Page last updated Mar 13, 2020|
PGC 57532 (= ESO 137-001)
A magnitude 14(?) spiral galaxy (type SB(rs)c? pec) in Triangulum Australe (RA 16 13 27.2, Dec -60 45 51)
Physical Information: Based on a recessional velocity relative to the Cosmic Microwave Background of 4545 km/sec (and H0 = 70 km/sec/Mpc), PGC 57532 is about 210 to 215 million light years away. Given that and its apparent size of about 1.45 by 0.55 arcmin (from the images below), it is about 90 thousand light years across. However, the most striking feature of the galaxy is an enormous stream of stars and gas that have been ripped out of the core of the galaxy as a result of "ram-pressure stripping", which has an apparent size of about 3.75 by 1.85 arcmin (from the images below), or about 230 to 235 thousand light years. This phenomenon is caused by a rapid motion of the galaxy through superheated intergalactic gas filling the otherwise apparently empty regions between the multitude of galaxies which form the rich cluster in which the galaxy resides (in this case, the Abell 3627 cluster of galaxies, also called the Norma Cluster). The superheated intergalactic gas typically has almost as much mass as all the galaxies in such clusters, but is so hot and so widely dispersed that it can only be observed in X-radiation, and the longest part of the blue streak shown in the HST image is also false-color X-radiation emitted by gas from the galaxy superheated by the force of its collision with the intergalactic gas which stripped it from its host galaxy. Aside from tearing gas from PGC 57532 (and stars formed from the compression of that gas), the ram-pressure of the intergalactic gas has caused a tremendous surge of star formation inside the galaxy as well, making it a so-called "starburst" galaxy, as evidenced by its bright core and dark dust lanes highlighted by the light of the core.
Above, a 12 arcmin wide DSS image centered on PGC 57532
Below, a 1.2 by 1.6 arcmin wide HST image of the galaxy (Image Credit NASA, ESA, CXC)
Below, a 3.2 arcmin wide HST image of the galaxy and its extended stream of "stripped" gas and stars
(Image Credit the same as for the cropped image above)
PGC 57678 (= Arp 2)
A magnitude 13.3 spiral galaxy (type SB(s)dm?) in Hercules (RA 16 16 17.9, Dec +47 02 43)
: Based on a recessional velocity relative to the Cosmic Microwave Background of 745 km/sec (and H0
= 70 km/sec/Mpc), PGC 57678 is about 35 million light years away, somewhat closer than a roughly 50 million light year distance obtained from the only reasonable redshift-independent distance estimate. Given that and its apparent size of about 2.6 by 2.2 arcmin (from the images below), PGC 57678 is about 25 to 30 thousand light years across.
Arp Atlas Usage
: This galaxy is one of six chosen by Halton Arp for his Atlas of Peculiar Galaxies as an example of spiral galaxies of low surface brightness, hence its designation as Arp 2
(in his notes Arp states that it is a low surface brightness dwarf, with a bright knot in one arm which appears almost stellar).
Above, a 12 arcmin wide SDSS image centered on PGC 57678
Below, a 3 arcmin wide SDSS image of the galaxy
Below, a 2.5 by 3.5 arcmin wide image of the galaxy (Image Credit HST/ESA, Julianne Dalcanton, Judy Schmidt)