Motivation for the Survey: APS Diameters

The APS Catalog of the POSS I provides an excellent sample of galaxies for studies of inclination effects on galaxy properties because of the isophotal nature of its diameters. Most previous studies have used either the UGC or RC3, both of which have demonstrable diameter-inclination effects.

The Diameter-Inclination effect (first noted by Huizinga [1994] in the ESO-LV), occurs because visual diameters are measured to a deeper isophote for face-on galaxies compared to edge-on galaxies. This is believed to be due to the eye's ability to follow "patchy" light distributions along the edges of spirals. The presence of this effect may have profoundly affected the outcome of several previous studies of the influence of inclination on observed galaxy properties which assumed that the catalog diameters were isophotal.

We used the APS to answer the following questions:

1) Are the APS reported major-axis diameters roughly isophotal?

Yes. We cross-identified Courteau (1997) galaxies with galaxies in the APS database. Courteau performed extensive photometric analysis of CCD images of UGC galaxies with ellipticities greater than 0.3. If we assume his diameters are isophotal, then the APS major-axis diameters are also isophotal (with considerable plate-to-plate scatter), as shown in Figure 3(a). In fact, the APS E (Red) diameters correspond well to Courteau's 23 mag arcsec^-2 diameters.

2) Can we confirm the diameter-inclination effect believed seen in the UGC by Courteau (1997) and Giovanelli & Haynes (1994)?

Yes, if we assume the APS diameters are isophotal (as established above), then there is a definite trend of APS to UGC diameter ratio versus ellipticity as shown in Figure 3(b). A best-fit line to the diamter ratio for > 0.3 indicates a slope of 0.44, while for < 0.3, the slope is 0.0, within error. This is the first direct comparison of UGC B band diameters with isophotal diameters in a similar bandpass, and it is a sample of over 5000 cross-identified galaxies. See Figure 3(b).

3) Does the RC3 exhibit a diamter-inclination effect?

Yes. Again assuming the isophotal nature of APS diamters, the RC3 shows the same trend of decreasing APS to RC3 diameter ratio versus increasing ellipticity for > 0.3 as the UGC, although the slope is a shallower 0.34. This is illustrated in Figure 3(c).

Warning: Use of galaxy catalogs utilizing visual diameters for determination of galaxy diameter functions or to investigate internal extinction in galaxies can lead to erroneous results, especially given the fact that this bias in diameters is inclination dependent.

An example - Most previous studies using the UGC that the outer parts of galaxies must be optically thick because a diameter-limited sample appears to show a flat ellipticity distribution. This is however based on a 'biased diameter'-limited sample, when true isophotal diameters (obtained using the APS and shown in Figure 4) are used, the sample shows a strong preference for highly-inclined galaxies, counter to the optically thick scenario.

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