Yoshiaki Hagiwara

Dense gas in nearby galaxies - XV. Hot ammonia in NGC 253, Maffei 2 and IC 342

The detection of NH3 inversion lines up to the ( J; K) = (6; 6) level is reported toward the central regions of the nearby galaxies NGC 253, Maei 2, and IC 342. The observed lines are up to 406 K (for (J; K)= (6; 6)) and 848 K (for the (9; 9) transition) above the ground state and reveal a warm (Tkin = 100:::140 K) molecular component toward all galaxies studied. The tentatively detected ( J; K) = (9; 9) line is evidence for an even warmer (>400 K) component toward IC 342. Toward NGC 253, IC 342 and Maei 2 the global beam averaged NH3 abundances are 1 2 10 8 , while the abundance relative to warm H2 is around 10 7 . The temperatures and NH3 abundances are similar to values found for the Galactic central region. C-shocks produced in cloud-cloud collisions can explain kinetic temperatures and chemical abundances. In the central region of M 82, however, the NH3 emitting gas component is comparatively cool (30 K). It must be dense (to provide sucient NH3 excitation) and well shielded from dissociating photons and comprises only a small fraction of the molecular gas mass in M 82. An important molecular component, which is warm and tenuous and characterized by a low ammonia abundance, can be seen mainly in CO. Photon dominated regions (PDRs) can explain both the high fraction of warm H2 in M 82 and the observed chemical abundances.

VLBI Study of Water Maser Emission in the Seyfert 2 Galaxy NGC 5793. I. Imaging Blueshifted Emission and the Parsec-Scale Jet

We present the first result of VLBI observations of the blueshifted water maser emission from the type 2 Seyfert galaxy NGC 5793, which we combine with new and previous VLBI observations of continuum emission at 1.7, 5.0, 8.4, 15, and 22 GHz. Maser emission was detected earlier in single-dish observations and found to have both red- and blueshifted features relative to the systemic velocity. We could image only the blueshifted emission, which is located 3.6 pc southwest of the 22 GHz continuum peak. The blueshifted emission was found to originate in two clusters that are separated by 0.7 mas (0.16 pc). No compact continuum emission was found within 3.6 pc of the maser spot. A compact continuum source showing a marginally inverted spectrum between 1.7 and 5.0 GHz was found 4.2 pc southwest of the maser position. The spectral turnover might be due to synchrotron self-absorption caused by a shock in the jet owing to collision with dense gas, or it might be due to free-free absorption in an ionized screen, possibly the inner part of a disk, foreground to the jet. The water maser may be part of a maser disk. If so, it would be rotating in the opposite sense to the highly inclined galactic disk observed in CO emission. We estimate a binding mass within 1 pc of the presumed nucleus to be on the order of 107 M☉. Alternatively, the maser emission could result from the amplification of a radio jet by foreground circumnuclear molecular gas. In this case, the high blueshift of the maser emission might mean that the masing region is moving outward away from the molecular gas surrounding an active nucleus.

A search for extragalactic H

We report the result of an on-going survey for 22 GHz H2O maser emission towards infrared luminous galaxies. The observed galaxies were selected primarily from the IRAS bright galaxy sample. The survey has resulted in the detection of one new maser. The new maser was discovered towards the [U]LIRG/merger galaxy NGC6240, which contains a LINER nucleus. This is the first detection of an H2O maser towards this class of galaxy, they are traditionally associated with OH megamaser sources. The detected maser emission is highly redshifted (~ 260-300 km/s) with respect to the adopted systemic velocity of the galaxy, and we identified no other significant emission at velocities<+- 500 km/s relative to the systemic velocity. The presence of high-velocity maser emission implies the possible existence of a rotating maser disk formed in the merging process. The large maser luminosity (~ 40 Lo) suggests that an active galactic nucleus could be the energy source that gives rise to the water emission. Alternatively, the maser emission could be associated with the previously observed double radio source in the centre of the galaxy. Interferometric observations with high angular resolution will be able to clarify the origin of the new maser.We report the result of an on-going survey for 22 GHz H2O maser emission towards infrared luminous galaxies. The observed galaxies were selected primarily from the IRAS bright galaxy sample. The survey has resulted in the detection of one new maser. The new maser was discovered towards the [U]LIRG/merger galaxy NGC6240, which contains a LINER nucleus. This is the first detection of an H2O maser towards this class of galaxy, they are traditionally associated with OH megamaser sources. The detected maser emission is highly redshifted (~ 260-300 km/s) with respect to the adopted systemic velocity of the galaxy, and we identified no other significant emission at velocities < +- 500 km/s relative to the systemic velocity. The presence of high-velocity maser emission implies the possible existence of a rotating maser disk formed in the merging process. The large maser luminosity (~ 40 Lo) suggests that an active galactic nucleus could be the energy source that gives rise to the water emission. Alternatively, the maser emission could be associated with the previously observed double radio source in the centre of the galaxy. Interferometric observations with high angular resolution will be able to clarify the origin of the new maser.