NGC 3921

{{short description|Interacting galaxy in the constellation Ursa Major}}

{{Infobox galaxy

| name = NGC 3921

| image = Evolution in slow motion.jpg

| image_size = 280px

| caption = NGC 3921, imaged by the Hubble Space Telescope

| epoch = J2000

| constellation name = Ursa Major

| ra = {{RA|11|51|06.863}}{{cite journal |doi=10.1086/498708 |last1=Skrutskie |first1=Michael F. |last2=Cutri |first2=Roc M. |last3=Stiening |first3=Rae |last4=Weinberg |first4=Martin D. |last5=Schneider |first5=Stephen E. |last6=Carpenter |first6=John M. |last7=Beichman |first7=Charles A. |last8=Capps |first8=Richard W. |last9=Chester |first9=Thomas |last10=Elias |first10=Jonathan H. |last11=Huchra |first11=John P. |last12=Liebert |first12=James W. |last13=Lonsdale |first13=Carol J. |last14=Monet |first14=David G. |last15=Price |first15=Stephan |last16=Seitzer |first16=Patrick |last17=Jarrett |first17=Thomas H. |last18=Kirkpatrick |first18=J. Davy |last19=Gizis |first19=John E. |last20=Howard |first20=Elizabeth V. |last21=Evans |first21=Tracey E. |last22=Fowler |first22=John W. |last23=Fullmer |first23=Linda |last24=Hurt |first24=Robert L. |last25=Light |first25=Robert M. |last26=Kopan |first26=Eugene L. |last27=Marsh |first27=Kenneth A. |last28=McCallon |first28=Howard L. |last29=Tam |first29=Robert |last30=Van Dyk |first30=Schuyler D. |last31=Wheelock |first31=Sherry L. |title=The Two Micron All Sky Survey (2MASS) |journal=The Astronomical Journal |date=1 February 2006 |volume=131 |issue=2 |pages=1163–1183 |bibcode=2006AJ....131.1163S |s2cid=18913331 |url=https://ui.adsabs.harvard.edu/abs/2006AJ....131.1163S/abstract |issn=0004-6256 |doi-access=free }}

| dec = {{DEC|+55|04|43.38}}

| z = 0.019667{{cite web|url=http://nedwww.ipac.caltech.edu/cgi-bin/nph-objsearch?objname=NGC+3921&extend=no&out_csys=Equatorial&out_equinox=J2000.0&obj_sort=RA+or+Longitude&of=pre_text&zv_breaker=30000.0&list_limit=5&img_stamp=YES|title=NED results for object NGC 3921|publisher=National Aeronautics and Space Administration / Infrared Processing and Analysis Center|access-date=28 May 2017}}

| h_radial_v = 5896 km/s

| dist_ly = {{cvt|85.19|Mpc|Mly|lk=on|order=flip}}{{cite journal|bibcode=2007ApJ...655..790C|doi=10.1086/510201|arxiv=astro-ph/0610732|title=Groups of Galaxies in the Two Micron All Sky Redshift Survey|journal=The Astrophysical Journal|volume=655|issue=2|pages=790–813|year=2007|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.|s2cid=11672751}}

| group_cluster =

| type = (R')SA0/a(s) pec

| size = {{cvt|55240|pc|ly|lk=on|order=flip}}POSS1 103a-O values used.

| appmag_v = 12.64{{cite simbad|title=NGC 3921|access-date=28 May 2017}}

| appmag_b = 13.4

| absmag_v = −22.09

| size_v = 2.1{{prime}} × 1.3{{prime}}

| notes =

| names = Arp 224, UGC 6823, MGC+09-20-009, PGC 37063

}}

NGC 3921 is an interacting galaxy in the northern constellation of Ursa Major. Estimates using redshift put it at about 59 million light years (18 megaparsecs) from Earth. It was discovered on 14 April 1789 by William Herschel,{{cite web|url=http://cseligman.com/text/atlas/ngc39.htm#3921|title=New General Catalogue objects: NGC 3900 - 3949|author=Seligman, Courtney|website=cseligman.com|access-date=28 May 2017}} and was described as "pretty faint, small, round" by John Louis Emil Dreyer, the compiler of the New General Catalogue.{{cite web|url=http://spider.seds.org/ngc/ngc.cgi?NGC+3921|title=Revised NGC Data for NGC 3921 - Hartmut Frommert - SEDS|website=seds.org|access-date=28 May 2017}}

NGC 3921 is the remnant of a galaxy merger. The two progenitor galaxies are thought to have been disk galaxies that collided about 700 million years ago. The image shows noticeable star formation and structures like loops, indicative of galaxies interacting.{{cite web|url=http://www.spacetelescope.org/images/potw1537a/|title=Evolution in slow motion | ESA/Hubble|website=www.spacetelescope.org|date=2015|access-date=28 May 2017}} Because of this, NGC 3921 was included in Halton Arp's Atlas of Peculiar Galaxies under the designation Arp 224.

Being a starburst galaxy, NGC 3921 has important features. One of them is an ultraluminous X-ray source, designated X-2, with an X-ray luminosity of {{val|8e39}} erg/s.{{cite journal|bibcode=2012ApJ...758...28J|doi=10.1088/0004-637X/758/1/28|arxiv=1208.4502|title=The Nature of the Bright Ulx X-2 in Ngc 3921: Achandraposition Andhstcandidate Counterpart|journal=The Astrophysical Journal|volume=758|issue=1|pages=28|year=2012|last1=Jonker|first1=P. G.|last2=Heida|first2=M.|last3=Torres|first3=M. A. P.|last4=Miller-Jones|first4=J. C. A.|last5=Fabian|first5=A. C.|last6=Ratti|first6=E. M.|last7=Miniutti|first7=G.|last8=Walton|first8=D. J.|last9=Roberts|first9=T. P.|s2cid=59330131}} Additionally, two candidate globular clusters have been detected within NGC 3921. They are both fairly young, and about half as massive as Omega Centauri, demonstrating that mergers of gas-rich galaxies can also create more metal-rich globular clusters.{{cite journal|bibcode=2004AJ....128..202S|doi=10.1086/421851|arxiv=astro-ph/0404424|title=Keck Spectroscopy of Two Young Globular Clusters in the Merger Remnant NGC 3921|journal=The Astronomical Journal|volume=128|issue=1|pages=202–210|year=2004|last1=Schweizer|first1=François|last2=Seitzer|first2=Patrick|last3=Brodie|first3=Jean P.|s2cid=14568163}}