Molecular gas around low-luminosity AGN in late-type spirals

¹ Research and Scientific Support Department, European Space Agency, Keplerlaan 1, 2200 AG Noordwijk, The Netherlands
e-mail: tboeker@rssd.esa.int
² Max-Planck-Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
³ Departamento de Física Teórica y del Cosmos, Facultad de Ciencias, Universidad de Granada, Spain

Received: 15 May 2011
Accepted: 26 July 2011

Abstract

We have studied the molecular gas in the vicinity of low-luminosity active galactic nuclei (AGNs) in three bulge-less spiral galaxies: NGC 1042, NGC 4178, and NGC 4395. The (1−0) and (2−1) transitions of gaseous carbon monoxide (CO) are clearly detected within the central kpc of all three galaxies. In the case of NGC 4395, this constitutes the first reported detection of CO. In general, the CO emission is faint, as may be expected from their less-than-spectacular star formation activity. Interestingly, however, both face-on galaxies in our sample (which allow an unimpeded view of their nucleus) show an elevated intensity ratio CO(2−1)/CO(1−0) when compared to similar late-type spirals without an AGN. We discuss that this is unlikely due to a very compact CO source. Instead, we speculate that even energetically weak AGN can impact the physical state of the surrounding gas. We do not detect any tracers of dense molecular gas such as HCN or HCO+, but the sensitivity of our observations allows us to establish upper limits that lie at the low end of the range observed in more energetic AGN. The derived gas density is less than n_H2 ≈ 2 × 10³  cm^-3 which is significantly lower than in most other nearby galaxies. The scarcity of dense gas suggests that the conditions for star formation are poor in these nuclei.