NGC 931

{{Infobox galaxy

| name = NGC 931

| image = NGC 931 -HST-606.png

| image_size = 250px

| caption = NGC 931 by Hubble Space Telescope

| credit = NASA/ESA

| epoch = J2000

| type = Sbc {{cite web

| title=NASA/IPAC Extragalactic Database

| work=Results for NGC 931

| url=http://ned.ipac.caltech.edu/cgi-bin/nph-objsearch?objname=NGC+931

| access-date=2019-01-18 }}

| ra = {{RA|02|28|14.5}}

| dec = {{DEC|+31|18|42}}

| dist_ly = 159 ± 30 Mly (48.7 ± 9.3 Mpc)

| z = 0.016652 ± 0.000020

| h_radial_v = 4,992 ± 6 km/s

| appmag_v = 13.5 {{cite web |title=Revised NGC Data for NGC 931 |url=http://spider.seds.org/ngc/revngcic.cgi?NGC931 |website=spider.seds.org |access-date=25 November 2018}}

| size_v = 3.9{{prime}} × 0.8{{prime}}

| constellation name = Triangulum

| notes = Seyfert galaxy

| names = UGC 1935, Mrk 1040, KUG 0225+310, CGCG 504-089, MCG +05-06-049, PGC 9399

}}

NGC 931 is a spiral galaxy located in the constellation Triangulum. It is located at a distance of circa 200 million light-years from Earth, which, given its apparent dimensions, means that NGC 931 is about 200,000 light years across. It was discovered by Heinrich d'Arrest on September 26, 1865.{{cite web |last1=Seligman |first1=Courtney |title=NGC 931 (= PGC 9399) |url=https://cseligman.com/text/atlas/ngc9.htm#931 |website=Celestial Atlas |access-date=19 November 2018}} It is classified as a Seyfert galaxy.

Characteristics

The nucleus of NGC 931 has been found to be active and it has been categorised as a type I Seyfert galaxy due to its narrow H-beta emission line.{{cite journal |last1=Osterbrock |first1=D. E. |last2=Shuder |first2=J. M. |title=Emission-line profiles in Seyfert 1 galaxies |journal=The Astrophysical Journal Supplement Series |date=May 1982 |volume=49 |pages=149 |doi=10.1086/190793|bibcode=1982ApJS...49..149O }}{{cite journal |last1=Osterbrock |first1=D. E. |last2=Dahari |first2=O. |title=Spectra of Seyfert galaxies and Seyfert galaxy candidates |journal=The Astrophysical Journal |date=October 1983 |volume=273 |pages=478 |doi=10.1086/161385 |bibcode=1983ApJ...273..478O|doi-access=free }} The most accepted theory for the energy source of active galactic nuclei is the presence of an accretion disk around a supermassive black hole. The mass of the black hole in the centre of NGC 931 is estimated to be 10^{7.64 ± 0.40} (17- 110 million) {{solar mass|link=yes}} based on the stellar velocity dispersion.{{cite journal |last1=Zhou |first1=Xin-Lin |last2=Zhang |first2=Shuang-Nan |last3=Wang |first3=Ding-Xiong |last4=Zhu |first4=Ling |title=Calibrating the Correlation Between Black Hole Mass and X-Ray Variability Amplitude: X-Ray Only Black Hole Mass Estimates for Active Galactic Nuclei and Ultra-Luminous X-Ray Sources |journal=The Astrophysical Journal |date=10 February 2010 |volume=710 |issue=1 |pages=16–23 |doi=10.1088/0004-637X/710/1/16|arxiv=0912.2636 |bibcode=2010ApJ...710...16Z |s2cid=118336945 }}{{cite journal |last1=De Marco |first1=B. |last2=Ponti |first2=G. |last3=Cappi |first3=M. |last4=Dadina |first4=M. |last5=Uttley |first5=P. |last6=Cackett |first6=E. M. |last7=Fabian |first7=A. C. |last8=Miniutti |first8=G. |title=Discovery of a relation between black hole mass and soft X-ray time lags in active galactic nuclei |journal=Monthly Notices of the Royal Astronomical Society |date=21 May 2013 |volume=431 |issue=3 |pages=2441–2452 |doi=10.1093/mnras/stt339|doi-access=free |arxiv=1201.0196 |bibcode=2013MNRAS.431.2441D |s2cid=18276159 }}

NGC 931 has been found to emit radiowaves, ultraviolet and X-rays. Observations by ASCA revealed the X-ray spectrum 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.{{cite journal |last1=Reynolds |first1=C. S. |last2=Fabian |first2=A. C. |last3=Inoue |first3=H. |title=ASCA observations of the Seyfert 1 galaxies Mrk 1040 and MS 0225.5 + 3121 |journal=Monthly Notices of the Royal Astronomical Society |date=15 October 1995 |volume=276 |issue=4 |pages=1311–1319 |doi=10.1093/mnras/276.4.1311|arxiv=astro-ph/9504095 |bibcode=1995MNRAS.276.1311R |doi-access=free }} The soft element has been identified as warm absorbing material. The galaxy was further observed by XMM-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.{{cite journal |last1=Tripathi |first1=Shruti |last2=Misra |first2=Ranjeev |last3=Dewangan |first3=Gulab |last4=Rastogi |first4=Shantanu |title=Soft time lags in the X-ray emission of Mrk 1040 |journal=The Astrophysical Journal |date=1 August 2011 |volume=736 |issue=2 |pages=L37 |doi=10.1088/2041-8205/736/2/L37|arxiv=1106.6139 |bibcode=2011ApJ...736L..37T |s2cid=44236594 }}

A detail X-ray spectrum of NGC 931 was obtained by Chandra X-ray Observatory. It revealed the presence of many absorption lines from neon, magnesium, and silicon, with a variety of ionisation 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.{{cite journal |last1=Reeves |first1=J. N. |last2=Braito |first2=V. |last3=Behar |first3=E. |last4=Fischer |first4=T. C. |last5=Kraemer |first5=S. B. |last6=Lobban |first6=A. |last7=Nardini |first7=E. |last8=Porquet |first8=D. |last9=Turner |first9=T. J. |title=High-resolution X-Ray Spectroscopy of the Seyfert 1 Galaxy Mrk 1040. Revealing the Failed Nuclear Wind with Chandra |journal=The Astrophysical Journal |date=28 February 2017 |volume=837 |issue=1 |pages=23 |doi=10.3847/1538-4357/aa5da1|arxiv=1702.00461 |bibcode=2017ApJ...837...23R |hdl=2381/45349 |s2cid=118860247 |hdl-access=free |doi-access=free }}

Supernova

One supernova has been observed in NGC 931, SN 2009lw. The supernova was discovered by W. Li, S. B. Cenko, and A. V. Filippenko during the Lick Observatory Supernova Search on 24.24 November 2009, when it had an apparent magnitude of 18.8.{{cite web |title=Electronic Telegram No. 2175: Supernova 2009lw in NGC 931 |url=http://www.cbat.eps.harvard.edu/cbet/002100/CBET002175.txt |publisher=Central Bureau for Astronomical Telegrams |access-date=3 July 2019 |date=15 February 2010}} It was identified as a type-Ib or possibly a type-IIb supernova a few months past maximum light.{{cite web |last1=W. Li, S. B. Cenko, and A. V. Filippenko |title=Electronic Telegram No. 2047: Silverman, J. M.; Cenko, S. B.; Filippenko, A. V. |url=http://www.cbat.eps.harvard.edu/cbet/002000/CBET002047.txt |publisher=Central Bureau for Astronomical Telegrams |access-date=3 July 2019 |date=25 November 2009}}

Nearby galaxies

NGC 931 has been identified as a member of the NGC 973 group, one of the largest galaxy groups of the Perseus–Pisces Supercluster,{{cite web |title=The Perseus-Pisces Supercluster |url=http://www.atlasoftheuniverse.com/superc/perpsc.html |website=www.atlasoftheuniverse.com}} with at least 39 galaxies identified as its members. Other members of the group include NGC 917, NGC 940, NGC 969, NGC 973, NGC 974, NGC 978, NGC 987, NGC 983, NGC 1060, NGC 1066, NGC 1067, and UGC 2105.{{cite journal |last1=Crook |first1=Aidan C. |last2=Huchra |first2=John P. |last3=Martimbeau |first3=Nathalie |last4=Masters |first4=Karen L. |last5=Jarrett |first5=Tom |last6=Macri |first6=Lucas M. |title=Erratum: "Groups of Galaxies in the Two Micron All Sky Redshift Survey" (ApJ, 655, 790 [2007]) |journal=The Astrophysical Journal |date=October 2008 |volume=685 |issue=2 |pages=1320–1323 |doi=10.1086/590385|bibcode=2008ApJ...685.1320C |doi-access=free }} On the other hand, Garcia recognised NGC 931 as the largest galaxy in a galaxy group known as the NGC 931 group, which also included NGC 940.{{cite journal |last1=Garcia, A. M. |title=General study of group membership. II - Determination of nearby groups |journal=Astronomy and Astrophysics Supplement Series |date=1993 |volume=100 |issue=1 |pages=47–90 |url=http://cdsarc.u-strasbg.fr/ftp/J/A+AS/100/47/table2.dat |issn=0365-0138 |bibcode=1993A&AS..100...47G}}

A smaller companion galaxy, measuring 0.2 by 0.1 arcminutes, is superimposed on the galaxy, lying about 0.35 arcminutes towards the north.Nilson, P. (1973) Uppsala General Catalogue of Galaxies, Acta Universitatis Upsalienis, Nova Regiae Societatis Upsaliensis, Series V: A Vol. 1