Distance: 60,000,000 light-years (18.4 Mpc) Image Size = 7 x 7 arcmin Visual Magnitude = 8.9

X-Ray: ROSAT Ultraviolet: ASTRO-1 Visible: DSS Visible: Color © AAO

Messier 77 is a beautiful face-on spiral galaxy that is slightly larger in size than our own Milky Way Galaxy. Its nucleus emits peculiar spectral-line features that suggest a massive object at its center, presumably an invisible black hole.

Visible: DSS and Visible: Color © AAO

We begin our study of M 77 by examining the visible-light images above. The first observation to make is that the galaxy disk appears to be saturated (over-exposed) in these images. Hence we lose our ability to clearly discern the inner structure of the spiral arms and to trace the obscuring dust within the arms. On the other hand, these images reveal the presence of a faint outer ring encircling the galaxy disk. Upon close examination, you should be able to see dark dust in the northwest (upper right) quadrant of the photos, well beyond the inner disk.

Now turn your attention to the east (left) sideof the optical pictures. The brightest point of light, in the 8 o'clock position, is a foreground star within the Milky Way. And so is the point of light at the lower-left edge of the photo. Do you see fuzzy blobs of light between these two foreground stars? These are actually regions of massive star formation within Messier 77. Note that these areas of star formation are located in the outer disk of M 77. Within spiral galaxies, large amounts of gas and dust (the raw materials for stars) are often found within the outer disk and spiral arms.

Near-Infrared: 2MASS and Visible: Color © AAO

Now take a look at the near-infrared view of Messier 77 (above left). The total exposure time for the 2MASS photo is only 7.8 seconds and hence its sensitivity is less than that of the visible-light image taken in Australia (above right). This is easily confirmed by two observations. First, the size of the central bright emission region is reduced in area from the large saturated region of the visible photo. Second, the surrounding density of faint stars is vastly reduced in the near-infrared image. Furthermore, the outer disk and rings of M77 have apparently vanished, a consequence of the short exposure time and the faint level of luminosity in these areas.

Mid-Infrared: IRAS and Far-Infrared: IRAS

The mid- and far-infrared images of Messier 77 are essentially unresolved. In other words, the angular size of the detectors is larger than the source of emission, and hence we cannot discern any significant details about the source. In each case, we see a point-like source of infrared emission (red is brightest). Because of the peculiar rectangular shape of the IRAS detectors, the point source emission is stretched into an ellipse. If the IRAS sensors had been square, the emission shape would be circular instead.

Radio: NVSS and X-Ray: ROSAT

Even though the above images were obtained at opposite ends of the electromagnetic spectrum, they appear very similar. This is partly due to the fact that both photographs utilize similar false-color schemes to depict the intensity of emission. In both images, the red emission is clearly the brightest, and the outlying purple and blue colors denote random noise. We are left to conclude that these images are also unresolved, and that both photographs reveal nothing more than a strong central source of radio and x-ray emission. Most spiral galaxies emit significant synchrotron radio energy, with local intensity maxima in the central nucleus and within the spiral arms. The amount of x-ray energy measured in spiral galaxies depends on the nature of its central nucleus. In a normal spiral galaxy, little x-ray emission is typically seen. In active galaxies, however, the nucleus often emits strong x-ray radiation. The source of this emission is ultimately a black hole, and the x-rays we see are produced by hot gas falling into the gravitational clutches of the black hole.

Ultraviolet: ASTRO-1 UIT

Finally, we arrive at the fascinating ultraviolet image of Messier 77, which clearly reveals the underlying spiral structure of this galaxy. Ultraviolet light is produced by hot and young stars. The high density of such stars in the nucleus of M 77 produces the central emission peak in the UV image. About one arcminute to the northwest (upper right), however, we also can distinguish patches of UV emission which are likely due to widespread regions of massive star formation within the inner disk of M 77. Other such regions, with diminished UV intensity, are also seen throughout the spiral arms.

NED technical data for M77