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Object of the Week May 14th, 2023 – NGC 2276 in Cepheus
Arp 25 = NGC 2276 = KPG 127a = VV 1189a = Part of Arp 114
Peculiar Galaxy in Cepheus
RA: 07 27 14
Dec: +85 45 16
Mag: 11.4
Size: 2.3' x 1.9'
Background and Science
An NGC 2000-something object now?? Don't you worry, this object is less than 5° away from the north pole and should be well-placed for most of the northern hemisphere readership of this forum.
NGC 2276 was discovered by August Winnecke on June 26, 1876 and independently by Wilhelm Tempel0. This peculiar spiral galaxy appears first in Halton Arp's famous catalog as the 25th entry under spiral galaxies with one heavy arm. It appears again along with its physical companion NGC 2300, together forming Arp 114, placed under the category of elliptical galaxies perturbing spirals. As observed by Dr. Arp, the morphology and the presence of NGC 2300 nearby conjure up a picture of what's going on. The article accompanying the Hubble image confirms, "a neighboring galaxy to the right of NGC 2276 ... is gravitationally tugging on its disk of blue stars, pulling the stars on one side of the galaxy outward to distort the galaxy's normal fried-egg appearance."
Attachment 5126
Hubble Space Telescope image of NGC 2276; Credit NASA/ESA/Hubble/P. Sell/L. Shatz
NGC 2276 is part of the NGC 2300 group, consisting of NGC 2300, NGC 2276, NGC 2268, perhaps IC 455 and UGC 36701,2. This group of galaxies is estimated to lie about 33 Mpc (107 Mly) from us. With the X-Ray space telescope ROSAT, a diffuse X-ray emission was discovered from this cluster in 19931, whose geometric center is roughly coincident with that of the cluster. The hot gas is believed to be held in place by dark matter, with estimates of the amount thereof being anywhere between 7--25x the regular matter1. This hot X-ray emitting gas forms the intragroup medium (IGM) or intracluster medium (ICM), which is a common feature in clusters of galaxies including the Virgo cluster3.
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ROSAT X-Ray Image of NGC 2300 group showing the hot X-ray emitting gas; Credit: NASA
In a highly-cited 2006 paper4, it was suggested that NGC 2276 was supersonically moving through the IGM and the ram pressure from the IGM has resulted in a bow-shock like feature appearing on the western edge. A number of bright X-ray point sources were detected on the western edge, where strong star forming activity is taking place. The paper also notes "Contrary to previous claims, we conclude that tidal interactions are not essential to explain ... the morphology of the galaxy ...". Ram pressure is the formal term for the force of the air that a motorcyclist feels when riding: the extra pressure exerted on a surface when a fluid is brought to a halt by the surface. Ram pressure can cause the gas in galaxies to be stripped out of the galaxy while the stars plow through it less affected, in a process called ram-pressure stripping, and is believed to be one of the reasons why the central regions of galaxy clusters have fewer spiral galaxies3.
A 2012 paper5 contradicts the 2006 study: "For NGC 2276, we do not find clear indications of a shock. Although ram-pressure effects seem to be visible, the main driver of the observed distortions is most likely tidal interaction." An excellent amateur-friendly summary of this state of affairs as of 2013 is a "Galaxy of the Month" article by Owen Brazell for the Webb Society.
However, a 2015 paper2 went back to attributing the disturbed morphology to ram pressure and viscous stripping, the latter ostensibly a result of the viscosity of the gases causing the IGM to cling to and tug on the galaxy's gas6. They used numerical simulations turning the IGM on and off, and concluded that tidal distortion was not as significant as the IGM's effects in reproducing the observed morphology of NGC 2276 from the simulation.
The ram-pressure or the interaction with NGC 2300, whichever the cause, it seems like NGC 2276 is churning stars out at an unusually high rate, with the western edge having the highest rate7. The 2018 study reporting on this attributed the drastic variation in star-forming rates across the galaxy to both effects.
Visual Observation
High star formation in a galaxy enthuses deep-sky observers because it suggests the presence of bright HII regions. Sure enough, the Hodge-Kennicutt atlas catalogs 72 HII regions in the galaxy. I was not able to figure out which designations correspond to the most prominent visual knots, but Steve Gottlieb has already done that for us. His notes report a positive observation of two non-stellar HII regions, NGC 2276:[HK83] 69 and NGC 2276:[HK83] 63 with his 24", and a stellar knot NGC 2276:[HK83] 24.
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DSS2 Blue Image of NGC 2276 and NGC 2300; Also seen is the bright star HD 51141 which detracts from the view. Credit: MAST/STScI
I took a look at this fascinating galaxy during the January new moon this year, with my 18". This was from Bortle 4 conditions perhaps, at Pinnacles National Park in California. Not much of the exciting morphology of this galaxy was visible in my 18", wherein it appeared as a low surface brightness, roundish glow. Careful observation faintly revealed the asymmetric nature of the galaxy, with the region closer to the bright star HD 51141 (west) being brighter. NGC 2300 on the other hand was an easy, high surface brightness glow, slightly elongated.
Steve's observation report on Jimi Lowrey's 48" is tantalizing: as is expected from the fine instrument and skies, he was able to pick out the spiral arms and several knots. I hold out hope of experiencing this in the coming week, if only Fort Davis weather co-operates.
I haven't observed this with an 8" scope, but I shall once again defer to Steve Gottlieb, who has succeeded. Whereas it might not reveal all of its detail in small aperture scopes, perhaps knowing the fascinating astrophysics behind the faint patch of light might make this an attractive target for you; so no matter the aperture of your light bucket,
GIVE IT A GO,
AND LET US KNOW!
[0] Prof. Courtney Selgiman's notes on NGC 2276
[1] J. S. Mulchaey, D. S. Davis, R. F. Mushotzky, D. Burstein, ApJ 404 L9-L12 (1993)
[2] A. Wolter, P. Esposito, M. Mapelli, F. Pizzolato, E. Ripamonti, MNRAS 448 Issue 1, pp. 781-791 (2015)
[3] Astrophysics for Physicists (book), Prof. Arnab Rai Choudhuri
[4] J. Rasmussen, T. J. Ponman, J. S. Mulchaey, MNRAS 370 Issue 1, pp. 453-467 (2006)
[5] M. Wezgowiec et. al. A&A 544, A99 (2012)
[6] P. E. J. Nulsen, MNRAS 198 Issue 4, pp. 1007-1016 (1982)
[7] N. Tomicic et. al., ApJ Letters 869: L38 (2018)
