2 Observations

We observed 3 overlapping $\sim$ $6.5' \times 6.5'$ areas (see Fig. 1) in the southern arm of M33 with the 2.5 m Nordic Optical Telescope (NOT) on La Palma. We used ALFOSC in direct image operating mode and with filters that isolate the H$\alpha$ and [OIII] 5007 Å lines, as well as the adjacent continuum regions. The characteristics of the filters used are presented in Table 1.

We usually take three images of the same field in order to eliminate cosmic rays and obtain deeper images after addition. In Table 2 we present the observations log, integration times and filters used.

The sky conditions were good and the seeing was better than 1 $.\!\!^{\prime\prime}$2 during the whole run. As its detector, ALFOSC has a $2k \times 2k$ CCD with 15 $\mu$m-wide pixels, so that the spatial scale of the images is $\sim$0 $.\!\!^{\prime\prime}$2/pixel.

At the beginning and end of each night we took images of the spectrophotometric standard stars BD +28 4211, Feige 110 and G191-B2B with the same filters. The standards belong to the catalogue by Oke (1990), with magnitudes corrected by Colina & Bohlin (1994) and the transformation from magnitudes to fluxes by Hamuy et al. (1992). All the data can be found in Walsh (1998). We used these images to calibrate the observations of M33.

Figure 1: The observed area of the southern arm of M33 is outlined over a NGS-POSS image of the galaxy. The orientation of the image is N up and E to the left.

 

 

Table 1: Filters used in the observations. Given are central wavelengths and half widths at half maximum in nanometers, the peak transmission and the identification used by the NOT.

Name	$\lambda _{\rm c}$	FWHM	T max.	NOT
	nm	nm	%	ident.
[O III]	501.0	4.3	60	NOT 40
Blue cont.	521.4	10.5	77	NOT 67
H$\alpha$	656.4	3.3	66	NOT 21
Red cont.	678.8	4.5	73	NOT 54

 

 

Table 2: Observations. The M33 fields are identified as M33 I, II and III. The field centres are given for epoch J2000.0.

Object	Filter	Int. time	Date	Object	Filter	Int. time	Date
		(s)				(s)
BD +28 4211	H$\alpha$	60($\times$4)	21/X/1998	BD+ 28 4211	H$\alpha$	60($\times$4)	22/X/1998
	Red cont	45($\times$4)			Red cont.	60($\times$4)
	[O III]	45($\times$4)			[O III]	45($\times$4)
	Blue cont.	20($\times$4)			Blue cont.	20($\times$3)
M33 I	Blue cont.	600($\times$3)		Feige 110	Red cont.	60($\times$3)
1:33:50 30:35:30	[O III]	600($\times$3)			[O III]	60($\times$3)
	Red cont.	600($\times$3)			Blue cont.	60($\times$3)
	H$\alpha$	600($\times$2)			H$\alpha$	60($\times$3)
M33 II	H$\alpha$	600($\times$3)		M33 III	H$\alpha$	600($\times$3)
1:33:38 30:35:28	Red cont.	600($\times$3)		1:33:38 30:32:26	Red cont.	600($\times$3)
	[O III]	600($\times$3)			[O III]	600($\times$3)
	Blue cont.	600($\times$3)
				G191-2B2	H$\alpha$	25($\times$4)
M33 III	Blue cont.	600($\times$3)			Blue cont	25($\times$3)
1:33:38 30:32:26					[O III]	25($\times$3)
G191-2B2	Blue cont.	25($\times$4)			Red cont.	25($\times$3)
	[O III]	25($\times$4)
	Red cont.	25($\times$4)
	H$\alpha$	25($\times$4)