| NGC 931 | |
|---|---|
 NGC 931 byHubble Space Telescope | |
| Observation data (J2000epoch) | |
| Constellation | Triangulum |
| Right ascension | 02h 28m 14.5s[1] |
| Declination | +31° 18′ 42″[1] |
| Redshift | 0.016652 ± 0.000020[1] |
| Heliocentric radial velocity | 4,992 ± 6 km/s[1] |
| Distance | 159 ± 30Mly (48.7 ± 9.3Mpc)[1] |
| Apparent magnitude (V) | 13.5[2] |
| Characteristics | |
| Type | Sbc[1] |
| Apparent size (V) | 3.9′ × 0.8′[1] |
| Notable features | Seyfert galaxy |
| Other designations | |
| UGC 1935,Mrk 1040, KUG 0225+310, CGCG 504-089,MCG +05-06-049,PGC 9399[1] | |
NGC 931 is aspiral galaxy located in the constellationTriangulum. It is located at a distance of circa 200 millionlight-years from Earth, which, given its apparent dimensions, means that NGC 931 is about 200,000 light years across. It was discovered byHeinrich d'Arrest on September 26, 1865.[3] It is classified as aSeyfert galaxy.
The nucleus of NGC 931 has been found to beactive and it has been categorised as a type ISeyfert galaxy due to its narrowH-betaemission line.[4][5] The most accepted theory for the energy source of active galactic nuclei is the presence of anaccretion disk around asupermassive black hole. The mass of the black hole in the centre of NGC 931 is estimated to be 107.64 ± 0.40 (17- 110 million)M☉ based on the stellarvelocity dispersion.[6][7]
NGC 931 has been found to emitradiowaves,ultraviolet andX-rays. Observations byASCA revealed the X-rayspectrum was composed of soft and hard emission. The hard element was identified as a strong and wide fluorescent FeKa line, which is created when X-rays meet an optically cold thick material.[8] The soft element has been identified as warm absorbing material. The galaxy was further observed byXMM-Newton, where it was observed that there were significant fluctuations and time lags in the flux changes observed both in the soft and hard elements.[9]
A detail X-ray spectrum of NGC 931 was obtained byChandra X-ray Observatory. It revealed the presence of manyabsorption lines fromneon,magnesium, andsilicon, with a variety ofionisation states. These lines were attributed to low ionisation gases surrounding the nuclear X-ray source. No significant outflowing gas was detected in the large scale.[10]
Onesupernova has been observed in NGC 931, SN 2009lw. The supernova was discovered by W. Li, S. B. Cenko, and A. V. Filippenko during theLick Observatory Supernova Search on 24.24 November 2009, when it had anapparent magnitude of 18.8.[11] It was identified as atype-Ib or possibly atype-IIb supernova a few months past maximum light.[12]
NGC 931 has been identified as a member of the NGC 973 group, one of the largest galaxy groups of thePerseus–Pisces Supercluster,[13] with at least 39 galaxies identified as its members. Other members of the group includeNGC 917,NGC 940,NGC 969,NGC 973,NGC 974,NGC 978,NGC 987,NGC 983,NGC 1060,NGC 1066,NGC 1067, andUGC 2105.[14] On the other hand, Garcia recognised NGC 931 as the largest galaxy in agalaxy group known as the NGC 931 group, which also included NGC 940.[15]
A smaller companion galaxy, measuring 0.2 by 0.1 arcminutes, is superimposed on the galaxy, lying about 0.35 arcminutes towards the north.[16]
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