5.6.1 Galactic Extinction

We want to correct the galaxy magnitudes and surface brightnesses for galactic extinction (i.e. for the extinction that the Milky Way causes). The galactic extinction in a given filter can be calculated as a constant times the reddening, e.g. $A_B = 4 \cdot E(B-V)$.

Burstein & Heiles (1982) have produced reddening maps for almost the entire sky with $\vert b\vert > 10^\circ$. 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 A_B, calculated from the reddening from Burstein & Heiles (1982) as $4 \cdot E(B-V)$. The actual numbers come from the fortran function extinction which D. Burstein has made publicly available via anonymous ftp^5.1. The data are given in pixels of size $0.3^\circ \times 0.6^\circ$ in (l,b).

 

 

The IRAS 100$\mu$ flux did not show a strong correlation with the Burstein & Heiles (1982) A_B values. Therefore, we did not use the IRAS data to determine A_B.

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 $26.494^\circ \leq b < 27.094^\circ$, a linear interpolation between the values just below and above was done:

$$A_B = 4 \cdot E(B-V) = \left\{ \begin{array}{ll} 0.1965 + x \... ... & 269.697^\circ < l \leq 269.997^\circ \\ \end{array}\right.$$ (5.4)

where we have used the auxiliary quantity x:

$$x = \frac{b-26.194^\circ}{1.2^\circ}.$$ (5.5)

Which pixel a given value of (l,b) belongs to is calculated by extinction as the integer values of [l/0.3 + 1.01] and [(b-10.)/0.6 + 1.51]. From these expressions the limits on l (and b) given in Eq. () can be derived.

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 $A_B = 4 \cdot E(B-V)$ 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 A_B = 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 A_B = 0.28 at most (cf. Fig. ) it is strange how these authors can get A_B = 0.32.

Eight galaxies in the no data strip are in common with JFK95a. These authors list A_B = 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 A_B = 0.17 or A_B = 0.32 for the galaxies in the no data strip.

We calculate the extinction in Gunn r (A_r) and Johnson U (A_U) from A_B following Seaton (1979), see Table . The extinction values for the HydraI galaxies are given in Table .

 

Table: Galactic Extinction in the Different Filters

Filter	Galactic extinction	Reference
Gunn r	0.625AB	Seaton (1979)
Johnson B	AB
Johnson U	1.183AB	Seaton (1979)