NGC 4691 is a barred spiral galaxy in the constellation Virgo. The galaxy lies about 70million light years away from Earth, which means, given its apparent dimensions, that NGC 4691 is approximately 60,000 light years across.[1] It was discovered by German-British astronomer William Herschel on 17 April 1784.[3]
Characteristics
The galaxy has a prominent bar with some bright knots. From the bar emerge two diffuse low-surface-brightness spiral arms which are devoid of knots. The arms form a nearly complete outer ring.[4] Dust is visible across the inner regions of the galaxy. The dust lanes are generally perpendicular to the major axis of the bar.[5] The galaxy is seen face-on. The total molecular gas of the galaxy based on CO emission is estimated to be about 9×108M☉. Two large molecular clouds are visible along the bar in each side of the nucleus and one more is seen perpendicularly to the bar, located within the inner Lindblad resonance.[6]
The nucleus of the galaxy has been categorised as active, however there is no evidence of it being active in optical wavelengths, but it is a star forming region (HII region) instead.[7][8] A broad H-alpha component has been observed in the central region of the galaxy that is blueshifted by about 500 km/s in respect to the galaxy, indicating it is an outflow that pushes gas away from the plane of the galaxy. It is possible the outflow was created by supernova remnants in the circumnuclear star forming regions.[9]
Supernova
One supernova has been observed in NGC 4691. SN 1997X was discovered by Masakatsu Aoki on 1 February 1997 at an apparent magnitude of 13.6.[10][11] It got as bright as magnitude 13.5, making it the brightest supernova of 1997.[12] It has been identified spectrographically as a type Ic supernova about one week post maximum.[13][14]
↑Eskridge, Paul B.; Frogel, Jay A.; Pogge, Richard W.; Quillen, Alice C.; Berlind, Andreas A.; Davies, Roger L.; DePoy, D. L.; Gilbert, Karoline M.; Houdashelt, Mark L.; Kuchinski, Leslie E.; Ramirez, Solange V.; Sellgren, K.; Stutz, Amelia; Terndrup, Donald M.; Tiede, Glenn P. (November 2002). "Near-Infrared and Optical Morphology of Spiral Galaxies". The Astrophysical Journal Supplement Series. 143 (1): 73–111. arXiv:astro-ph/0206320. Bibcode:2002ApJS..143...73E. doi:10.1086/342340.
12Sandage, A., Bedke, J. (1994), The Carnegie Atlas of Galaxies. Volume I, Carnegie Institution of Washington
↑Garcia-Barreto, J. A.; Franco, J.; Guichard, J.; Carrillo, R. (September 1995). "The Central Regions of NGC 4691: A Remarkable Structure with a Fast Outflow near the Nucleus". The Astrophysical Journal. 451: 156. Bibcode:1995ApJ...451..156G. doi:10.1086/176207.
↑Nakano, S.; Aoki, M.; Kushida, Y.; Kushida, R.; Suntzeff, N. B.; Clocchiatti, A.; Phillips, M. M.; Kim, D. -W.; Covarrubias, R.; Garnavich, P.; Kirshner, R.; Berlind, P. (4 February 1997). "Supernova 1997X in NGC 4691". International Astronomical Union Circular (6552): 1. ISSN0081-0304.
↑"SN1997X". Transient Name Server. IAU. Retrieved 4 June 2025.
↑Gómez, G.; López, R. (January 2002). "Nebular Spectra of the Type I[CLC]c[/CLC] Supernovae 1997B and 1997X". The Astronomical Journal. 123 (1): 328–336. doi:10.1086/324636.
