Galaxies Behind A Galaxy

Today’s post comes from science team member Bill Keel from the University of Alabama.  While Bill’s work is well-known within the Galaxy Zoo community, we thought it would be great for Bill to explain what science he and team member Benne Holwerda (ESA-ESTEC) hope to address with the background galaxies from the Andromeda Project.

This is a briefing on background. On deep background, really deep background. Among the things participants in the Andromeda Project mark are very distant background galaxies shining through Andromeda. In part, this is to make sure they don’t get confused with star clusters (or even nebulae) in the Andromeda galaxy itself. In addition, we can learn about the absorbing dust in the foreground by seeing what it does to the light of background galaxies. This is parallel to one of the projects carried out under Galaxy Zoo, finding examples of galaxies backlighting one another so we can map their dust. But in this case, of course the foreground galaxy is enormously closer, and there are vast numbers of faint background galaxies. This means we learn something different.

We know that spiral galaxies are dusty – a wide-field view of Andromeda shows this nicely, with the dark dust clouds along the spiral arms clearly outlined against the light of more distant stars. The amount and location of the dust control how it affects a galaxy’s overall light, and give clues to a galaxy’s past history of star formation (since dust not only participates in the cooling of gas to make new stars, but can be produced in the late stages of some stars’ lives). The overall dust properties of the whole spiral population are well addressed by the Galaxy Zoo search for backlit galaxies (whose results complement what we’re learning from far-infrared studies by such space telescopes as IRAS, Akari, Spitzer and Herschel). But seeing galaxies through Andromeda brings us something new – tracing small-scale structure in the dust clouds, right down to the resolution limit of Hubble’s images. We can learn about large-scale dust concentrations from both infrared data (where we see the dust shining by the radiation it gives off when slightly heated from absorbing starlight) and from the colors of stars in the Hubble PHAT images themselves. But both these techniques run out when we get to very small features – the resolution in the far-infrared is still too coarse, and there are usually not enough stars to outline such clouds. Hence, go to the background.

Here’s an example of what we particularly hope to find. The spiral galaxy IC 4633 is seen partly silhouetted behind a dust cloud in our own Milky Way. The cloud is close enough to some luminous stars to be shining more brightly by reflected light than we’d like, so the contrast is reduced, but it shows the principle – seeing a piece of a dust cloud against a background light source.


A ground-based gri color composite image of the galaxy IC4633.  This galaxy is partially obscured by a dust lane that resides in our host galaxy, the Milky Way.

As so often happens, we stand on the shoulders of our predecessors. In setting up the work on background galaxies, we got a head start from a catalog compiled by Paul Hodge (University of Washington) and Karl Krienke (Seattle Pacific University) of obvious background galaxies in an earlier set of Hubble images in various parts of Andromeda. This gave us an excellent starting point in knowing how many to expect, how bright they had to be to show detail, and thus what we were getting in to.

Colleague Benne Holwerda and I will be looking at the background galaxy results, hoping to see some where we see this fine structure in the dust. Even if we don’t, the data on background galaxies will tell us what kinds of filaments and tendrils in the dust are not there, which is likewise interesting information.

You can learn more about Bill Keel at, or read his Galaxy Zoo blog entries at


4 responses to “Galaxies Behind A Galaxy”

  1. Jean Tate says :

    Very cool!

    Zooites have already noted ~100+ background galaxies, in various Talk threads/discussions. Where can we find the “catalog compiled by Paul Hodge (University of Washington) and Karl Krienke (Seattle Pacific University) of obvious background galaxies in an earlier set of Hubble images in various parts of Andromeda”? It might be very interesting to see which of the ones zooites have found are in that catalog, and which are not!

    • lcjohnso says :

      The paper that Bill was referring to can be found here: This work surveyed Hubble Space Telescope images that were scattered across the disk of M31, before PHAT’s systematic survey began. There is some overlap between the two sets of images, but the catalog that will be produced from the Andromeda Project data will include mostly new object identifications.

  2. zutopian says :

    I would like to mention following.:

    “PHAT Stellar Cluster Survey I. Year 1 Catalog and Integrated Photometry”
    Authors: L. Clifton Johnson, Anil C. Seth, Julianne J. Dalcanton et al.

    “During the cluster search, we also identified 370 putative background galaxies.”
    “As introduced in Section 3.1, we present a catalog of objects visually identified as background galaxies in Table 6.”
    (Submitted on 13 Apr 2012)

    • lcjohnso says :

      Hi zutopian – As part of the Year 1 catalog paper we did include a small sample of galaxies that we identified while searching for star clusters. However, the Andromeda Project galaxy identifications will be awesome for a few reasons: 1) We expect Andromeda Project participants to identify 3-4 times more galaxies than our team found during the first due to the greatly increased search area. 2) At the time the science team was not explicitly trying to identify the background galaxies, so that sample is likely not complete (as we state in the paper). AP users will do a much better job of identifying the galaxies!

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