Astronomy and Astrophysics – Astronomy
Scientific paper
May 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011iaus..280p.107b&link_type=abstract
The Molecular Universe, Posters from the proceedings of the 280th Symposium of the International Astronomical Union held in Tole
Astronomy and Astrophysics
Astronomy
Scientific paper
Using the IRAM 30m telescope we observed and detected HCN and HCO^+ 1-0 towards 6 positions along the major axis of the nearby spiral galaxy M33, at radial distances of up to 3.4 kpc. M33, viewed at an intermediate inclination from a distance of 840 kpc, is an ideal source to study these molecules in an environment not affected by an active galactic nucleus (AGN). Unlike the Galaxy, which is seen edge-on, M33's inclination of 56 degrees allows to determine precise positions and velocities inside its large scale disk. Furthermore, at this distance M33 is still near enough that individual giant molecular clouds (GMCs) can be resolved by the 28" beam of the IRAM 30m at the frequencies of HCN and HCO^+, which translates to 114 pc spatial resolution. HCN, HCO^+, and 13CO were observed at an RMS level of about 0.7 mK at 2 km/s resolution. Additionally, the frequencies of the isotopmeres of HCN and HCO^+, HNC and HOC^+ also fall within the bandwidth of our observation setup. By stacking all observed spectra, paying the price of losing the positional information, we could lower the baseline RMS even further to 0.27 mK and detected HNC and HOC^+ as well as CCH at a very low level of ≈ 3 σ. Having high critical densities they trace the dense gas and thus also star formation (SF). HCN shows a tight correlation with SF that holds for galactic GMCs to the nuclear environment of distant star-burst (SB) galaxies as has been shown by Gao & Solomon 2004 and Wu et al. 2005. Furthermore, the ratio of HCO^+ to HCN is a popular tool in extra-galactic astronomy to investigate the predominant radiation mechanism (X-Ray - AGN , UV - SB) in galactic nuclei. Interestingly, we find HCN-to-HCO^+ ratios in the range of 0.7-2.8. This includes typical values found towards AGN and SB dominated galactic nuclei suggesting that on local scale this interpretation does not hold, since M33 is neither dominated by intense X-ray radiation nor by the influence of a nearby AGN. These observations are embedded in the Herschel open key time project HERM33ES that targets the major axis of M33 to observe the major FIR gas cooling lines, e.g. CII and OI. In the scope of this project a huge data set of complementary observations has been obtained that give a wealth of information on the regions we studied in HCN and HCO^+.
Buchbender Christof
Kramer Carsten
Rosolowsky Erik
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