Atomic and molecular gas in the starburst galaxy NGC 4945

Abstract

Abstract. Spatial and kinematical correlations between the H i and12 CO(2−1) emission of the southern spiral galaxy NGC 4945 are studied with a common angular resolution of ∼23 (corresponding to 750 pc at D = 6.7 Mpc) and a velocity resolution of ∼7 km s −1 . The 1.4 GHz continuum emission is also observed. The H i kinematics yield a galaxy mass of ∼1.4×1011 M within radius R = 380 , with molecular and neutral atomic gas each contributing ∼2%, respectively. A central continuum source of size 7 . 6 × 3 . 4 (250 × 110 pc) is enveloped by a molecular cloud of mass 1.5 × 10 9 M for R ≤ 7 . 5, and is rapidly rotating with Vrot ∼ 160 km s −1 . H i emission from the central region at velocities |V −Vsys| > 200 km s −1 may be related to optically detected gas that is believed to trace an outflow directed towards the halo. Nuclear H i absorption at V -Vsys ∼ +80 km s −1 suggests inflow towards the centre, that was so far only seen in molecular lines. H i features at each end of the major axis (|R| ∼ 600 ) are interpreted as spiral arms that are viewed tangentially and that also cause prominent emission features in the radio continuum, H i, and CO further inside the galaxy. A central elongated region showing non-circular motions is interpreted as a bar which fuels the nuclear starburst. The H i and CO position-velocity data have been analysed using linear resonance theory, and possible locations of resonances are identified.

Detection of the H92αrecombination line from NGC 4945

“…The total area of line emission is 1.5 times the naturally-weighted beam area (8.6 times the uniformly-weighted beam area), or 4500 pc 2 . The H91α + H92α spectrum integrated over the line emitting region shows a strong line detection with significance of 52σ, with centroid at 581 km s −1 in good agreement with the CO systemic velocity of 585 km s −1 (Chou et al 2007) and with the velocity of the HI absorption towards the nucleus of 585 km s −1 , though significantly higher than the HI systemic velocity inferred from the whole galaxy of 557 km s −1 (Ott et al 2001). The line FWHM is 220 km s −1 after deconvolving the instrumental velocity resolution of 42 km s −1 .…”

“…This was seen also in CO(2−1) by Ott et al (2001) with SEST at 23 resolution. However, inflow was not seen in the H91α + H92α dynamics within 4 of the nucleus.…”

“…H I kinematics were studied by Ott et al (2001) with the ATCA at 3.5 resolution. They saw HI in emission over the disk, with an extent of 22 × 4 (43 kpc × 7.8 kpc) and in absorption towards the nuclear continuum emission.…”

“…CO has been imaged at resolution from 43 to 15 with SEST by Dahlem et al (1993), Ott et al (2001), and Mauersberger et al (1996), and interferometrically at 4 with the Submillimeter Array (SMA) by Chou et al (2007). All groups found the CO to be strongly concentrated towards the centre in a disk of molecular material 16 × 11 (310 pc × 210 pc).…”

THE VARIABLE HARD X-RAY EMISSION OF NGC 4945 AS OBSERVED BYNUSTAR

“…Madejski et al (2000) and Done et al (2003) suggested that the megamaser spots are aligned with the absorbing disk/torus at 0.1-1 pc. In addition to this dense absorbing disk, the nucleus is completely embedded in a column of ∼10 23 cm −2 at ∼25 pc, which may be due to a bar-driven gas inflow (Ott et al 2001) or perhaps a high-latitude extension of the disk/torus. The extreme absorption region is surrounded by an extended, dusty, lower absorption region on scales of 50-100 pc in the starburst region.…”