NGC 7172 | |
---|---|
Observation data (J2000 epoch) | |
Constellation | Piscis Austrinus |
Right ascension | 22h 02m 01.9s[2] |
Declination | −31° 52′ 11″[2] |
Redshift | 0.008683 ± 0.000040 [2] |
Heliocentric radial velocity | 2,603 ± 12 km/s[2] |
Distance | 110 Mly (34 Mpc)[2] |
Apparent magnitude (V) | 11.9 |
Characteristics | |
Type | Sa pec [2] |
Apparent size (V) | 2.5′ × 1.4′[2] |
Notable features | Seyfert galaxy |
Other designations | |
ESO 466- G 038, AM 2159-320, MCG -05-52-007, PGC 67874[2] |
NGC 7172 is a spiral galaxy located in the constellation Piscis Austrinus. It is located at a distance of about 110 million light years from Earth, which, given its apparent dimensions, means that NGC 7172 is about 100,000 light years across.[2] It was discovered by John Herschel on September 23, 1834.[3]
NGC 7172 is a spiral galaxy that is seen edge-on. A thick dark dust lane runs across the galaxy, obscuring the nucleus of the galaxy.[1][4] The galaxy appears in deep photographs to be tidally distorted, and a diffuse tail is extending towards the north-west.[5]
The nucleus of NGC 7172 was originally considered to be normal,[6] but later observations revealed that it emitted X-rays[7][8] and was a powerful infrared source, which exhibited variation.[5] These findings indicated that NGC 7172 has an active galactic nucleus which is obstructed in optical wavelengths.[5] The nucleus was classified as a type 2 Seyfert galaxy in the optical, but observations in other wavelengths suggest it is type 1 Seyfert galaxy.[9] The source of activity of the nucleus is an accretion disk around a supermassive black hole. The mass of the supermassive black hole in the centre of NGC 7172 is estimated to be 5.5×107 M☉ based on the MBH–σ⋆ relation[10] or (1.03±0.35)×107 M☉ based on the X-ray scaling method.[11]
NGC 7172 has been found to exhibit variability in X-rays. As observed by the Advanced Satellite for Cosmology and Astrophysics (ASCA), the galaxy exhibited short term variability of about 30% in the time scale of hours, which is common for Seyfert I galaxies. It also exhibits long term variability, as it dimmed three to four times between 1995 and 1996.[12] This was also observed by BeppoSAX.[13] The FeKα line appeared constant when observed with BeppoSAX, but appeared to variate by a factor of two by ASCA, and thus it is associated with an accretion disk.[14] The hard X-ray spectrum appears similar to that of a Seyfert I galaxy, indicating that the nucleus is seen though a Compton-thin absorber.[15]
In infrared observations of the nucleus dominates the spectrum of the active nucleus, with 3.4-μm carbonaceous dust absorption detected, but no 3.3-μm PAH emission, indicating that the nucleus is obstructed. A strong obsurption feature is observed, probably the 9.7-μm silicate dust absorption line.[16] X-rays and mid-infrared have similar absorption columns.[17] It is possible that an outflow towards the southwest is detected in radiowaves.[9]
NGC 7172 is part of the Hickson Compact Group 90. NGC 7172 lies 6 arcminutes north of the core of the compact group, which is comprised by the elliptical galaxies NGC 7173, and NGC 7174, and disturbed spiral galaxy NGC 7176; these three galaxies lie within 6 arcminutes from each other.[18]
The compact group is surrounded by a more loose group, with 19 galaxies of similar redshift within 1.5 degrees from the core group.[18] Garcia identified as members of this group the galaxies NGC 7154, ESO 404- 12, NGC 7163, ESO 466- 36, ESO 466- 46, ESO 404- 27, NGC 7187, IC 5156, ESO 404- 39, and ESO 466- 51.[19] Other nearby galaxies include NGC 7135 and its group, NGC 7204, and NGC 7208.[20]