Andreas Heithausen

Molecular and atomic clouds associated with infrared cirrus in Ursa Major

Observations of CO and H I revealed that in Ursa Major the high-latitude far-infrared cirrus emission discovered by the Infrared Astronomical Satellite comes from molecular and atomic clouds. These clouds differ sufficiently from the large clouds in the Galactic plane so that the ratio of H/sub 2/ column density to velocity-integrated CO radiation temperature, N(H/sub 2/)/W(CO), derived from Galactic plane surveys, may not apply to them. On the assumption of a constant gas-to-dust ratio, it is argued that the cirrus emission in Ursa Major is a good mass tracer, since both the atomic and the molecular gas are probably optically thin at visual wavelengths, and the grains are heated not by local sources but by the background field of Galactic starlight. The N(H/sub 2/)/W(CO) ratio thus derived for those diffuse clouds, is significantly lower than the ratio applicable to Galactic plane surveys. 29 references.

The Polaris Flare - Extensive molecular gas near the north celestial pole

CO observations with the CfA 1.2 m telescope reveal an extensive molecular cloud at high Galactiic latitude in the direction of the north celestial pole. The object, the Polaris Flare, covers 80 percent of the 50 sq deg surveyed, making it larger in apparent size by an order of magnitude than previously known cirrus clouds. The Polaris Flare is at a distance of about 240 pc and has a total mass of about 5500 solar masses. Its distance, 100 micron intensity, N(H2)/W(CO) ratio, and gravitational stability are similar to those of other high-latitude clouds. IRAS 100 micron emission over more than 40 percent of the sky is comparable to that toward the Flare, suggesting that very sensitive CO surveys at high latitudes can detect molecular gas in a much larger fraction of the sky than previously suspected. 24 refs.

Small-area molecular structures without shielding

Using the IRAM 30 m telescope two molecular structures have been detected which cover very small areas, FWHM 1 0 . The clouds have velocities ofvlsr 5k m s 1 and linewidth ofv 0: 8k m s 1 ; thus they belong most likely to the Milky Way. Applying standard conversion factors one finds that even at the upper distance limit of 2200 pc the structures are low mass objects (M= (1 6) 10 4 ( d 100 pc ) 2 M) which are not gravitationally virialized. H I 21 cm line data towards the clouds show no prominent HI clouds. The total H I column densities for both structures are below N(HI) 2:1 10 20 cm 2 , corresponding to AV 0:2 mag, assuming a standard gas-to-dust ratio. IRAS 100m data towards the structures show also only low emission, consistent with low extinction. Unless there is unseen cold dust associated with the structures this shielding is too low for the structures to survive the interstellar radiation field for a long time. The detection of 2 such structures in a rather limited sample of observations suggests that they could be a rather common feature in the interstellar medium, however, so far not recognized as such due to the weakness of their lines and their small extent.

Cold dust and its relation to molecular gas in the dwarf irregular galaxy NGC 4449

We present observations of the dwarf irregular galaxy NGC 4449 at 850m and 450m obtained with SCUBA at the JCMT. The distribution of the cold dust agrees well with that of the CO and H emission. To explain the integrated mm- through far-infrared continuum spectrum three dust components are required, with temperatures of 16 K, 39 K and 168 K, respectively. The dust mass is dominated by the cold dust component; we derive a total dust mass of3:8 10 6 M, and with the local gas-to-dust ratio of130 a total gas mass of MHI+H2 4:9 10 8 M. Comparison with the HI mass leads to a total molecular gas mass of3:4 10 8 M. We derive a conversion factor of the CO line intensity to molecular hydrogen column density XCO= NH2 =ICO which is at least 11 times larger than the Galactic value. These values are in accord with the lower metallicity of NGC 4449.

Gravitationally bound cores in a molecular cirrus cloud

Using MAMBO at the IRAM 30 m telescope we have observed a dense core in the cirrus cloud MCLD 123.5+24.9 in the dust continuum emission at 250 GHz. The core is detected as an elongated filament with an extent of 4.′2 × 0.′7, corresponding to 0.18 pc×0.03 pc at an adopted distance of 150 pc. We find a close correlation between the continuum emission and previously observed CO (1→0) line emission. Using standard dust models we derive hydrogen column densities of up to 10 cm−2. The total mass for the filament is about 0.66M . We also present observations of the HC3N (3→2), (4→3), and (10→9) emission lines obtained with the MPIfR 100 m and the IRAM 30 m telescopes. The distribution is very different from the dust continuum and the CO (1→0) line emission. HC3N is concentrated in two distinct clumps located at the ends of the filament seen in the other tracers. Based on a LVG analysis of the HC3N transitions we derive column densities of N(HC3N)/∆v ≈ 1013cm−2/ km s−1 and volume densities of n(H2) ≈ 10 cm−3. We find that the HC3N clumps have masses of 0.13 and 0.19M . Our data demonstrate that the cirrus cloud cores are gravitationally bound, and that they show chemical structure indicating different evolutionary stages within the cloud.

The Discovery of a Molecular Complex in the Tidal Arms near NGC 3077

We present the discovery of a giant molecular complex (r ≈ 350 pc, Mvir ≈ 107 M☉) in the tidal arms southeast of NGC 3077, a member of the M81 triplet. The complex is clearly detected in the 12CO (J = 1 → 0) transition at five independent positions. The position relative to NGC 3077, the systemic velocity (vhel ≈ 14 km s-1), and the cloud-averaged line width (Δv ≈ 15 km s-1) indicate that the object is not related to Galactic cirrus, but is extragalactic. The tidal H I arm in which the molecular complex is located has a total H I mass of Mvir ≈ 3 × 108 M☉. This tidal material was presumably stripped off the outer parts of NGC 3077 during the closest encounter with M81, about 3 × 108 yr ago. After the complex detected along a torn-out spiral arm of M81 by Brouillet et al., it is the second of its kind reported so far. Based on published optical observations, we have no evidence for ongoing star formation in the newly detected molecular complex. Since the system has all the ingredients to form stars in the future, we speculate that it might eventually resemble the young dwarf galaxies in the M81 group.

Carbon monoxide (CO) in the Magellanic irregular galaxy NGC 55 and the N(H2)/W(CO) ratio in irregular galaxies

Results are presented from a search of NGC 55 and from mapping of NGC 55 in the J = 1-0 CO line. It is shown that the extent of the CO cloud is about 975 pc along the major axis and 390 pc perpendicular to the major axis. It is estimated that the total molecular mass of the cloud is about 8 x 10 to the 7th solar masses. It is found that the factor that transforms the velocity integrated CO line, W(CO), into a column density of molecular H, N(H2), is similar to the value found for the SMC and about 20 times higher than values for the Galaxy. It is concluded that, for irregular galaxies, the dependence of N(H2)/W(CO) on metallicity is inversely proportional to the nitrogen abundance. 24 refs.

NEUTRAL ATOMIC CARBON IN INTERMEDIATE-VELOCITY CLOUDS

Using the Heinrich-Hertz-Telescope we have detected for the —rst time the [C I] line tran3P 1 ] 3P 0 sition at 492 GHz of neutral atomic carbon from two intermediate-velocity clouds (IVCs): IVC 135]54[46 and the Draco Nebula. Both clouds are located at high Galactic altitudes, more than 200 pc above the Galactic plane. The width of the [C I] line is quantitatively comparable to that of the 12CO rather than to that of the 13CO line, as commonly found for Galactic plane clouds. This indicates that neutral atomic carbon within the diUuse medium at high Galactic altitudes —lls a signi—cantly larger volume in comparison to the situation in dark or denser cirrus clouds close to the Galactic plane. Moreover, our observations suggest that for IVCs cooling by the [C I] line is as important as or even more important than that by the low level CO lines. PDR models used to approximate the observed line intensity ratios indicate that the molecular gas in high-altitude IVCs is exposed to a weak interstellar radiation —eld while its density must be high. . . . . ..

On the nature of dust clouds in the region towards M 81 and NGC 3077

There is some controversy on the nature of dust clouds found in direction of the interacting galaxy triplett M81, M82, and NGC3077. Are they associated with the tidal arms seen in HI around those galaxies or are they simply Galactic foreground clouds? Data from the SPIRE instrument onboard HERSCHEL and MIPS onboard of SPITZER are used to derive physical parameters for the dust clouds. These observions are compared to CO clouds previously mapped with the IRAM and the FCRAO radio telescopes.SPIRE and MIPS maps show several dust clouds north of M81 and south of NGC3077. Modelling of the dust emission provides total hydrogen column densities between 1.5 and 5.0 * 10^20 cm^-2. Dust temperatures are between 13 to 17K. No significant difference in the dust emission can be found between individual clouds. It is shown that CO line emission provides the best clues on the origin of those clouds. Most of the clouds seen towards M81 are associated with small-area molecular structures (SAMS), i.e. tiny CO clouds of Galactic origin. The clouds seen towards NGC3077 are partly associated with the tidal arms and are partly in the Galactic foreground associated with SAMS.

Detection of H 2 in UV Absorption in the LMC

Based on measurements with the ORFEUS far UV echelle spectrograph, we present the first detection of H-2 in absorption in the Large Magellanic Cloud (LMC) in the spectrum of the star LH 10:3120.