses aa.cls, submitted to Astronomy and
Astrophysic
13CO 1-0 imaging of the Medusa merger, NGC4194
Studying molecular gas properties in merging galaxies gives important clues
to the onset and evolution of interaction-triggered starbursts. The CO/13CO 1-0
line intensity ratio can be used as a tracer of how dynamics and star formation
processes impact the gas properties. The Medusa (NGC~4194) merger is
particularly interesting to study since its LFIR/LCO ratio rivals that of
ultraluminous galaxies (ULIRGs), despite the comparatively modest luminosity,
indicating an exceptionally high star formation efficiency (SFE) in the Medusa
merger.
Interferometric OVRO observations of CO and 13CO 1-0 in the Medusa show the
CO/13CO intensity ratio increases from normal, quiescent values (7-10) in the
outer parts (r>2 kpc) of the galaxy to high (16 to >40) values in the central
(r<1 kpc) starburst region. In the centre there is an east-west gradient where
the line ratio changes by more than a factor of three over 5" (945 pc). The
integrated 13CO emission peaks in the north-western starburst region while the
central CO emission is strongly associated with the prominent crossing
dust-lane. We discuss the central east-west gradient in the context of gas
properties in the starburst and the central dust lane. We suggest that the
central gradient is mainly caused by diffuse gas in the dust lane. In this
scenario, the actual molecular mass distribution is better traced by the 13CO
1-0 emission than the CO. The possibilities of temperature and abundance
gradients are also discussed. We compare the central gas properties of the
Medusa to those of other minor mergers and suggest that the extreme and
transient phase of the Medusa star formation activity has similar traits to
those of high-redshift galaxies.Comment: 7 pages, 4 figures, accepted for publication in Astronomy and
Astrophysic
Copolyamides of nylon-4,6 and nylon-4,T
Copolyamides of nylon-4,6 and nylon-4,T were prepared by a two-step method: (1) a prepolymerization in an autoclave (40 min at 210°C) and (2) a postcondensation in the solid state (4 h, 260°C). On these materials was studied the melting behavior with DSC, the crystalline structure with WAXS, the water absorption, and the mechanical properties with a torsion pendulum. In these copolyamides the order was found to remain high, but the crystalline structures of -4,6 and -4,T were not isomorphous. The torsion moduli increased with -4,T content both at RT and at 140°C
CN and HNC Line Emission in IR Luminous Galaxies
We have observed HNC 1-0, CN 1-0 and 2-1 line emission in a sample of 13 IR
luminous (LIRGs, L_IR > 10E11 Lo) starburst and Seyfert galaxies. HNC 1-0 is
detected in 9, CN 1-0 is detected in 10 and CN 2-1 in 7 of the galaxies. We
also report the first detection of HC3N (10-9) emission in Arp220. The
excitation of HNC and CN emission requires densities n > 10E4 cm-3. We compare
their intensities to that of the usual high density tracer HCN. The
I(HCN)/I(HNC}) and I(HCN)/I(CN) 1-0 line intensity ratios vary significantly,
from 0.5 to >6, among the galaxies. This implies that the actual properties of
the dense gas is varying among galaxies who otherwise have similar I(CO)/I(HCN)
line intensity ratios. We suggest that the HNC emission is not a reliable
tracer of cold (10 K) gas at the center of LIRGs, as it often is in the disk of
the Milky Way. Instead, the HNC abundance may remain substantial, despite high
gas temperatures, because the emission is emerging from regions where the HCN
and HNC formation and destruction processes are dominated by ion-neutral
reactions which are not strongly dependent on kinetic temperature. We find five
galaxies (four AGNs and one starburst) where the I(HCN)/I(HNC) intensity ratio
is close to unity. In other AGNs, however, I(HCN)/I(HNC}) is >4. The CN
emission is on average a factor of two fainter than HCN, but the variation is
large and there seems to be a trend of reduced relative CN luminosity with
increasing IR luminosity. One galaxy, NGC3690, has a CN luminosity twice that
of HCN and its ISM is thus strongly affected by UV radiation. We discuss the
I(HCN)/I(HNC) and I(HCN)/I(CN) line ratios as indicators of starburst
evolution.Comment: 12 pages, 4 figures. Accepted for publication in Astronomy and
Astrophysic
An Origin of the Huge Far-Infrared Luminosity of Starburst Mergers
Recently Taniguchi and Ohyama found that the higher CO to CO
integrated intensity ratios at a transition =1--0, CO)CO) , in a sample of starburst merging
galaxies such as Arp 220 are mainly attributed to the depression of CO
emission with respect to CO. Investigating the same sample of galaxies
analyzed by Taniguchi & Ohyama, we find that there is a tight, almost linear
correlation between the dust mass and CO luminosity. This implies that
dust grains are also depressed in the high- starburst mergers, leading to
the higher dust temperature () in them because of the relative
increase in the radiation density. Nevertheless, the average dust mass () of the high- starburst mergers is higher significantly than that of
non-high galaxies. This is naturally understood because the galaxy mergers
could accumulate a lot of dust grains from their progenitor galaxies together
with supply of dust grains formed newly in the star forming regions. Since
(FIR) given the dust emissivity law, , the increases in both and
explain well why the starburst mergers are so bright in the FIR. We discuss
that the superwind activity plays an important role in destroying dust grains
as well as dense gas clouds in the central region of mergers.Comment: 10 pages (aaspp4.sty), 3 postscript figures (embedded). Accepted for
publication in Astrophysical Journal Letter
Star-formation in the central kpc of the starburst/LINER galaxy NGC1614
A high angular resolution, multi-wavelength study of the LINER galaxy NGC1614
has been carried out. OVRO CO 1-0 observations are presented together with
extensive multi-frequency radio continuum and HI absorption observations with
the VLA and MERLIN. Toward the center of NGC1614, we have detected a ring of
radio continuum emission with a radius of 300 pc. This ring is coincident with
previous radio and Paschen-alpha observations. The dynamical mass of the ring
based on HI absorption is 3.1 x 10E9 Msun. The peak of the integrated CO 1-0
emission is shifted by 1" to the north-west of the ring center and a
significant fraction of the CO emission is associated with a crossing dust
lane. An upper limit to the molecular gas mass in the ring region is 1.7 x 10E9
Msun. Inside the ring, there is a north to south elongated 1.4GHz radio
continuum feature with a nuclear peak. This peak is also seen in the 5GHz radio
continuum and in the CO. We suggest that the R=300 pc star forming ring
represents the radius of a dynamical resonance - as an alternative to the
scenario that the starburst is propagating outwards from the center into a
molecular ring. The ring-like appearance probably part of a spiral structure.
Substantial amounts of molecular gas have passed the radius of the ring and
reached the nuclear region. The nuclear peak seen in 5GHz radio continuum and
CO is likely related to previous star formation, where all molecular gas was
not consumed. The LINER-like optical spectrum observed in NGC1614 may be due to
nuclear starburst activity, and not to an Active Galactic Nucleus (AGN).
Although the presence of an AGN cannot be excluded.Comment: Accepted by Astronomy and Astrophysics, 12 pages, 10 figure
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