3 Image reduction

The images were reduced with IRAF in the standard way (bias subtraction, flat-field correction and cosmic ray removal). We usually took three consecutive images of the same field in each filter (see Table 2). About 20 stars in the field were used as control points to calculate and correct the geometric distortion between images of the same field. We found that there were small shifts in the consecutive images, even though we did not move the telescope between them and used the same guide star in both images. This can be explained by mechanical flexures and torsions in the instrument, by atmospheric refraction, or by a combination of both. The images of the same field and filter were added together after correction of these shifts to obtain deeper images.

We subtract the continuum images from the on-line images in order to isolate the emission objects and areas. For this operation we scale the on-line images to account for the differently filter bandwidth, system throughput and stellar fluxes as function of wavelength. We used the observations of the standard stars to find the scale factor between the continuum and on-line images. The scale factors obtained are 0.72 for the H$\alpha$ observations and 0.38 for the [O III] images. However, the standard stars that we observed are high temperature objects (BD+28 4211, Op; Feige 110, DOp; G191 B2B, DA0). In a galaxy like M33 we will find very different stars, and in principle each has a different scale factor. If we then assume a single scale factor to equalize on- and off-line images we perhaps include some false emission objects that were not properly subtracted.

As an example, consider the use of black bodies from 5000 to 50000 K and the tabulated response of the filters we use. The [O III/blue continuum] ratio would vary from 0.34 to 0.40 and the [H$\alpha$/red continuum] ratio would go from 0.74 to 0.82, a $\sim$12 and $\sim$15%, respectively. Using the Strömgren y filter as continuum would result in ratios of 0.19-0.29 for [O III/y] and 0.16-0.08 for [H$\alpha$/y]. These are variations of $\sim$35% and 100%. These numbers illustrate nicely the importance of choosing a continuum filter close to the line wavelength and give us an idea about the possible range of variations of the scale factors.

Therefore we used $\sim$10 stars in each observed field to check that the scale factors found with the standard stars are not biased. We first matched the PSF of the stellar images with the IRAF task PSFMATCH and check that these test stars were not in evident emission regions. The average [H$\alpha$/red continuum] ratio found was $0.72~ \pm~ 0.02$ for regions II and III (note that black bodies do not account for the stellar absorption lines). For region I the ratio was $0.47 \pm 0.1$, because in this case we added together only two images instead of three (see Table 2). The [O III/blue continuum] ratio found was $0.38~ \pm ~ 0.06$ for all images. The small variation in the average of the scale factors reflects the fact that the chosen continuum regions are close to the observed emission lines and that the difference in emission in the spectral regions observed are not too high. The possibility that by chance all our test objects have similar spectral characteristics is rulled out, because the color of these objects in our continumm images (i.e. the difference in magnitude between our blue and red continua), span almost 2 mag. We conclude that for the combination of continuum and on-line filters that we used the application of a single scale factor was safe (within the quoted errors).

We also used the images of the spectrophotometric standard stars acquired during the run to calibrate the M33 images in flux. The standard star images were reduced with IRAF in the standard way and the integrated fluxes (in counts) were found with APPHOT. The published spectrophotometric measurements were convolved with the transmission function of the filters. We scaled the counts s^-1 with the integrated flux and found the conversion factors. These conversion factors were applied to the aperture photometry of the detected emission objects (also obtained with APPHOT) giving the fluxes reported in Table 3. We did not apply any extinction correction to the fluxes that we obtain, neither by atmospheric nor galactic nature. The atmospheric extinction differs in each individual image. Taking a "average'' extinction curve and the "average'' air mass of the images used, we can estimate that the H$\alpha$ images have an atmospheric extinction of $\sim$0.06 mag and the extinction of the [O III] images is $\sim$0.14 mag. This fact should be taken into account when comparing our results with other fluxes reported.

The positions of the sources were computed using selected isolated stars measured with APM. We obtain a solution with typical errors in the position of the order of 0 $.\!\!^{\prime\prime}$3. To check the astrometric solution that we obtained we found in our images the position of OB (Massey et al. 1995) and WR stars (Massey & Johnson 1998) reported in the literature. We found a good correlation when we compared these with the finding charts of the objects.

 

 

Table 3: Candidate planetary nebulae in the southern arm of M33. Coordinates are for epoch J2000.0, and fluxes are in units of 10^-15 erg cm^-2 s^-1.

M 33	RA	Dec	H$\alpha$	[O III]	m[O III]	Name	m[O III]
PNc			Flux	Flux		Magrini	Magrini
1	1:33:27.8	30:34:28.7	1.23	1.41	23.4	40	22.2
2	1:33:28.5	30:37:45.4	0.55	1.66	23.2	39	22.2
3	1:33:30.1	30:32:36.4	1.10
4	1:33:30.7	30:31:39.2	2.72
5	1:33:31.4	30:37:20.5	3.14
6	1:33:31.9	30:30:32.4	1.10	1.60	23.3	45	22.5
7	1:33:32.2	30:31:46.0	2.47	0.95	23.8
8	1:33:32.7	30:32:08.3	3.47
9	1:33:33.2	30:37:34.3	2.35	2.21	22.9
10	1:33:33.8	30:32:15.2	4.72
11	1:33:33.8	30:32:58.8	0.78
12	1:33:34.2	30:32:45.2	1.72
13	1:33:34.6	30:37:00.6	3.19
14	1:33:34.9	30:32:24.3	0.43
15	1:33:35.3	30:32:53.6	7.24	0.63	24.3
16	1:33:36.2	30:35:19.9	0.79
17	1:33:36.8	30:31:40.4	0.26	1.41	23.4	48	23.18
18	1:33:37.8	30:33:52.1	0.89	0.49	24.5
19	1:33:37.9	30:32:05.5	1.47
20	1:33:38.6	30:33:01.9	0.76	1.94	23.0	51	22.4
21	1:33:40.2	30:37:48.8	1.12	2.53	22.8
22	1:33:42.3	30:37:17.1	1.49
23	1:33:42.3	30:37:38.6	1.40	2.73	22.7	54	21.65
24	1:33:43.4	30:35:24.7	1.86	0.42	24.7
25	1:33:43.8	30:33:26.0	0.44	1.46	23.4
26	1:33:44.3	30:36:22.2	10.24	0.47	24.6
27	1:33:47.3	30:37:10.7	6.42
28	1:33:48.3	30:33:15.1	2.23	2.30	22.9
29	1:33:48.6	30:35:47.6	1.38	4.08	22.2	62	21.73
30	1:33:49.4	30:32:05.6	0.91	3.30	22.5	65	21.87
31	1:33:49.4	30:32:27.5	1.54	0.59	24.3
32	1:33:50.9	30:33:43.2	1.18
33	1:33:50.9	30:37:10.5	3.74			66	23.56
34	1:33:51.4	30:35:50.7	11.03	1.12	23.6
35	1:33:51.8	30:32:59.7	1.85
36	1:33:52.2	30:36:03.6	1.63
37	1:33:52.5	30:33:59.1	1.02
38	1:33:52.7	30:37:37.7	2.37	7.74	21.5	68	20.99
39	1:33:53.1	30:34:00.5	4.23
40	1:33:53.6	30:33:23.5	2.77	0.93	23.8
41	1:33:54.7	30:36:05.1	2.14	4.33	22.2	69	21.78
42	1:33:54.9	30:37:44.0	6.80	0.94	23.8	70	24.00
43	1:33:55.7	30:34:53.6	0.85
44	1:33:57.1	30:36:46.9	0.79	2.97	22.6	73	22.13
45	1:33:57.6	30:32:23.1	1.01	0.46	24.6
46	1:33:58.4	30:36:23.7	2.70	1.07	23.7
47	1:33:58.7	30:34:59.6	5.39	2.21	22.8
48	1:33:59.4	30:34:51.2	3.20	0.64	24.2