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Astron. Astrophys. 326, 915-918 (1997)

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1. Introduction

The HI Tully-Fisher relation has been the most successful and widely applied tool to measure distances to galaxies (e.g., Tully & Fisher 1977; Aaronson et al. 1986; Pierce & Tully 1988). CO linewidth has been also used in the Tully-Fisher relation instead of HI linewidth. HI linewidth almost coincides with CO linewidth for galaxies in the Coma cluster and other nearby clusters (Dickey & Kazes 1992), for field galaxies (Sofue 1992; Schöniger & Sofue 1994) and for Virgo cluster galaxies (Schöniger & Sofue 1997).

Because the beamsize of CO observations is much sharper than that for HI, we are able to resolve individual galaxies at higher redshift, and avoid contamination by other galaxies in one beam for CO observations. Observations for the CO-line Tully-Fisher relation have been performed using the Nobeyama Radio Observatory (NRO) 45-m telescope (HPBW = 15"), and CO linewidths have been obtained for galaxies at redshift [FORMULA] 29,000 km s [FORMULA] (Sofue et al. 1996), at which the CO beamsize is still small enough to distinguish galaxies.

HI gas extends far from the center of galaxies beyond the optical radius, while it is deficient in the central region (Bosma 1981). On the other hand, the molecular gas is known to be more concentrated in a better correlation with the optical disk (Young & Scoville, 1982). Since the atomic and molecular gases in galactic disk are distributed separately in radius (Sofue et al.1995; Honma et al.1995), the HI linewidths may be more strongly disturbed by galaxy-galaxy interaction, which is inevitable in rich clusters of galaxies and at high redshifts. Interacting galaxies are generally excluded from a sample of nearby galaxies for the Tully-Fisher relation. However, when we measure distances to farther galaxies, we may overlook features of interaction and overestimate the distances.

In this paper we examine the correlation between CO and HI linewidths of nearby galaxies and discuss the Tully-Fisher relation for interacting galaxies.

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© European Southern Observatory (ESO) 1997

Online publication: April 8, 1998