arXiv subjects
Publications and source records attributed to A. Tarchi.
We present 1.4 GHz HI absorption line observations towards the starburst in NGC2146, made with the VLA and MERLIN. The HI absorption has a regular spatial and regular velocity distribution, and does not reveal any anomaly as a sign of an encounter with another galaxy or of a far-evolved merger.
We report the first detection of a water megamaser in a radio-loud galaxy, 3C403. This object has been observed as part of a small sample of FRIIs with evidence of nuclear obscuration. The isotropic luminosity of the maser is ~1200 L(sun). With a recessional velocity of cz ~ 17680 km/s it is the most distant water maser so far reported. The line arises from the densest interstellar gas component ever observed in a radio-loud galaxy. Two spectral features are observed, likely bracketing the systemic velocity of the galaxy. These may either arise from the tangentially seen parts of a nuclear accretion disk or from dense warm gas interacting with the radio jets.
We report the first results of an ongoing survey at 22 GHz with the 100-m Effelsberg telescope to search for water maser emission in bright IRAS sources. We have detected water vapor emission in IC 342. The maser, associated with a star forming region ~10-15 arcsec west of the nucleus, consists of a single 0.5 km/s wide feature and reaches an isotropic luminosity of 0.01 L(sun) (D = 1.8 Mpc). Our detection raises the detection rate among northern galaxies with IRAS point source fluxes S(100micron) > 50 Jy to 16%.
We report the detection of 22 GHz water vapor emission toward the starburst galaxy NGC2146, made using the Effelsberg 100-m telescope. Interferometric observations with the Very Large Array (VLA) show that a part of the emission originates from two prominent sites of star formation that are associated with compact radio continuum sources, likely ultra-compact HII regions. It is concluded that the emission arises from the most luminous and distant water `kilomasers' detected so far. Our data increase the number of water maser detections in northern galaxies (Dec > -30 deg) with 100 micron IRAS point source fluxes > 50 Jy to 18%.
The detection of 22 GHz water vapor emission from IC342 is reported, raising the detection rate among northern galaxies with 100 micron IRAS point source fluxes > 50 Jy to 16%. The maser, associated with a star forming region 10-15 arcsec west of the nucleus, consists of a single 0.5 km/s wide feature and reaches an isotropic luminosity of 0.01 solar luminosity (D = 1.8 Mpc). If the time variability is intrinsic, the maser size is < 1.5 x 10^(16) cm (< 0.5 mas) which corresponds to a brightness temperature of > 10^(9) K. The linewidth, luminosity, and rapid variability are reminiscent of the 8 km/s super maser in Orion-KL. A velocity shift of 1 km/s within two weeks and subsequent rapid fading is explained in terms of a chance alignment of two dense molecular clouds. Observations at 22 GHz toward Maffei2 are also reported, yielding a 5 sigma upper limit of 25 mJy for a channel spacing of 1.05 km/s.
We have used MERLIN, at 1.4 and 5 GHz, to search for radio supernovae (RSNe) and supernova remnants (SNRs) in the unobscured irregular dwarf galaxy NGC1569, and in particular in the region of its super star clusters (SSCs) A and B. Throughout NGC1569 we find some 5 RSNe and SNRs but the SSCs and their immediate surroundings are largely devoid of non-thermal radio sources. Even though many massive stars in the SSCs are expected to have exploded already, when compared with M82 and its many SSCs the absence of RSNe and SNRs in and near A and B may seem plausible on statistical arguments. The absence of RSNe and SNRs in and near A and B may, however, also be due to a violent and turbulent outflow of stellar winds and supernova ejected material, which does not provide a quiescent environment for the development of SNRs within and near the SSCs.
X-ray and optical observations have shown that the supernova explosions and stellar winds of the starburst in NGC 2146 produce an outflow of hot material along the minor axis. This outflow is expected to have a more or less conical shape, on either side of the galactic plane, and cone walls of cooler material where the outflow is in shock contact with halo gas. We attempt to determine the geometry (diameter at the base and opening angle) and the physical parameters (velocity and density) of the material in the cone walls from the optical emission line and radio observations presented here, and from published X-ray, radio, and optical observations. We compare the outflow of NGC 2146 with the outflow of M 82.
We present a high-resolution 5-GHz radio continuum map of the starburst galaxy NGC 2146 made with MERLIN and the VLA, in a search of radio supernovae and supernova remnants expected to be already produced by the most massive stars in the starburst. At 5 GHz, about 20 point sources were detected earlier by Glendenning & Kronberg (1986) in the central 800 pc of NGC 2146. Our observations with higher sensitivity and resolution made with MERLIN and the VLA confirms the detection of 18 sources, and resolves 7 of them. Additional 1.6-GHz MERLIN observations disclose 9 sources coincident in position with those detected at 5 GHz, which allows us to derive their spectral indices. Only 3 sources have indices consistent with synchrotron emission from supernova remnants or radio supernovae, while the others have very steep inverted spectra. We suggest that the sources with positive spectral index are optically thick ultra-compact and/or ultra-dense HII regions with high electron densities and high emission measures (EM > 10^(7) cm^(-6) pc). Minimum energy requirements indicate that these regions may contain up to 1000 equivalent stars of type O6. When compared with M 82, the galaxy NGC 2146 lacks however a large number of supernova remnants. We suggest that NGC 2146 is experiencing a burst of star formation stronger than that in M 82, but being in a younger phase. We may, however, not exclude an alternative scenario in which strong free-free absorption at 1.6 GHz in foreground ionized gas with very high emission measures (EM > 10^(8) cm^(-6) pc) hides a certain number of supernova remnants, thus rendering for some sources the observed inverted spectra.