Object of the Week, March 9th 2025 – KPG 148, a pair of galaxies in Gemini

[akarsh]

KPG 148
Pair of Galaxies in Gemini
Comprising of:

NGC 2480
Galaxy in Gemini
Type: Im/BCD
RA (ICRS): 07 57 10
Dec (ICRS): +23 46 47
Mag: 14.1 (estimated using Sloan (g+r)/2)
Size: 0.6' x 0.2'

NGC 2481
Galaxy in Gemini
Type: S0
RA (ICRS): 07 57 14
Dec (ICRS): +23 46 04
Mag: 12.8 (estimated using Sloan (g+r)/2)
Size: 1.2' x 0.4'

I came across this pair upon the suggestion of my friend Gautham Ramachandra. Apparently this fell on his radar because a NASA post on Instagram confused NGC 2481 for the much more popular NGC 2841 in Ursa Major. Such a happy coincidence!

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Sloan Digital Sky Survey Image of the pair

This isolated pair of galaxies in Gemini neither shows up on the Arp nor on the Vorontsov-Vel'yaminov catalogs. At first, they do not appear to be anything more than a chance alignment. The image from the Legacy Survey was very suggestive of distortion in NGC 2480, but NGC 2481 appears only mildly warped. The halos almost touch. My best guess from the Legacy image was that they are not interacting, and NGC 2480 must have been the product of a recent minor merger with some other galaxy. Of course, I'm not a galaxy morphologist.

The obvious next check is to look at distances. NGC 2480 has no redshift-independent distances listed in NED, but the redshift of z = 0.007822 puts it at a comoving distance of about 110 Mly. NGC 2481 also does not have redshift-independent dsitances listed, but has a redshift of z = 0.007195, corresponding to a comoving distance of 100 Mly. Hmm, kinda far.

However, when doing my due diligence for this OOTW, I discovered this paper (https://academic.oup.com/mnras/article/494/2/1751/5804806), which indeed claims that the two galaxies are interacting. They used cameras optimized for low surface brightness imaging at the prime focus of two 0.7m telescopes to survey the haloes of several nearby galaxies in visible light. They brand NGC 2481 as a flocculent spiral, not a lenticular (as quoted in the data above from SIMBAD). They go on to state "Also, we can see a distorted structure of NGC 2480 and some other signatures of their close interaction: tidal streams, plums, a highly flared disc of NGC 2480, and a U-warped stellar disc of NGC 2481. The outer shape of NGC 2481 is oval, but the orientation is far from pure edge-on."

Here is the image they have in the paper:
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Fascinating, so they must really be interacting! I went back to the Legacy Survey image after adjusting the gamma on my laptop monitor and whoa! I can see a lot of diffuse tidal streams, and a long tidal tail going from NGC 2481. So it was there all along in the Legacy images. For your convenience, I've also stretched the Legacy Survey image below

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Screenshot from the DESI Legacy Imaging Survey

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Screenshot from the DESI Legacy Imaging Survey, stretched in GIMP to emphasize the faint tidal features

Gautham and I looked at this pair with the 12-inch f/4 travelscope prototype that I was beta-testing at a recent star party in south India. At 203x, NGC 2481 appeared bright, somewhat elongated, continuously visible to direct vision. NGC 2480 appeared very very dim, I got only 3--4 flashes.

At least NGC 2480 must show some interesting structure in large telescopes.

As always,

GIVE IT A GO AND LET US KNOW!

[Clear Skies]

This is Holmberg 89 (https://clearskies.eu/csog/downloads/dsfootw2025/#10).

A single observation for this pair in my log (https://clearskies.eu/astronomy/observing/observinglog/), in the freezing night of 8 February 2018 (https://clearskies.eu/astronomy/observing/sessions/sessions2018/8feb2018/) in Hohenwoos, Germany. 14" SCT @ 168x / 29'; temperature was -9C degrees Celsius (16 degrees Fahrenheit) at the time of that observation:


Both galaxies are visible.

NGC2481 is the SE galaxy: Clearly elongated NNE to SSW, suddenly brighter in a small, round core, no nucleus visible. A mag. 14.5 star is superimposed SSW of the middle (4UCAC569-042129).

NGC2480 is the NW galaxy: Much fainter than NGC2481 is, elongated NW to SE, only visible with AV and at the limit of visibility with AV. Approximately half the size of NGC2481.

I did not observe the star on the northwestern edge of NGC2481.

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[lamperti]

Back in 1993 with a 13" at 240x:"NGC 2481 had a very obvious bright core. Elongation was easy to see. Companion NGC 2480 was quite faint and averted vision was needed to see it."

[Steve Gottlieb]

Interesting new galaxy feature: "plums"! I'm surprised the referee or editor didn't catch this typo.

By the way, both William and John Herschel missed NGC 2480, and it was discovered with Lord Rosse's 72-inch. An "interesting" observation was made in 1867 by Sir Robert Ball, which mentions a possible connection or bridge. I've added some phrases to make it more readable than the original shorthand.


"A very interesting pair. At first [NGC 2481] looked something like a star on a bad night, but the single lens [eyepiece] brought it out well. [NGC 2481] is bright, very small, elongated south-preceding to north-following, much brighter in the middle. [NGC 2480] is faint, little extended? [It is] close north-preceding [NGC 2481] and perhaps joined to it by a nebulous band. They form an angle of about 50° ± with one another."