3 Object selection

We made extensive use of the NASA Extragalactic Database (NED) to search for known galaxies in the area covered by the observations. We measured the H$\alpha$ + [N II] fluxes for all galaxies with known radial velocities, thus up to $r' \approx 15.5$ for Abell 1367 and $r' \approx 16.5$ for Coma.

The narrow band filter used did not cover the whole velocity interval of the clusters. In order to avoid large uncertainties in the determination of fluxes and equivalent widths, we measured only galaxies for which the filter transmitance was larger than 0.5.

 

 

Table 1: Journal of the observations.

Field	RA	Dec.	Exp.	Filter
	(J2000)	(J2000)	s
26th April 2000
Coma 1	12:59:24.75	+27:58:49.89	$3\times 1200$	[S II]
			$3\times 300$	r'
Coma 2	13:01:24.45	+27:58:52.12	1200	[S II]
			300	r'
28th April 2000
Coma 3	13:01:24.26	+28:28:52.12	$3\times 1200$	[S II]
			$3\times 300$	r'
Coma 4	12:59:24.57	+28:58:49.89	$3\times 1200$	[S II]
			$3\times 300$	r'
26th April 2000
A1367 1	11:41:35.83	+19:58:21.44	$3\times 1200$	[S II]
			$3\times 300$	r'
A1367 2	11:43:35.61	+19:58:20.73	$3\times 1200$	[S II]
			$3\times 300$	r'
28th April 2000
A1367 3	11:45:35.40	+19:58:20.10	$3\times 1200$	[S II]
			$3\times 300$	r'
A1367 4	11:43:35.56	+19:28:20.73	$3\times 1200$	[S II]
			$3\times 300$	r'

Figure 3: Histograms of the H$\alpha$ fluxes (left) and equivalent widths (right) of the emitting galaxies. Shaded bins correspond to the galaxies in Abell 1367.

Visual inspection of the net H$\alpha$ + [N II] frames allowed us to identify faint galaxies with non-negligible net H$\alpha$ + [N II] emission. For these galaxies, there is no estimate of their velocities in NED. A population of faint galaxies ( $r' \geq 17.2$) showed up, most of them belonging to Abell 1367. Their H $\alpha \,+$ [N II] fluxes are low ( $-15.53 < \log F(\mbox{H}\alpha\, + \mbox{[N{\sc ii}]}) < -13.83$ erg s^-1 cm^-2) but their H$\alpha$ equivalent width is in the range $9 < EW(\mbox{H}\alpha \,+ \mbox{[N{\sc ii}]}) < 418$ Å. The search was performed for all objects visible on the NET-frames, but, in order to avoid spurious detections, we considered only objects with $F(\mbox{H}\alpha \,+ \mbox{[N{\sc ii}]})/\sigma_{\rm flux} > 5$ as reliable detections (see Col. 9 of Tables  4 and 5 for the definition of $\sigma_{\rm flux}$). Since for some of them, their redshift is unknown, both the H$\alpha$ fluxes and equivalent widths were computed assuming that their velocity coincides with the average velocity of the cluster^,.

In total 41 and 22 H$\alpha$ emitting galaxies were detected in Abell 1367 and Coma respectively. These are listed in Tables 4 and 5, arranged as follows:

Figure 4: Comparison between our fluxes (left) and equivalent widths (right) and the values reported in the literature for some galaxies in common. Fluxes are expressed in erg s-1 cm-2 and equivalent widths in Å.

• Column 1: galaxy designation.
• Column 2: CGCG name (Zwicky et al. 1961-1968).
• Column 3: other source designation.
• Columns 4, 5: celestial coordinates (J2000).
• Column 6: radial velocity in km s^-1. Galaxies flagged with $\dagger$ are those detected in H$\alpha$ but with unknown redshift. For these objects, the mean velocity of the cluster was adopted. Galaxies flagged with $\dagger\dagger$ are those for which a spectroscopic follow-up was carried out in order to measure their redshifts (Gavazzi et al. 2002, in prep.).
• Column 7: magnitude in the r' band.
• Column 8: Log of the H$\alpha +$[N II] flux, in erg s^-1 cm^-2.
• Column 9: error ( $\sigma_{\rm flux}$) of the H$\alpha +$[N II] flux. The uncertainty includes three contributions: the Poisson photon counts error, the uncertainty on the background and the photometric uncertainty, which is assumed as 10% of the net flux. These errors were determined separately on the ON and OFF-band frames, and combined using the standard error propagation.
• Column 10: H$\alpha +$[N II] equivalent width in Å.
• Column 11: error in the H$\alpha +$[N II] equivalent width, computed similarly to $\sigma_{\rm flux}$ (Col. 9) except that the error on the absolute flux scale does not affect the equivalent width.

Figure 3 shows the histograms of H$\alpha$ fluxes and equivalent widths of the emitting galaxies.

In order to check the quality of the photometry, we compared our fluxes and equivalent widths with those taken from the literature (see Table 6). Figure 4 shows the plots of the H$\alpha$ fluxes and equivalent widths reported in other works vs. ours. The linear regressions found for both plots are the following:

$$\log F(\mbox{H}\alpha+\mbox{[N{\sc ii}]})_{\rm this~work} = 0.34(\pm 0.91) + 1.03(\pm 0.07) \times \log F(\mbox{H}\alpha+\mbox{[N{\sc ii}]})_{\rm literature}$$ (1)

$$\log EW(\mbox{H}\alpha+\mbox{[N{\sc ii}]})_{\rm this~work} = 0.24(\pm 0.11) + 0.85(\pm 0.07) \times \log EW(\mbox{H}\alpha+\mbox{[N{\sc ii}]})_{\rm literature}.$$ (2)

Both plots show a discordant point, which corresponds with galaxy CGCG 097-114. This galaxy was measured by Kennicutt et al. (1984), Moss et al. (1988), Moss et al. (1998) and Gavazzi et al. (1998). There is agreement in the flux between our data and that of Kennicutt et al. (1984) but not in the equivalent width. The measurements by Moss et al. (1988, 1998) are consistent with each other, but not with other sources. The equivalent width by Gavazzi et al. (1998) is fairly consistent with ours.