Object of the Week, October 3, 2021 – IC 275 in Perseus

Object of the Week, October 3, 2021 – IC 275 in Perseus

Very compact Galaxy Trio

PGC 11389 (RA: 03 00 55.9 Dec: +44 21 00. Size: 0.4x0.4’ Mag: 17.3)
PGC 11388 (RA: 03 00 55.8 Dec: +44 20 47 Size: 0.2x0.2’ Mag: 17.5)
PGC 11390 (RA: 03 00 58.7 Dec: +44 21 59 Size: 0.2x0.2’ Mag: 17.5)

Posted in place of Akarsh Shima (we switched places for this week only)

This object was originally catalogued as a single IC object as it was probably not resolved when first discovered. After taking a look at this relatively unknown trio a little while ago, I can see why. The three members are so close together that it occupies a space of 35-40”. That is arc seconds, not arc minutes. Just think, Jupiter at opposition is about 50” apparent diameter. It would obscure the entire group! So you need to crank the magnification to see the individual members.

I could not find any professional work on this group, but sure that it is out there. I’m interested in learning a little more about this group. Is it interacting? Optical trio? I’m pretty sure that it is interacting based on the halo surrounding all three galaxies. I’ve kinda out of date on finding research papers in the astrophysical world.

Notes from my most recent observation with a 22” reflector under mag 7+ skies at a site at 7,800 ft above sea level can be found below. I’ve observed this object several times, including once with my old 16” reflector. It was a very faint unresolved group at 610x through the 16”.

22” (230, 383, 591 and 690x) – This IC galaxy is actually a very compact triple galaxy system arranged in a 30-60-right triangle that occupies a very small space of 35-40” across. This trio is resolved at 591 and 690x. Requires some patience sitting at the eyepiece to see all three members.
PGC 11389 – Very faint very small galaxy located at the right angle of the triangle. 0.1’ across.
PGC 11388 – Extremely faint very small round glow. <0.1’ across. Sitting at the 60 degree angle and just 15” due south of NGC 11389.
PGC 11390 – Extremely faint very small round glow. At the 30 degree angle and 35” due east of PGC 11389.

SDSS image
Attachment 4468

Anyhow, as always, give it a shot and let us know.

I've made one observation of IC 275 with my 24", though only at 375x. I first picked up the glow at 200x and called it "faint, roundish, ~24" diameter, low irregular surface brightness."

When I examined it at 375x, I was confident that 2 or 3 extremely faint stellar or quasi-stellar nuclei were glimpsed and made a diagram of the orientation. Checking the SDSS at home, my diagram matched PGC 11388 and PGC 11389, which are separated by just 15" N-S. The third object on my diagram turned out to be a mag 15.8 star, which happens to lie only 18" NW of PGC 11389! So, I apparently missed PGC 11390, which is 30" E of PGC 11389 and perhaps the faintest of the trio.

Neither SIMBAD or NED shows any research (just inclusion in a few catalogues) and SDSS doesn't have individual redshifts. So, basically an unstudied trio.

Nice trio. Checked my notes, I wrote:
27", 419x, NELM 7m0+, Seeing II-III - immediately noticeable as one faint glow in the searching eyepiece; 419x shows three easy to separate glows; PGC 11388/11389 shows as two tight direct vision glows, which shows a laminar and somewhat concentrated image; PGC 11390 shows as fainter, but steady visible concentrated glow; two still fainter glows 3' W

sketch: 27", 419x, NELM 7m0+, Seeing II-III
Attachment 4469
home

Interestingly Courtney Seligman gives the following info

GC 11388 = A magnitude 16.5 elliptical galaxy (type E1?) at RA 03 00 55.9, Dec +44 20 47
PGC 11389 = A magnitude 16.3 elliptical galaxy (type E1 pec?) at RA 03 00 55.9, Dec +44 21 01
PGC 11390 = A magnitude 15.6 elliptical galaxy (type E0?) at RA 03 00 58.6, Dec +44 20 59
Historical Identification: Per Dreyer, IC 275 (Swift list VIII (#9), 1860 RA 02 51 40, NPD 46 12.8) is "most extremely faint, pretty small, round, between 2 stars". Swift's Catalogue No. 8 note adds "Faint star near to west, double star near to southwest, following of 2", the other being IC 274. The position precesses to RA 03 00 56.2, Dec +44 20 48, on the southern side of the brightest of the three galaxies listed above, and the stars to the south and southwest make the identification certain. The main question about the identification has to do with the statement "between 2 stars", which does not seem to apply to the group unless Swift thought the eastern member of the triplet was a faint star. For that reason some think that only the western pair (PGC 11388 and 11389) should be IC 275, but it is general practice to apply the term to all three galaxies, as I have done here.
Physical Information: Based on a recessional velocity of 9735 km/sec, PGC 11389 is about 450 million light years across. Given that and its apparent size of 1.1 by 1.0 arcmin, it is about 140 thousand light years across. Presuming the other galaxies are at the same distance (LEDA lists the same recessional velocity for PGC 11388 and 11389, but NED only assigns the recessional velocity to PGC 11389), PGC 11388's apparent size of 0.45 by 0.4 arcmin corresponds to about 60 thousand light years, and PGC 11390's apparent size of 0.45 by 0.45 arcmin corresponds to about 60 thousand light years.

It's certainly possible that Swift only caught the combined glow of the western two galaxies (unresolved), but I doubt he would have resolved the eastern galaxy as a star. Swift observed at only 132x, using a custom made wide-field eyepiece with a field of 33' (implying an apparent field of ~70°). The three galaxies would be too tight to resolve at that power and almost certainly the eastern one would be too faint to see individually.

When he described a "nebula" (galaxy) as "between 2 stars", the separations of the stars could easily be 15' apart, so there's really no telling which ones he had in mind.

Steve