Astron. Astrophys. 354, 823-835 (2000)

3. Construction of the HII region catalogue

In Fig 1 we give a plot of the spatial distribution of the ionized gas. In all we catalogued 547 HII regions, giving their positions, fluxes in H, and equivalent radii. Before obtaining the catalogue we flagged the foreground stars, which are relatively easy to distinguish from HII regions by their regular circular shapes prior to continuum subtraction, and because they show much higher continuum intensities than the HII regions. To select an HII region against background H, we specified that a region must contain at least nine contiguous pixels in a non-filamentary configuration, each with an intensity at least three times the rms noise level of the local background. An object not meeting this criterion was not distinguishable from noise, and was treated as noise. This selection criterion led automatically to observational lower limits to the luminosity of the detected HII regions, and the minimum radius of a catalogued region which are, respectively, erg s^-1 and pc. There were three complicating effects to take care of when identifying the HII regions and quantifying their observable parameters. Firstly, many regions overlap on the image, which is of course a projection of the disc structure onto a plane. We did not attempt to analyze the fraction of these overlaps implying physical contact, we adopted the solution proposed by Rand (1992), and followed in Knapen et al. (1993), and later in Rozas et al. (1996a) of counting each peak in H as coming from a separate HII region. The flux of each region was then estimated by integrating over the pixels which could reasonably be attributed to a given region. Using this method, one will miss some HII regions which are too weak to be detected in the vicinity of stronger emitters close by, which will affect the lower luminosity end of the LF, but has a negligible effect on the true determination of the LF above a definable completeness limit (Rand 1992). Secondly, an HII region is not necessarily circular, and we adopted as an effective radius for each region the mean of the maximum and minimum radii measured. Thirdly the presence of diffuse H around an HII region may lead to difficulty in defining its edges, introducing systematic errors above all for the faintest regions (Zurita et al. 2000, in preparation). The detection and cataloguing of the HII regions were performed using a suite of programs developed for this purpose by C. Heller, used also in Rozas et al. (1999a) to analyze the regions in NGC 7479. The program identifies a region, measures the position of its centre, obtains its area in pixels, and computes its total H flux, integrating over all the pixels of the

region, and subtracting the sky value. The program will be described in detail in a forthcoming paper (Heller et al. 2000, in preparation). As the background was variable across the frame, we picked some 150 background areas with a circular aperture of radius 3 pixels, and the background value applied to a given HII region was that corresponding to the mean value of the nearest selected area. We ran a series of trials, which showed that the uncertainties in the fluxes of the regions caused by background variation and uncertainty in defining the edges of a region range from 10% for the faintest regions catalogued to 1% for the brightest. The analysis code is automatic, but then allows interactive fine adjustments to the catalogue, by deleting, adding, separating, or rounding the regions by hand, (the latter if the automatic process produced very extended very faint wings of irregular shape). The final number of regions catalogued in NGC 3359 is 547. For all of these we found coordinate offsets from the nucleus in R.A. and dec and deprojected distances to the centre of the galaxy in arcsec, using the inclination angle and position angle given by Grosbol (1985), (, PA=170^o) which are in fair agreement with those derived from our kinematic data (Rozas et al. 1999b). We also measured the diameter, and the H luminosity in erg s^-1, for each HII region. The catalogue is available via the CDS or directly from the authors. In Fig 2 we give a schematic diagram of the positions of the HII regions in the disc of NGC 3359, on a deprojected RA-dec grid, centred on the nucleus of the galaxy.

Fig. 2. Schematic representation of the positions and luminosities of the catalogued HII regions in NGC 3359. Radii of the circles are proportional to .

© European Southern Observatory (ESO) 2000

Online publication: February 25, 2000
helpdesk.link@springer.de