After posting an image of Stephan's Quintet in September, it occurred to me that it could be fun and instructive to also measure and compare the spectra of two galaxies in the group, one with an "active" nucleus, and the other with a quiescent (or "normal") nucleus, and I spent the next month working to do just that!

The image above has a closeup of the Quintet overlaid with the two spectra: the active galaxy that I measured is NGC 7319 (classified in part as a Seyfert galaxy of type 2), and the quiescent galaxy is NGC 7318a (classified as a peculiar E2). Arrows point from the spectra to the associated galaxy. They are part of a true quartet of interacting galaxies, which is at a distance of about 300 million light-years (the two other members of the interacting quartet are NGC 7318b and NGC 7317, while the remaining galaxy in the Quintet, NGC 7320, which is on the lower left, is in the foreground, only 40 million light-years away). The image and the spectra can be viewed separately by choosing one of the other "Revisions" ("B" for the image, and "D" for the spectra). Note that the spectra have been arbitrarily normalized to "1" in the neighbourhood of 6600 Angstroms, and take note as well of the different scales along the vertical axes. 

The spectra were obtained using a Shelyak LISA spectrograph, integrating for about 8 hours for each galaxy (although useful results came from just an hour or two), with the data acquired over the course of seven nights in September and October. It is obvious from the spectra that the galaxies are in dramatically different states (analysis of the spectra follows below), and I find it remarkable that such richly-detailed results can be obtained for such distant galaxies using "amateur" equipment! Reduction and calibration of the raw data was done using Shelyak's Demetra software package, along with Christian Buil's ISIS package for some further processing. A Gaussian filter was used to smooth over smaller scale noise.

For what it's worth, I also decided to reprocess the image from my September post using Russell Croman's BlurXTerminator (BXT), and used a crop of the re-done image to display with the new spectra. The improvement from using BXT is very noticeable, and in case it might be of interest, I've included the same crop of my September post (which used conventional deconvolution) as Revision "C". Both versions are displayed here at the full resolution of my imaging system, which is about 0.47"/pixel, although I had downsampled the image for my September post by 75%, since I didn't think it was sharp enough at that level of processing. The dimensions of the crops are about 12' x 11'.

The spectrum of NGC 7319 is dominated by very strong emission lines, as is typical of active galactic nuclei, with H-alpha and H-beta Balmer lines and "forbidden" transitions in Oxygen, Nitrogen, and Sulphur standing out (for another interesting example of the spectrum of an active galaxy, see my post on the Quasar Markarian 205, which has a curious scientific backstory, while an extremely useful atlas of amateur spectroscopic measurements can be found here). Also noticeable at the plotted scale is the sodium "D" absorption line (actually a blend of two lines that have not been separately resolved), although other shallower absorption lines are also present. To illustrate the galaxy's redshift, I've identified the rest-frame wavelength of the H-beta transition, which is clearly separated from the observed peak. The redshift as computed from the sharp peak wavelengths of the emission lines agrees very well with the known value, which is about 2.25% of the speed of light, or about 6700 km/sec.

The spectrum of NGC 7318a is comprised almost entirely of absorption lines, with few if any recognizable emission lines, and is typical of quiescent elliptical galaxies (a classic professional spectrophotometric atlas of galaxies can be found here). I've identified the elements responsible for a number of prominent absorption lines, as well as a dense set of absorption lines due to Oxygen molecules in our atmosphere (so-called telluric lines) that form a characteristic pattern between about 6900A and 7200A (other examples of telluric spectra taken with amateur equipment can be found in the "amateur" atlas). The redshift of the galaxy can again be estimated by comparison with known rest wavelengths, and at 2.22% of the speed of light differs only slightly from that of NGC 7319. To illustrate the redshift, I've identified the rest-frame wavelength of the blended Magnesium (triplet) line. 

These results also illustrate the fact that distinctive features in the spectrum of an active galaxy are generally much easier to observe than those of a normal galaxy of comparable apparent magnitude, since much of the energy is concentrated in a few very prominent emission lines, compared with the far shallower absorption lines of a typical quiescent galaxy. Here, the emission lines of the Seyfert tower over the continuums of both galaxies, which have similar apparent magnitudes.

As a final note, I had a hard time finding published optical spectra of these two galaxies, but did come across one professional paper on the Quintet with some useful plots that provided a sanity check on my results.