Page last updated Sep 25, 2017|
A magnitude 14.5(?) irregular galaxy (type dIm?) in Sextans (RA 10 39 25.2, Dec +01 43 07)
Physical Information: Based on a recessional velocity of 705 km/sec (and H0 = 70 km/sec/Mpc), PGC 31693 is about 30 to 35 million light years away, in good agreement with a single redshift-independent distance estimate of 30 to 35 million light years. Given that and its apparent size of about 0.65 by 0.65 arcmin (from the images below), the galaxy is about 6 thousand light years across (hence its classification as a "dwarf" irregular), while its fainter outer regions span about 0.95 by 0.9 arcmin, or about 9 thousand light years. It is known as a starburst galaxy because it is undergoing a period of unusually intense star formation, resulting in a large number of hot bright young stars. It is also an "emission-line" galaxy, because those young stars are heating and lighting up surrounding clouds of gas and dust, causing them to fluorescence at wavelengths corresponding to their chemical composition.
Above, a 12 arcmin wide SDSS image centered on PGC 31693
Below, a 1.2 arcmin wide SDSS image of the galaxy
Below, a 1.2 arcmin wide HST image of the galaxy
(Image Credit ESA/Hubble & NASA; Acknowledgement Luca Limatola)
A magnitude 15.5(?) irregular galaxy (type dIrr?) in Antlia (RA 10 43 27.3, Dec -37 02 38)
Designation Notes: On DSS images the faint northwestern extension of PGC 31961 looks like it could be a separate object (which LEDA lists as PGC 629236); but that is merely part of PGC 31961, and is not recognized by other databases. Also, although the PGC 31961 designation is recognized by almost all databases, the HST image lists the galaxy as ESO 376-16 (a designation also recognized by almost all databases).
Position Note: The position listed above is for the bright southeastern region of PGC 31961; the center of the galaxy is a little to the northwest of that.
Physical Information: The galaxy type is considered uncertain, with some references suggesting that it is a disorganied spiral galaxy (type dSBm? pec). Based on a recessional velocity of 670 km/sec (and H0 = 70 km/sec/Mpc), PGC 31961 is about 30 million light years away, in fair agreement with redshift-independent distance estimates of about 22 to 23 million light years. The redshift-independent distance is considered almost certain, as the relatively small distance and sparse population of the galaxy allows an accurate study of individual stars in the galaxy, and provides a tight fit to the red giant branch of HR Diagrams for objects of known distance. As a result, PGC 31961 is used as one of the primary distance indicators for most of the galaxies that are relatively close to our own galaxy. Based on a distance of about 23 million light years, the galaxy's apparent size of about 0.9 by 0.5 arcmin (from the images below) corresponds to only about 6 thousand light years, making it a dwarf galaxy, regardless of its type.
Above, a 12 arcmin wide DSS image centered on PGC 31961
Below, a 1.6 arcmin wide HST image of the galaxy; the "bright" star at top is only magnitude 11.4
(Image Credit ESA/Hubble & NASA)