Dust in Spiral Galaxies

There has been considerable controversy about the dust content of spiral galaxies, with some new results casting doubt on the traditional assumption that its effects on emerging light are relatively minor. Trying to avoid some of the pitfalls of modelling and statistics on such complex systems, Ray White III and I have been exploring the use of galaxy overlaps to measure the dust extinction in an unusually direct way. As background to the current HST and ISO observations, you may wish to peruse a WWW poster paper on our ground-based results that Ray prepared for a 1996 meeting in South Africa. It's important to understand dust in galaxies because it can have a profound effect on how we see them, and thus on how we interpret such diverse matters as the amount of dark matter galaxies contain, how may quasars do or don't exist in the early Universe, and how the different components of interstellar matter interact with each other and with stars at the beginning and end of their lives. More details (and lots of them) can be found in my course lecture on dust in galaxies. For a nice example of using dust to trace mass transfer between interacting galaxies, see the section on HST imaging and interpretation of NGC 1409/10.

HST and ISO Results

Read the press release on the HST and ISO results presented at the January 1998 meeting of the American Astronomical Society. The accompanying pictures are available in several forms:

a GIF (82k) of the press-release montage
HST Color composite image of AM0500-620, in full-resolution TIFF(718K), GIF(142K), or JPEG (31K). For comparison, you can see our earlier ground-based picture
Color-composite HST image of AM1316-241, again as full-resolution TIFF(1200K), GIF(238K), or JPEG(53K)

Some panels from our 1998 AAS poster paper are available, to see yet more detail. Read the abstract, the background and technique, or the summary of results. From HST imagery, we generated these opacity maps for the two spirals backlit by E/S0 galaxies:

AM0500-620 opacity image AM1316-241 opacity image
This stuff is discussed in a paper about to be submitted (or maybe by now it has been) to the Astrophysical Journal. One of the most noteworthy results is that HST resolves most of the dust structure, so that we finally measure extinction curves with shapes quite close to what is typical for dust in the Milky Way (perhaps not surprising for luminous spiral galaxies, but our preconceptions always need confrontation with facts). We also tabulate both cumulative and differential distributions of the extinctions we measure, by area. Two overlapping pairs were observed in our ISO project to compare dust masses measured using the completely different emission and absorption techniques.

We've gotten more adventurous in this work, and are now trying to unscramble the absorption in the overlapping (that's a mild word, they're almost exactly superimposed) galaxy pair NGC 3314. The data for this formed part of the May 2000 Hubble Heritage picture, reproduced below (and could I help it if the new data for the Heritage production are scientifically useful?). An especially interesting aspect of this object is that we can now see the background nucleus and estimate the dust extinction closer to the foreground nucleus than any other direct technique allows (about a factor of 10 in the I band; it would be much larger in blue light but we have great trouble measuring its feeble contribution). How do we know that's the background nucleus? It matches the location seen in near-IR images, like one we got with the NASA Infrared Telescope Facility.

HST image of NGC 3314

Back to Bill Keel's home page or Ray White's home page.

keel@bildad.astr.ua.edu (Bill Keel)
Last updated: May 2000