NGC 4921

{{Short description|Galaxy in the constellation Coma Berenices}}

{{Sky|13|01|26.1196|+|27|53|09.602}}

{{Infobox Galaxy

| name = NGC 4921

| image = NGC 4921 by HST.jpg

| caption= NGC 4921 in a combination of near-infrared and yellow light

| epoch = J2000

| type = SB(rs)ab{{cite web

| title=NASA/IPAC Extragalactic Database

| work=Results for NGC 4921

| url=http://nedwww.ipac.caltech.edu/

| access-date=2009-03-05 }}

| ra = {{RA|13|01|26.1196}}

| dec = {{DEC|+27|53|09.602}}

| dist_ly = ca. 320 Mly

| z = 0.018286; 5,482 km/s{{cite journal

| author=Haynes, Martha P. | display-authors=etal

| title=21 CM H1 Line Spectra of Galaxies in Nearby Clusters

| journal=Astronomical Journal | volume=113 | pages=1197–1211

|date=April 1997 | doi=10.1086/118337 | bibcode=1997AJ....113.1197H| doi-access=free }}

| constellation name = Coma Berenices

| group_cluster = Coma Cluster

| appmag_v = 13.04

| size=~{{convert|90.07|kpc|ly|abbr=on|lk=on|order=flip}} (estimated)

| size_v = 2.5{{prime}} × 2.2{{prime}}

| names = {{odlist | UGC= 8134 | PGC= 44899 | CGCG= 160-095 | MCG= +05-31-098}}

}}

NGC 4921 is a large barred spiral galaxy in the Coma Cluster, located in the constellation of Coma Berenices. It was discovered by German-British astronomer William Herschel on 11 April 1785.{{cite web | url = https://cseligman.com/text/atlas/ngc49.htm#4921 | title = New General Catalogue Objects: NGC{{nbsp}}4921| last = Seligman | first = Courtney | website = Celestial Atlas | access-date = 30 November 2024}} It is about 320 million light-years from Earth.{{cite web|url=http://www.universetoday.com/2009/02/05/deep-hubble-view-of-unusual-fluffy-galaxy-–-and-beyond/ |archive-url=https://web.archive.org/web/20090208150840/http://www.universetoday.com/2009/02/05/deep-hubble-view-of-unusual-fluffy-galaxy-%E2%80%93-and-beyond/ |url-status=dead |archive-date=2009-02-08 |title=Deep Hubble View of Unusual "Fluffy" Galaxy – and Beyond |last=Atkinson |first=Nancy |date=2009-02-05 |publisher=Universe Today |access-date=2009-02-05 }} The galaxy has a nucleus with a bar structure that is surrounded by a distinct ring of dust that contains recently formed, hot blue stars. The outer part consists of unusually smooth, poorly distinguished spiral arms.{{cite web

| first=Lars Lindberg | last=Christensen

| title=Exceptionally deep view of strange galaxy

| date=2009-02-05 | publisher=European Space Agency

| url=http://www.esa.int/esaSC/SEMA1NXPXPF_index_0.html

| access-date=2009-02-12 }}

In 1976, the Canadian astronomer Sidney Van den Bergh categorized this galaxy as "anemic" because of the low rate at which stars are being formed. He noted that it has "an unusually low surface brightness and exhibits remarkably diffuse spiral arms". Nonetheless, it is the brightest spiral galaxy in the Coma Cluster.{{cite journal

| author=van den Bergh, S.

| title=A new classification system for galaxies

| journal=Astrophysical Journal | volume=206

| date=1976-06-15 | pages=883–887

| doi=10.1086/154452

| bibcode=1976ApJ...206..883V| doi-access=free}}

This galaxy is located near the center of the cluster and has a high relative velocity (7,560 km/s){{cite journal

| author1=Poggianti, Bianca M.|author1-link=Bianca Poggianti | display-authors=4 | author2=Bridges, Terry J. | author3=Komiyama, Y. Yagi, M. | author4=Carter, Dave | author5=Mobasher, Bahram | author6=Okamura, S. | author7=Kashikawa, N. | title=A Comparison of the Galaxy Populations in the Coma and Distant Clusters: The Evolution of k+a Galaxies and the Role of the Intracluster Medium

| journal=The Astrophysical Journal | volume=601

| issue=1

| pages=197–213 | date=2004-01-20 | doi=10.1086/380195 | bibcode=2004ApJ...601..197P|arxiv = astro-ph/0309449 | s2cid=17429859 }} compared to the mean cluster velocity. When examined at the 21 cm wavelength Hydrogen line, NGC 4921 was found to be strongly H I deficient, indicating that it is low in hydrogen. The distribution of hydrogen has also been deeply perturbed toward the SE spiral arm and is less extended than the optical disk of the galaxy. This may have been caused by interaction with the intergalactic medium, which is stripping off the gas via ram pressure.{{cite journal

| author=Bravo-Alfaro, H. | author2=Cayatte, V. | author3=van Gorkom, J. H. | author4=Balkowski, C. | title=VLA HI Imaging of the brightest spiral galaxies in Coma.

| journal=The Astronomical Journal | volume=119

| issue=2

| pages=580–592 |date=February 2000

| doi=10.1086/301194 | bibcode=2000AJ....119..580B|arxiv = astro-ph/9912405 | s2cid=118308523 }} Some filaments of gas that have been stripped away are falling back to the galaxy, making NGC 4921 possibly the first observational evidence of this fallback.{{cite journal|doi=10.3847/1538-4357/ac1793|title=Molecular Gas Filaments and Fallback in the Ram Pressure Stripped Coma Spiral NGC 4921|year=2021|last1=Cramer|first1=W. J.|last2=Kenney|first2=J. D. P.|last3=Tonnesen|first3=S.|last4=Smith|first4=R.|last5=Wong|first5=T.|last6=Jáchym|first6=P.|last7=Cortés|first7=J. R.|last8=Cortés|first8=P. C.|last9=Wu|first9=Y.-T.|journal=The Astrophysical Journal|volume=921|issue=1|page=22|arxiv=2107.11731|bibcode=2021ApJ...921...22C|s2cid=236428596 |doi-access=free }}