Burstein & Heiles (1982) have produced reddening maps for almost the entire sky with . The appropriate section for HydraI is shown in Figure . As can be seen, there is a strip with no data going right through HydraI. This is more clear in Figure . Here, the 64 sample galaxies are plotted in galactic coordinates. Overlaied are the values of AB, calculated from the reddening from Burstein & Heiles (1982) as . The actual numbers come from the fortran function extinction which D. Burstein has made publicly available via anonymous ftp5.1. The data are given in pixels of size in (l,b).
The IRAS 100 flux did not show a strong correlation with the Burstein & Heiles (1982) AB values. Therefore, we did not use the IRAS data to determine AB.
The following scheme was used to determine the extinction
for the 64 sample galaxies:
For the 30 galaxies located pixels for which Burstein & Heiles
(1982) do make a reddening prediction, the value in that pixel was used,
i.e. no interpolation was done.
For the 34 galaxies located pixels for which Burstein & Heiles (1982)
do not make a reddening prediction, i.e. for the galaxies with
a linear interpolation
between the values just below and above
An example of what the interpolation looks like is shown in Figure . It is unfortunate, that the missing data should appear in a place, where the extinction is changing rapidly.
Burstein & Heiles (1984) give for the galaxies in RC2 (de Vaucouleurs, de Vaucouleurs, & Corwin 1976) and UGC (Nilson 1973), with E(B-V) calculated in almost the same way as in Burstein & Heiles (1982). Of our 34 sample galaxies in the Burstein & Heiles (1982) no data strip, four are in Burstein & Heiles (1984). All four have AB = 0.32 listed, with a note that they are in areas without H I data, and that an interpolation to neighboring H I values have been done. Since the neighboring pixels have AB = 0.28 at most (cf. Fig. ) it is strange how these authors can get AB = 0.32.
Eight galaxies in the no data strip are in common with JFK95a. These authors list AB = 0.17 for all eight galaxies, quoting Burstein & Heiles (1982, 1984).
We conclude that our interpolation scheme is more sensible to do use than either AB = 0.17 or AB = 0.32 for the galaxies in the no data strip.
We calculate the extinction in Gunn r (Ar) and Johnson U (AU) from AB following Seaton (1979), see Table . The extinction values for the HydraI galaxies are given in Table .
Properties of E and S0 Galaxies in the Clusters HydraI and Coma
Master's Thesis, University of Copenhagen, July 1997
Bo Milvang-Jensen (firstname.lastname@example.org)