Hot Dust Grains in the Interstellar Medium of Galaxies

In the mid-infrared part of the spectrum (3 - 20 microns), the radiation from our own galaxies and other galaxies is dominated by emission from very small interstellar dust grains heated to temperatures of 100-1000 degrees Kelvin. Mid-infrared emission is particularly strong in regions where there are many hot young stars. This means that mid-infrared imaging can be used to search for regions where stars are forming which are heavily obscured by dust in optical light. Infrared radiation is less affected by intervening dust than optical light, and so can be used to search deeply in dusty parts of galaxies for hidden young stars.

The Infrared Space Observatory (ISO) is by far the best telescope available at present for mid-infrared observations. To study the spatial distribution of the mid-infrared emission in galaxies, I used the ISO Camera (CAM) because of its imaging capability.

A mid-infrared image of the Sab galaxy NGC 4736, from Smith and Madden 1999, proceedings of Astrophysics with Infrared Arrays: A Prelude to SIRTF conference, ASP Conference Series, Volume 177, p. 216. Note the mid-infrared bright bulge surrounded by a circumnuclear ring. The radius of the ring is approximately 45 arcseconds (1.3 kpc).

An ISOCAM image for the nearby Sb galaxy NGC 7331 (from Smith 1998, Astrophysical Journal, 500, 181):
Visible LightH-alpha+[N II]ISOCAM 15 microns

The first image is the appearance of the galaxy in optical light. This shows the distribution of the *stars*. The second is how the galaxy looks when imaged through a narrow-band optical filter containing the hydrogen alpha line plus a nearby nitrogen line. This image shows the location of the unobscured *ionized gas*. These two optical images are from Rick Pogge's thesis.

The third image, the ISOCAM image, shows the distribution of hot dust grains. There is a prominent ring of mid-infrared light with a diameter of 0.5 arcminutes x 1.5 arcminutes (2.2 kpc x 6.6 kpc). One quarter of the mid-infrared radiation from this galaxy comes from this ring.

Note that the ring is much more prominent in the mid-infrared than in the H-alpha+[N II] image, even though both the ionized gas and the hot dust traces young massive stars. The reason for this is that the optical H-alpha+[N II] light is highly obscured by dust, so it only shows the location of H II regions without much dust in front of them. The mid-infrared radiation is not strongly affected by foreground dust, so it gives a much better picture of the true distribution of young stars in this galaxy. Notice the differences between these two images. The left (eastern) part of the ring is also quite bright in the H-alpha+[N II] map; the right (western) side, however, is much fainter. The bright H-alpha+[N II] sources on the western ring of the ring lie *outside* the ring and are not part of the ring. They only have faint counterparts in the mid-infrared image, and therefore are *intrinsically* less luminous than the star forming regions in the ring. They just look bright in the optical because they don't have much dust in front of them.

The center of the galaxy, which is so bright in the optical, is relatively faint in the mid-infrared. This is because most of the newly formed stars in this galaxy are in the ring, not in the center. The stars in the center are mostly old, and do not emit very much ultraviolet radiation. Some of the mid-infrared emission in the bulge of this galaxy is probably direct starlight, rather than dust emission.

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