NGC 7331, Deer Lick Galaxy Group, Pegasus

NGC 7331, also identified as UGC 12113, PGC 69327, and Caldwell 30, is an unbarred spiral galaxy in the constellation of Pegasus. It is the brightest and largest member of a visual field of galaxies called the Deer Lick Galaxy Group that includes NGC 7325, 7326, 7335, 7336, 7337, 7340, and LEDA 2051985, all of which lie in the background at distances between 290 and 400 million light years. The group was first documented in 1784 by William Herschel.

Based on its measurable features (redshift of 0.002732, apparent magnitude 9.48, and angular size 10.47) we can estimate the following NGC 7331 physical properties: actual diameter 115,000 ly, absolute magnitude -20.86, distance 38 million ly, and recession velocity due to the expansion of the Universe (Hubble flow) 818 km/s. The galaxy is remarkably similar to the Milky Way in size and luminosity, but not strictly speaking in morphology. Although both are spiral galaxies, NGC 7331 is unbarred, while the Milky Way is presently thought to be a barred spiral. Further, NGC 7331 has peculiar features which are most likely due to a merger of large component galaxies in the remote past. Unlike the majority of spiral galaxies, its central bulge and the spiral arm disk rotate in opposite directions, and the central bulge is eccentric toward the N relative to the disk and the stellar halo.

Dwarf galaxy LGG 459 lies in-line with the S edge of the NGC 7331 disk, may be merging with it, and may have gravitationally disrupted the S spiral arm into 4 or 5 branches. Although redshift based distance between the two galaxies is around 4 Mly (assuming redshift is due exclusively to Hubble flow), the actual distance between them could be much smaller if LGG 459 redshift were in part due to "peculiar velocity" through space toward the background, away from the observer.

Spectroscopic analysis of NGC 7331 central region revealed an Hii/LINER active galactic nucleus. Filho et al. (2004) reported core radio luminosity 10 times higher and X-ray emission 10,000 times higher than the Sagittarius A* source in the center of our Galaxy. These studies suggest the presence of an obscured central supermassive black hole of approximately 175 million Solar masses, about 44 times larger than the Milky Way's <a href="https://arxiv.org/abs/astro-ph/0401593" rel="noreferrer nofollow">arxiv.org/abs/astro-ph/0401593</a>

The distant members of the Deer Lick Group are gravitationally bound to each other and also to the nearby Stephan's Quintet (NGC 7320) Group. Even at the scale of this image, NGC 7337 and 7336 show evidence of tidal interactions. The S side of the field contains a very remote galaxy cluster whose members are annotated with letter G. Unfortunately, none of these galaxies carry identifiers in Simbad or NED extragalactic databases. Comparing their average angular size to that of the Deer Lick Group, I very roughly estimate their distance at 2 billion light years.

Image Details: -Remote Takahashi TOA 150 x 1105mm -Paramount GT GEM -25 x 300 sec subs, OSC, 2x drizzle, 50% linar crop