Molecular Gas in the Bulge and Ring of NGC 7331

Maps of the J=2-1 12CO emission from the SbII galaxy NGC 7331 show a low-contrast ring at a radius of about 3.5 kpc. There is no evidence for a pronounced central hole in the CO distribution as claimed by others. The molecular ring is just outside the radius of peak emission from warm dust, but coincides with the peak of colder dust emission. Various 12CO and 13CO transitions have been observed from three positions including the center, which was also observed in the 492 GHz transition. The line measurements have been modelled by emission from a clumpy mixture of low-density molecular gas at about T(kin) = 10 K and high-density molecular gas at temperatures of 10 K and 20 K. The CO to H2 conversion factor in NGC 7331 is lower than that in the Milky Way, and lowest in the center of NGC 7331. The total interstellar gas mass is dominated by molecular hydrogen in the bulge and in the ring, and by atomic hydrogen outside the ring. Total hydrogen mass densities in the ring are about twice those in the bulge. Total gas to dynamic mass ratios increase from 1% in the bulge to 3% outside the ring. The bulge molecular gas may have originated in mass loss from bulge stars, in which case the molecular ring is probably the consequence of evacuation efficiency decreases at the outer bulge edge.Comment: 12 pages, 4 figures, A&A in pres

HCN versus HCO+ as dense molecular gas mass tracer in Luminous Infrared Galaxies

It has been recently argued that the HCN J=1--0 line emission may not be an unbiased tracer of dense molecular gas (\rm n\ga 10^4 cm^{-3}) in Luminous Infrared Galaxies (LIRGs: $\rm L_{FIR}> 10^{11} L_{\odot}$) and HCO$^+$ J=1--0 may constitute a better tracer instead (Graci\'a-Carpio et al. 2006), casting doubt into earlier claims supporting the former as a good tracer of such gas (Gao & Solomon 2004; Wu et al. 2006). In this paper new sensitive HCN J=4--3 observations of four such galaxies are presented, revealing a surprisingly wide excitation range for their dense gas phase that may render the J=1--0 transition from either species a poor proxy of its mass. Moreover the well-known sensitivity of the HCO$^+$ abundance on the ionization degree of the molecular gas (an important issue omitted from the ongoing discussion about the relative merits of HCN and HCO$^+$ as dense gas tracers) may severely reduce the HCO$^+$ abundance in the star-forming and highly turbulent molecular gas found in LIRGs, while HCN remains abundant. This may result to the decreasing HCO$^+$/HCN J=1--0 line ratio with increasing IR luminosity found in LIRGs, and casts doubts on the HCO$^+$ rather than the HCN as a good dense molecular gas tracer. Multi-transition observations of both molecules are needed to identify the best such tracer, its relation to ongoing star formation, and constrain what may be a considerable range of dense gas properties in such galaxies.Comment: 16 pages, 4 figures, Accepted for publication in the Astrophysical Journa

A low-mass HI companion of NGC 1569?

High-sensitivity maps of the large-scale structure of atomic hydrogen in the starburst dwarf galaxy NGC 1569 show evidence for an HI cloud with a mass of 7*10**6 M_sun, at a projected distance of 5 kpc from the parent galaxy. This cloud may be a condensation in a low-column-density HI halo or a companion galaxy/HI-cloud. NGC 1569 and its companion are connected by a low surface brightness HI bridge. At the edge of NGC1569, the HI bridge coincides with H_alpha arcs, also detected in soft X-rays.Comment: 5 pages, 4 figures, 1 tabl

Detection of Neutral Carbon in the M 31 Dark Cloud D478

Emission from the 492 GHz CI tranition was detected towards the dark cloud D478 in M31. Using existing 12CO and 13CO measurements, models for the gas properties of D478 are discussed. The observed CO and C line ratios can be explained by two-component models (dense cores and tenuous envelopes); single-density models appear less likely. The models indicate temperatures T(kin) = 10 K. The beam-averaged C column density is 0.3 - 0.8 times that of CO, whereas the total carbon to hydrogen ratio N(C)/N(H) = 5-3 times 10**-4. The resulting CO-to-H2 conversion factor X is about half that of the Solar Neighbourhood. With temperatures of about 10 K and projected mass densities of 5-10 M(sun)/pc**2 there appears to be no need to invoke the presence of very cold and very massive clouds. Rather, D478 appears to be comparable to Milky Way dark cloud complexes such as the Taurus-Auriga dark cloud complex.Comment: 7 Pages, 1 Figure; accepted by A&

Detection of X-ray-Emitting Hypernova Remnants in M101

Based on an ultra deep (230 ks) ROSAT HRI imaging of M101, we have detected 5 X-ray sources that coincide spatially with optical emission line features previously classified as supernova remnants in this nearby galaxy. Two of these coincidences (SNR MF83 and NGC5471B) most likely represent the true physical association of X-ray emission with shock-heated interstellar gas. MF83, with a radius of ~ 134 pc, is one of the largest remnants known. NGC5471B, with a radius of 30 pc and a velocity of at least 350 km/s (FWZI), is extremely bright in both radio and optical. The X-ray luminosities of these two shell-like remnants are $\sim 1$ and $3 \times 10^{38} ergs/s$ (0.5-2 keV), about an order of magnitude brighter than the brightest supernova remnants known in our Galaxy and in the Magellanic Clouds. The inferred blastwave energy is $\sim 3 \times 10^{52} ergs$ for NGC5471B and $\sim 3 \times 10^{53}$ ergs for MF83. Therefore, the remnants likely originate in hypernovae, which are a factor of $\gtrsim 10$ more energetic than canonical supernovae and are postulated as being responsible for Gamma-ray bursts observed at cosmological distances. The study of such hypernova remnants in nearby galaxies has the potential to provide important constraints on the progenitor type, rate, energetics, and beaming effect of Gamma-ray bursts.Comment: 10 pages, 2 gif figures, Accepted for publication in Astrophysical Journal Letter