3 Observations

 

 

Table 2: Logbook of the observations.

Telescope	Date	Nights (ass./used)	CCD	Pixel size
2.5 m INT	17-20/2/1999	4/3.5	WFC	0.333
2.56 m NOT	29/4-1/5/2000	3/0.5	Loral/Lesser $2048 \times 2048$	0.188
2.56 m NOT	27-29/4/2001	3/1.5	Loral/Lesser $2048 \times 2048$	0.188

Narrow band imaging in the H$\alpha$ emission line ($\lambda$ = 6562.8 Å) of galaxies was obtained in 1999 at the 2.5 m INT and in 2000 and 2001 at the 2.56 m NOT telescope at El Roque de los Muchachos, La Palma, Canary Islands. The INT observations were carried out with the Wide Field Camera (WFC) attached at the prime focus of the telescope. The WFC is composed by a science array of four thinned AR coated EEV 4 K$\times$2 K CCDs, plus a fifth acting as autoguider. The pixel scale at the detectors is 0.33 arcsecpixel^-1, which gives a total field of view of about $34\times 34$ arcmin². Given the particular arrangement of the detectors, a squared area of about $11 \times 11$ arcmin² is lost at the top right corner of the field. The top left corner of detector #3 is also lost because of vignetting of the filters.

The f/11 2.56 m NOT telescope is equipped with a Loral/Lesser 2048$\times$2048 pixelCCD detector. The pixel size is 0.19 arcsec. At the adopted gain, the electron/adu conversion is 1 e^-/adu, with a readout noise of 6 e^-. A total of 4 nights at the INT and 6 at NOT were allocated to this project. Of these, 5.5 were totally or partly useful due to technical problems or weather limitations, as reported in Table 2 (logbook of the observations and CCD technical data). One galaxy (VCC 24) was observed at the INT in service mode.
Each galaxy was observed through one narrow band interferometric filter (see Table 3) centered at the redshifted H$\alpha$ line (ON), while the red continuum near H$\alpha$ (OFF) was generally taken in a broad band filter. The filters given in Table 3 are used as ON or OFF band for each target as specified in Cols. 2 and 3 of Table 4. The flux from the [NII] emission lines at $\lambda$6548 Å and $\lambda$6584 Å is included in the ON band observations. The typical integration time was of $3\times 1800$-1500 min ON- and $3\times 600$-200 min OFF-band for the INT observations, and $3\times 1500$-900 min ON- and $3\times 300$-900 min for the broad and narrow OFF-band observations at the NOT. NOT observations were obtained during good seeing conditions ($\sim$1 arcsec), fairly good at the INT (1.5-2 arcsec).

The observations were calibrated using the standard stars Feige 56, 66, 67 and BD 33+2642 from the catalogue of Massey et al. (1988) or from the IRAF compilation. Observations of the standard stars were repeated every 2 hours, with an integration time of 3-30 s. At least once per night a calibration star was observed on each CCD of the WFC to allow cross calibration of the different chips. The zero point of each galaxy is determined assuming an extinction law of slope 0.06 for the 1999 run, 0.1 for the other runs. Repeated measurements gave <10% differences, which we assume as the typical uncertainty of the photometry given in this work. All frames except one were obtained in photometric conditions. The determination of the H$\alpha$ equivalent width can however be achieved also in non photometric conditions using field stars to normalize the ON and OFF band images, as described in Paper I.