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Search Results: 1 - 10 of 325301 matches for " S. Huettemeister "
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Conversion Problems: How (Not) to Determine Molecular Masses in Dwarf Galaxies
S. Huettemeister
Physics , 2001,
Abstract: The determination of molecular gas masses in star forming dwarf irregular galaxies is crucial to assess the star formation process in these objects. But the derivation of the molecular gas content of dwarf galaxies has been a long-standing problem. CO, as the only practical tracer of cold molecular gas, has been (and to some extent still is) notoriously diffucult to detect. Yet, star formation clearly takes place in many dwarf irregulars. This conference contribution contrasts a number of methods commonly used to derive the molecular gas contents of dwarf galaxies based on CO observations: Procedures based on the virial theorem and those relying on radiative transfer arguments. It is shown that both classes of methods have serious drawbacks. Still, examples show that there seem to be real differences in the `correct' conversion factor both between and within star forming dwarf irregular galaxies.
Gas Properties in the Starburst Centers of Barred Galaxies
S. Huettemeister,S. Aalto
Physics , 2001,
Abstract: The physical properties of molecular gas can be deduced from the analysis of line intensity ratios, most commonly of the CO isotopomers 12CO and 13CO. Barred galaxies, especially those with central starbursts, provide an excellent laboratory to study the changing properties of the gas as it approaches the center. Four galaxies (UGC2855, UGC2866, NGC7479 and NGC4123) are presented as examples of the results that can be obtained. Special attention is given to the case of (abnormally) high 12CO/13CO R_12/13 line ratios (exceeding 20), which may indicate unusual gas properties. Qualitative scenarios for the structure of the ISM explaining a range of values of R_12/13 are discussed.
Complex molecular gas structure in the Medusa merger
S. Aalto,S. Huettemeister
Physics , 2000,
Abstract: High resolution OVRO aperture synthesis maps of the 12CO 1-0 emission in the `Medusa' galaxy merger (NGC4194) reveal the molecular emission being surprisingly extended. It is distributed on a total scale of 25$''$ (4.7 kpc) - despite the apparent advanced stage of the merger. The complex, striking CO morphology occupies mainly the center and the north-eastern part of the main optical body. The extended 12CO flux is tracing two prominent dust lanes: one which is crossing the central region at right angle and a second which curves to the north-east and then into the beginning of the northern tidal tail. The bulk of the 12CO emission (67%) can be found in a complex starburst region encompassing the central 2kpc. The molecular gas is distributed in five major emission regions of typical size 300pc. About 15% of the total 12CO flux is found in a bright region 1.5'' south of the radio continuum nucleus. We suggest that this region together with the kpc sized central starburst is being fueled by gas flows along the central dust lane. We discuss the merger history of NGC4194 and suggest that it may be the result of an early-type/spiral merger with a shell emerging to the south of the center. The total molecular mass in the system is estimated to be at most 2 X 10^9 M_sun (depending on the CO - H_2 conversion factor). The high 12CO/13CO 1-0 intensity ratio, ~20, indicates highly excited physical conditions in the interstellar medium showing that the starburst has a big impact on its surrounding ISM. At the current rate of star formation, the gas will be consumed within 40 million years.
A Molecular Tidal Tail in the Medusa Minor Merger
S. Aalto,S. Huettemeister,A. G. Polatidis
Physics , 2001, DOI: 10.1051/0004-6361:20010435
Abstract: We have detected CO 1-0 emission along the tidal tail of the NGC 4194 (the Medusa) merger. It is the first CO detection in the optical tail of a minor merger. Emission is detected both in the centre of the tail and at its tip. The molecular mass in the 33'' Onsala 20m beam is estimated to be >= 8.5 x 10^7 M_{sun} which is at least 4% of the total molecular mass measured so far in this system. We suggest that the emission is a molecular tidal tail which is part of the extended structure of the main body, and that the molecular gas was thrown out by the collision instead of having formed in situ from condensing atomic material. We find it unlikely that the emission is associated with a tidal dwarf galaxy (even if the future formation of such an object is possible), but high resolution HI, CO and optical observations are necessary to resolve the issue. The Medusa is very likely the result of an elliptical+spiral collison and our detection supports the notion that molecular gas in minor mergers can be found at great distances from the merger centre.
The molecular gas content of the advanced S+E merger NGC 4441 - Evidence for an extended decoupled nuclear disc?
E. Juette,S. Aalto,S. Huettemeister
Physics , 2009, DOI: 10.1051/0004-6361/200809686
Abstract: 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)
The discovery of a gas-rich bar in UGC 2855: A galaxy in a pre-starburst phase?
S. Huettemeister,S. Aalto,W. F. Wall
Physics , 1999,
Abstract: We present the first interferometric CO observations of the barred late-type galaxy UGC2855 and its companion UGC2866. UGC2855 is shown to belong to the rare class of galaxies with a long (~ 8kpc) continuous molecular bar. The velocity field along the bar is dominated by solid-body rotation and shows few perturbations. This, together with an almost constant and low ^12CO/^13CO line intensity ratio along the bar and in the center as well as only weak H alpha emission, indicate that the gas in the bar is not subjected to strong shocks, but surprisingly quiescent. In the central 3'', a high velocity feature consisting of a number of Giant Molecular Associations, is identified. We discuss possible scenarios of the nature of this structure in connection with the question of the presence of an Inner Lindblad Resonance (ILR) in the bar. We suggest that the bar of UGC2855 is a young object, possibly has no ILR, and that the mass concentration toward the center of the galaxy is just beginning. In contrast, the companion UGC2866 experiences a strong starburst. We compare the properties of the gas in the bar of UGC2855 to those we find in NGC7479. Despite superficial parallels, the two bars are very dissimilar objects in terms of ^12CO/^13CO line ratio, velocity field and H alpha activity. NGC7479 exhibits starburst characteristics, while we argue that UGC2855 is in a pre-burst stage.
Dense gas in nearby galaxies XI. H_2CO and CH_3OH: Molecular abundances and physical conditions
S. Huettemeister,R. Mauersberger,C. Henkel
Physics , 1997,
Abstract: Multilevel observations of formaldehyde (H_2CO) and methanol (CH_3OH) toward the nearby spiral galaxies NGC 253, Maffei 2, IC 342, M 82 and NGC 6946 are presented. H_2CO was detected in all galaxies (tentatively in NGC 6946). CH_3OH was detected in all objects with the notable exception of M 82. H_2CO line intensity ratios point out differences in gas density both between galaxies and within the central regions of individual objects. Model calculations show that the bulk of the gas emitting H_2CO in NGC 253 is at a density of ~10^4 cm^-3 , while the H_2CO lines in M 82 and IC 342 trace two different, spatially separated gas components with densities of <= 10^4 cm^-3 and ~10^6 cm^-3. The south-western molecular hotspot in M 82 and the center of IC 342 are the regions with the highest density. Methanol is subthermally excited in all galaxies, with the lowest excitation temperatures found in IC 342. The CH_3OH abundance in NGC 253 and the non-starburst nuclei of IC 342 and Maffei 2 are comparable. A map of the 3_k-2_k lines in NGC 253 shows that CH_3OH traces clumpy structures better than other molecules requiring high gas density to be excited. CH_3OH toward M 82 is at least an order of magnitude less abundant than in otherwise comparable galaxies. This confirms the existence of global chemical differences, and thus very large scale variations in the state of the molecular gas phase, even between galaxies commonly classified as starburst nuclei.
The Influence of the Starburst on the ISM in NGC 1569
S. Muehle,S. Huettemeister,U. Klein,E. M. Wilcots
Physics , 2001,
Abstract: A tremendous starburst, that ended about 4 million years ago, has had a very strong impact on the dwarf galaxy NGC 1569. Evidence for this event is still prominent in the different components of the galaxy: For example, photometric studies of the resolved stellar population close to the very luminous super-star clusters hint at past star formation rates of up to 3 M_solar/yr. In ROSAT observations, extended X-ray emission has been found at the same location. And the spectrum of the strong synchrotron emission shows a kink characteristic for a recently stopped starburst. We study the impact of the starburst on the neutral atomic and molecular gas. Our CO(3--2) map obtained with the Heinrich-Hertz Telescope reveals 3 giant molecular associations as well as a large amount of extended CO gas. The comparison of this map with interferometric and single-dish data of the CO(2--1) and CO(1--0) transitions indicates an unusually warm molecular gas phase. The HI distribution is remarkably smooth for a star-forming dwarf galaxy. Only one huge bubble has been found. It is located near the position of the super-star clusters. With our naturally weighted VLA data cube, we can confirm the existence of a low-mass HI cloud and a bridge connecting it to NGC 1569. This might be an HI cloud in the process of merging with the galaxy and being tidally disrupted. We also report the detection of the remnants of a shell, that can be traced best in the HI velocity field.
Changing Molecular Gas Properties in the Bar and Canter of NGC7479
S. Huettemeister,S. Aalto,M. Das,W. F. Wall
Physics , 2000,
Abstract: We present sensitive interferometric 12CO, 13CO and HCN observations of the barred spiral galaxy NGC7479, known to be one of the few barred galaxies with a continuous gas-filled bar. We focus on the investigation and interpretation of 12CO/13CO line intensity ratios, which is facilitated by having more than 90% of the flux in our interferometer maps. The global (9kpc by 2.5kpc) value of the 12CO/13CO ratio is high at 20 - 40. On smaller scales (~ 750 pc), the ratio is found to vary dramatically, reaching values > 30 in large parts of the bar, but dropping to values ~ 5, typical for galactic disks, at a 13CO condensation in the southern part of the bar. We interpret these changes in terms of the relative importance of the contribution of a diffuse molecular component, characterized by unbound gas that has a moderate optical depth in the 12CO(1 - 0) transition. This component dominates the 12CO along the bar and is also likely to play an important role in the center of NGC7479. In the center, the 12CO and the HCN intensity peaks coincide, while the 13CO peak is slightly offset. This can be explained in terms of high gas temperature and density at the 12CO peak position. Along the bar, the relation between the distribution of 12CO, 13CO, dust lanes and velocity gradient is complex. A southern 13CO condensation is found offset from the 12CO ridge that generally coincides with the most prominent dust lanes. It is possible that strong 13CO detections along the bar indicate quiescent conditions, downstream from the major bar shock. Still, these condensations are found close to high velocity gradients. In the central region, the velocity gradient is traced much more closely by 13CO than by 12CO.
A New High Resolution CO Map of the inner 2.'5 of M51 I. Streaming Motions and Spiral Structure
S. Aalto,S. Huettemeister,N. Z. Scoville,P. Thaddeus
Physics , 1999, DOI: 10.1086/307610
Abstract: [Abridged] The Owens Valley mm-Array has been used to map the CO 1--0 emission in the inner 2'.5 of the grand design spiral galaxy M51 at 2''-3'' resolution. The molecular spiral arms are revealed with unprecedented clarity: supermassive cloud complexes, Giant Molecular Associations, are for the first time resolved both along and perpendicular to the arms. Major complexes occur symmetrically opposite each other in the two major arms. Streaming motions can be studied in detail along the major and minor axes of M51. The streaming velocities are very large, 60-150 km/s. For the first time, sufficient resolution to resolve the structure in the molecular streaming motions is obtained. Our data support the presence of galactic shocks in the arms of M51. In general, velocity gradients across arms are higher by a factor of 2-10 than previously found. They vary in steepness along the spiral arms, becoming particularly steep in between GMAs. The steep gradients cause conditions of strong reverse shear in several regions in the arms, and thus the notion that shear is generally reduced by streaming motions in spiral arms will have to be modified. Of the three GMAs studied on the SW arm, only one shows reduced shear. We find an expansion in the NE molecular arm at 25'' radius SE of the center. This broadening occurs right after the end of the NE arm at the Inner Lindblad Resonance. Bifurcations in the molecular spiral arm structure, at a radius of 73'', may be evidence of a secondary compression of the gas caused by the 4/1 ultraharmonic resonance. Inside the radius of the ILR, we detect narrow (~ 5'') molecular spiral arms possibly related to the K-band arms found in the same region. We find evidence of non-circular motions in the inner 20'' which are consistent with gas on elliptical orbits in a bar.
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