1 Introduction

 The effect of cluster environment on the star formation properties of galaxies has long been a matter of debate. While some studies have suggested a lower star formation rate for cluster spirals as compared to the field (e.g. Gisler 1978; Dressler et al. 1985), other work has suggested a similar or enhanced rate, particularly for early-type spirals (e.g. Kennicut et al. 1984; Gavazzi et al. 1991; Moss & Whhitle 1993; Biviano et al. 1997). With the discovery that in distant rich clusters there is a high fraction of blue, star-forming galaxies, often with unusual morphology suggestive of mergers and tidal interactions (e.g. Lavery & Henry 1986; Thompson 1988; Couch et al. 1994), there is renewed interest in tidally-induced star formation by mergers and interactions in nearby clusters. In order to address these issues, Moss, Whittle and co-authors have completed an objective-prism survey of eight nearby clusters of galaxies to detect global H$\alpha$ + [Nii] emission as an indicator of the current rate of massive star formation. The survey technique is described by Moss et al. (1988), hereafter MWI, and initial results have been discussed by Moss & Whittle (1993) (see also Moss 1990; Moss et al. 1995; Moss & Whittle 1997; Moss et al. 1998). We have extended this survey to a ninth cluster, the Hydra I cluster, Abell 1060.

Abell 1060 is one of the nearest of the Abell clusters, at a redshift of z $\sim$ 0.01, and is the nearest large cluster beyond the Virgo and Fornax clusters. It has a high spiral fraction (e.g. Solanes & Salvador-Solé 1992) and is a relatively poor cluster, with a low density intracluster medium (Loewenstein & Mushotzkzy 1996) and low X-ray luminosity (Edge & Stewart 1991). Since it is the nearest of the clusters surveyed by us so far, it can be surveyed to a fainter limit in absolute magnitude. However, its proximity means that it has a large projected diameter on the sky, with one Abell radius, $1\, r_{\rm A}=2\hbox{$.\!\!^\circ$}3=1.5\, h^{-1}$ Mpc (where h is defined in terms of the Hubble constant $H_{0}=100h\ \rm{km\ s}^{-1}\rm{Mpc}^{-1}$). Whereas other clusters were surveyed in a region of radius 1.5 $r_{\rm A}$, the photographic plate size restricted survey of Abell 1060 to a region of radius somewhat less than one Abell radius (see Sect. 2.1). Richter (1989), hereafter R89, presents a catalogue of 581 galaxies in the cluster area, which contains a sample which is complete to the magnitude limit V₂₅=16.65, within 2$^\circ$ of the cluster centre. This is a convenient complete sample for the present H$\alpha$ survey, extending to a fainter apparent magnitude than the Zwicky Catalogue used to define samples for other clusters.

Cluster properties are summarised in Table 1. The (B1950.0) position of the central cluster galaxy NGC 3311 is given as the cluster centre in Cols. 2 and 3. The mean heliocentric radial velocity $\langle v_{\odot}\rangle$ and velocity dispersion $\sigma$ determined using $N_{\rm gal}$ individual galaxy redshifts are given in Cols. 4, 5, and 6 (Bird 1994). The Abell richness class (Abell et al. 1989) is given in Col. 7, the Bautz-Morgan and Rood-Shastry type classes are given in Cols. 8 and 9 respectively (Bautz & Morgan 1970; Struble & Rood 1982).

 

Table 1:   Cluster properties

The observations and data reduction are described in Sect. 2. Details of the observational technique are given in Sect. 2.1, and of the process of identifying the emission-line galaxies in Sect. 2.2, where a table of the detected emission-line galaxies (ELGs) is given. Measurements of radial velocities for the detected emission-line galaxies are presented in Sect. 2.3, and those of H$\alpha$ + N[II] equivalent widths and fluxes in Sect. 2.4, where measured H$\alpha$ + N[II] fluxes are also converted into effective star formation rates. A comparison of detected cluster emission-line galaxies in Abell 1060 with field galaxies and detected emission-line galaxies in other clusters is given in Sect. 2.5. Notes on individual galaxies are given in Sect. 2.6. Finally, we present a brief discussion of our results in Sect. 3.