Repeated exposures of the same field taken with opposite dispersion directions are useful to confirm the reality of emission features allowing both elimination of spurious detections, and confirmation of weak emission features, particularly in cases where the underlying continuum is too faint to be seen. Reversing the dispersion direction also enables redshift determination (see MWI). The seeing is critical to the detection of emission. Details of the exposures, both taken under good seeing conditions, are given in Table 2.
The fields covered by the two plates are not exactly coincident. The plate boundaries are shown in Fig. 1 as solid boxes. The coverage of R89 is also shown: the dashed box shows the area covered by the catalogue, the dashed circle the area within which it is complete to the magnitude limit V25 = 16.65. One Abell radius is indicated by the dotted circle. There are three distinct regions in Fig. 1: the region in which the plates overlap, the region covered by only one plate, and the region within which R89 is complete. The complete region is contained entirely within the overlap region.
|Figure 1: Survey coverage. Plate boundaries are indicated by solid lines. The area covered by R89 is shown by the dashed box, the area within which it is complete by the dashed circle. The dotted circle has a radius of 1.5 h-1 Mpc (1 Abell radius) and is centred on the central cluster galaxy NGC 3311. Galaxies in R89 are shown as dots, detected emission-line galaxies as filled and open circles|
The plates were searched systematically for galaxies showing H emission using a low-power binocular microscope. A visibility parameter (S strong, MS medium-strong, M medium, MW medium-weak, or W weak) describing how readily the emission is seen on the plate, and a concentration parameter (VD very diffuse, D diffuse, N normal, C concentrated, or VC very concentrated) describing the spatial distribution of the emission and the contrast with the underlying continuum, were assigned to each candidate following the scheme of MWI. Within the overlap region galaxies were only accepted as emission-line sources if they had been independently detected on both plates and the separate emission features were consistent with each other. Outside the overlap area, galaxies were accepted only if they showed strong emission. Galaxies detected in H are listed in Table 3 in order of increasing Right Ascension.
In order to construct a complete sample of surveyed galaxies for statistical analysis, it is necessary to correct for the effect of overlapping spectra. This is a particular problem at the relatively low galactic latitude () of Abell 1060. All galaxies within the region of completeness of R89 brighter than the limiting magnitude V25=16.65 were checked on both plates to ensure that their spectra were not overlapped by those of nearby objects. Table 4 lists both the identifications in R89 of the galaxies included in the complete sample, and those of galaxies which are deemed not to have been surveyed due to overlapping spectra. Those in the former list can be regarded as being confirmed as non emission-line galaxies if they do not appear in the table of detected emission-line galaxies (Table 3).
The objective prism spectra of the detected emission-line galaxies were digitised using the PDS (Plate Density Sensitometer) facility at the Royal Greenwich Observatory in order to measure line redshifts, equivalent widths, and fluxes.
Column 1. Identification number. An asterisk indicates that the object lies outside the overlap region and has therefore only been detected on one plate.
Column 2. Line 1, identification in R89. Line 2, identification in Wamsteker et al. (1985).
Column 3. Line 1, identification in Lauberts (1982). Line 2, NGC number.
Columns 5 & 6. (B 1950.0) Right Ascension and Declination. These are taken from R89 except No. 19 (Wamsteker et al. 1985), No. 23 (Lauberts 1982), No. 31 (van Driel et al. 1991), No. 33 (Lauberts 1982) and are quoted to the same precision as the source.
Column 7. V magnitude, V25 from R89.
Column 8. Morphological types from R89 (line 1) and Wang et al. (1991) (line 2) except No. 33 (Lauberts 1982). Wang's types should be more reliable than Richter's as they are based on a UK Schmidt IIIaJ plate. Note, though, that Wang's identification of the galaxy pair NGC 3314 as IG (interacting galaxy) is misleading (see text).
Column 9. Visibility parameter (S strong, MS medium-strong, M medium, MW medium-weak, W weak).
Column 10. Concentration parameter (VD very diffuse, D diffuse, N normal, C concentrated, VC very concentrated).
Column 12. An asterisk indicates a note: No. 7 appears to be tidally distorted. Lauberts (1982) notes "bright centre, faint disturbed envelope.'' No. 8 has an apparently double nucleus, and could be a merger remnant. No. 9 has peculiar morphology, exhibits several distinct emission regions. No. 12 is asymmetric. Lauberts (1982) notes an "extremely faint ring.'' No. 18, noted by Arp & Madore (1987) as a disturbed spiral, has a peculiar morphology and may have been tidally disrupted. Several emission regions are visible. No. 23 is noted as disturbed by Lauberts (1982) and ascribed "thick arms, knots, and dust patches, optical pair'' by Corwin et al. (1985). No. 24, NGC 3336, is a face-on Sc galaxy, with many individual HII regions visible in emission. No. 25 is noted by Arp & Madore as "close pair, end of chain.'' No. 32, NGC 3393, is a type 2 Seyfert (Storchi-Bergmann et al. 1995). Nos. 6, 13, 16, & 17 see Sect. 2.6.
As demonstrated by MWI, after Stock & Osborn (1980), it is possible to obtain redshift estimates for emission-line galaxies accurate to within a few hundred kms-1 using two Schmidt telescope objective prism plates taken with opposite dispersion directions. Following the method detailed in MWI, we obtained redshift estimates for all detected emission-line galaxies which we calibrated against available redshifts from the literature. Assuming that the dispersion of the prism combination is approximately constant over the narrow wavelength range of interest, the slope of this calibration gives a value for the dispersion of the 6 + 4 prism combination of 465 4 Å mm-1 between 6600 and 6800Å. The rms scatter about this relation of 340 kms-1 is slightly larger than that found by MWI of 205 kms-1; this may be partly due to the fact that MWI used a two-dimensional cross-correlation technique, whereas we used the one-dimensional analogue.
Heliocentric velocities determined using the two-plate technique for seven galaxies which do not have existing redshift determinations and the emission associated with the superposed galaxies NGC 3314 (see Sect. 2.6) are listed in Table 5.
The two-dimensional digitised spectral scans were reduced to one-dimensional spectra. These were used to determine equivalent widths of the blended H + [Nii] emission lines using the dispersion value derived from the two-plate redshift determinations (Sect. 2.3 above).
It was only possible to measure a reliable equivalent width for 13 of the 33 emission-line galaxies. Other galaxies had spectra which were overlapped by those of other objects, or emission or continuum which was too faint, or too diffuse, to be measured reliably.
Comparison between equivalent widths measured from the two plates shows a mean difference of 0()Å for the 11 galaxies for which equivalent widths were measured on both plates. The mean measured equivalent widths () are listed in Col. 3 of Table 6. A colon appended to the equivalent width value indicates some additional uncertainty in fitting the continuum.
H + [Nii] fluxes were measured in arbitrary units of photographic density, which is approximately proportional to intensity. We then used the flux calibration of Bessell (1979) to give H + [Nii] fluxes from the R magnitudes, where available in the literature, and the equivalent width measurements. These flux values were used to calibrate the photographic fluxes, which were then converted to luminosities using corrections for galactic extinction from Burstein & Heiles (1984) and the NASA Extragalactic Database (NED), the internal extinction correction given by de Vaucouleurs et al. (1991), and a distance corresponding to the cluster mean redshift for cluster members, or individual galaxy redshifts for background galaxies, using the velocity correction for Virgocentric infall given in R89 (Richter et al. 1987). H0 was taken as 50 km s-1 Mpc-1. The scatter in this calibration is in the log, corresponding to an uncertainty of 20% in luminosity.
The H + [Nii] luminosities from this calibration were corrected to give H luminosities using the conversion of (H + [Nii]) = 1.33 (H) from Kennicutt (1983). This was then converted to an effective star formation rate using the conversion factor of Alonso-Herrero et al. (1996) of L(H)/SFR = 3.101041 erg s-1/ yr-1, which corresponds to a Salpeter (1955) IMF with upper and lower mass cutoffs of 125 and 0.1 respectively. This conversion is highly IMF dependent.
Calibrated H + [Nii] fluxes and effective star formation rates are listed in Cols. 4 and 5 of Table 6. A star formation rate is not listed for NGC 3393 as it is a Seyfert. Despite the uncertainties in the star formation rate determination, the derived values of between a tenth of a solar mass and a few solar masses per year lie within the range expected for normal spirals. Nevertheless, a few galaxies appear to have an anomalously high star formation rate for their morphological type (see Sect. 2.5).
Previous work (Moss & Whittle 1993) has shown the usefulness of a distinction between diffuse emission described by the concentration classes D (diffuse) and VD (very diffuse), and compact emission described by the other three classes VC (very concentrated), C (concentrated), and N (normal). It has been found that compact emission is strongly associated with a disturbed morphology of the galaxy, and most likely results from tidally-induced star formation from galaxy-galaxy or cluster-galaxy interactions. Furthermore there is a strong correlation between cluster mean central galaxy density and the fraction of galaxies of types Sa and later with compact emission (Moss & Whittle 1997). This is illustrated in Fig. 2, where the central galaxy density is calculated from the number of galaxies with absolute magnitude within of the cluster centre, corrected for the effects of foreground and background contamination and cluster galaxies projected onto the central region. For Abell 1060, the central galaxy density calculated in this way is approximately 1.2 Mpc-3. However, the actual value is likely to be lower than this as the correction for projection was less accurate than was possible for more distant clusters. Within 120 arcmin () of the cluster centre 8 galaxies (14%) of types Sa and later have compact emission. This is in good agreement with the fraction of 12% for the lowest density bin in Fig. 2, indicating that the number of spirals detected with compact emission in Abell 1060 agrees with the expected value for a cluster of its richness.
A detailed comparison between the star formation rates of cluster galaxies in Abell 1060 and corresponding rates for field galaxies is not possible due both to the small sample of detected emission-line galaxies and incomplete measurements of H equivalent widths. Nevertheless it may be noted that a significant fraction of the detected cluster emission-line galaxies are early-type spirals which may be surprising in view of the detection limit of the survey technique of 20 Å (see MWI), and the expectation that Hequivalent widths for galaxies of types Sab and earlier in the field are less than 20 Å (Kennicutt & Kent 1983). Indeed, the three galaxies ESO 501-G17, ESO 501-G45, and R89 281, typed Sa or earlier by R89 and Wang et al. (1991), and known cluster members, have measured equivalent widths greater than 20Å, and can therefore be considered to have unusually high star formation rates. The cluster elliptical ESO 436-IG42 also has a very high equivalent width of 96Å(see Sect. 2.6). These results are consistent with an enhanced star formation rate in early-type cluster spirals found previously for other clusters (see Moss & Whittle 1993).
|Figure 2: The percentage of galaxies of types Sa and later showing compact emission plotted as a function of cluster central galaxy density. Filled circles indicate the eight clusters studied by Moss & Whittle (1997), the open circle indicates Abell 1060. Error bars indicate Poisson errors|
No. 13, NGC 3314A: This is the foreground member of the remarkable superposed galaxy pair NGC 3314, discussed in detail by Schweizer & Thonnard (1985) and Richter et al. (1982), which consists of two spiral galaxies of comparable angular size, one face-on and the other more nearly edge-on, whose centres are superposed almost exactly along the line of sight. Two redshifts have been determined for the pair from optical and 21 cm line measurements of 2851 and 4641kms-1. Both sets of authors identify NGC 3314A as both the foreground object (dust lanes in its disk obscure NGC 3314B) and as having the lower recession velocity. The two-plate redshift estimate (see Sect. 2.3) of 2600340kms-1 confirms that emission is detected in the foreground NGC 3314A, in agreement with the spectrum of Schweizer & Thonnard (1985).
Nos. 16 & 17, sESO 501-IG61 & 501-G62: These two galaxies are seen in close separation on the sky, 1 arcmin apart, and might consequently be taken for an interacting pair. As they lie outside the overlap area, a two-plate redshift measurement was not possible. However, a less accurate estimate can be made using a single plate, giving redshifts of 3200 kms-1 for ESO 501-IG61 and 10200 kms-1 for ESO 501-G62, with an adopted uncertainty of 550 kms-1 (MWI). This is accurate enough to identify ESO 501-IG61 as a possible cluster member and ESO 501-G62 as a non-member, and to confirm that they are not an interacting pair as might otherwise be inferred from their close separation. The R89 value of 3597 kms-1 has been adopted for the redshift of ESO 501-IG61.
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