Object of the Week, December 12, 2021 – NGC 750/751 = Arp 166 - Triangulum

[Paul Alsing]

Object of the Week, December 12, 2021 – NGC 750/751 = Arp 166 - Triangulum
R.A.: 01h57m32.8s
Dec.: +33°12'34"
Size: 1.5'x 1.4'; Magnitude: 12.9

NGC 750 and NGC 751 are interacting elliptical galaxies about 225 million light-years away in Triangulum that are well-placed for observing this month. The pair is listed as Arp 166 in the Atlas of Peculiar Galaxies (in the category Galaxies with diffuse filaments). It is interesting to note that Herschel discovered NGC 750 on September 12, 1784, and he did not resolve it as a pair, but rather as a single object… so it took another 66 years before the Irish astronomer Bindon Stoney was able to find NGC 751 hiding in the glare of its brighter big brother on October 11, 1850, using Lord Rosse's 72" telescope.

NGC_0750_SDSS.jpg

In my 25” f/5 Obsession dob I can see the cores of each galaxy lined up almost exactly north and south with NGC 750 being the northern component, and they are surrounded by what appears to be a common haze of nebulous galactic material, looking pretty much like most other amorphous elliptical galaxies, that is, without any structure or character to speak of. I would guess that the cores are about 20” apart. Wiki tells me there is a tidal tail but I saw no sign of it!

SkyTools Arp 166 CHART.jpg

There are a number of other galaxies in the area to explore, which is why I decided to write about Arp 166 in the first place… I should have said NGC 750/751 and friends!

NGC 761 is about 11’ NNE of NGC 750, a barred lenticular galaxy, mag 14.4, a barred lenticular galaxy, small, elongated, and faint but easy enough to see.

NGC 739 is about 9’ NW of NGC 750, and is another lenticular galaxy, even smaller than NGC 761 and even dimmer at mag 15… but still available with direct vision on a good night. There was apparently some discrepancy about the position of this guy and Steve Gottlieb has details in his great NGC notes… https://www.astronomy-mall.com/Adven...(11-30-17).htm

NGC 736 is about 14.5’ SW of NGC 750 and is almost a twin to NGC 750, being an elliptical galaxy a little brighter at mag 12.1, but he has (4) NGC galaxies close by that should all be in the same field of view with moderate magnification. NGC 740 is the brightest companion about 3.4’ SE, mag 14.8, NGC 738 is the closest and dimmest companion about 1.5’ NE mag 15.9, and NGC 733 is about 3.5’ WNW, at about mag 16… and with some ID issues, which are once again are addressed by Steve G. in his notes.

If you reference the chart you will see that there are still more galaxies in this area, so go ahead and see how many of them you can detect!

As an added bonus there is a nice double star about 4.5' SE of NGC 750, ADS 1562, mags 9.4and 9.9 with a separation of 1.7", an easy split on a night of good seeing.

As always, give it a go and let us know.

[Clear Skies]

... "and friends" is definitely right, as there is plenty to observe in the vicinity of this pair!

In 2017 I observed this area during an all-nighter in the French Ardennes. 14" SCT @ 168x / 29' for all observations.

Arp 166 (NGC750 & NGC751)

This pair also goes by VV' 189 - KPG 46 - Zwicky VI 123. Halton Arp noted a "Small spiral at end of plume" and Fritz Zwicky, too, noted a connection to a red elliptical. The SDDS image reveals PGC2027448 is (visually) connected, but if its redshift in NED is correct, it is in the distant background.

Both galaxies are visible. Without AV the galaxies are detached patches, with AV the halo's of the galaxies overlap. Faint galaxies but a nice view.

The northern galaxy NGC750 is round, larger and slightly brighter than the southern galaxy NGC751 is, gradually brighter in the middle, the nucleus is visible with AV.

The southern galaxy NGC751 is smaller and fainter than NGC750 is and is subtly elongated north to south, even in brightness, the nucleus is visible with AV. The nucleus of NGC751 is just a bit fainter than the nucleus of NGC750 is.

1/2 FoV to the SW are the galaxies NGC736 and NGC738, 1/3 FoV to the NNE is NGC761.

Rating 6/10

Arp 166-1.jpg Arp 166-2.jpg Arp 166-3.jpg

Click here to download the observing guide.

NGC761

A NW-SE elongated glow, clearly fainter than the galaxies NGC750 & NGC751 (Arp 166) that are 1/3 FoV to the SSW. With AV subtly brighter in the middle, no nucleus visible.

To the ENE is an elongated triangle of mag. 12.5 and fainter stars pointing SSW.

To the WNW is a white mag. 9 star (SAO55129). Almost 1/2 FoV to the NE is a white mag. 8.5 star with a mag. 13 star to its SW, ±230°/25" (not a double star in WDS - the mag. 8.5 is component A of the double star HDS267 but has a sep. of 0.2" WDS2017.8).

3/10

NGC739

A small, very faint, round patch, even in brightness, the nucleus is faintly visible with AV.

2/10

NGC736 & NGC738 (with the stars that form NGC737)

NGC736 (SW) and NGC738 (NE) are both visible.

NGC736 is a faint, round glow, suddenly brighter in the middle (quite a large, round core) with a bright nucleus that is visible without AV. To the NNE is a mag. 13.5 star with a mag. 14 star to its SSW: these stars form NGC737.
NGC738 is a small, faint, round patch, even in brightness, no change with AV.

4/10

NGC740

Sharing the FoV with the galaxies NGC736 and NGC738 to the NW: A SE-NW elongated streak, even in brightness, no change with AV.

2/10

NGC733

Sharing the FoV with the galaxies NGC736 and NGC738 to the ESE: To the WNW of NGC736 is an extremely faint, NE-SW elongated glow, at the limit of visibility with AV, even in brightness.

1/10

[Raul Leon]

Hi, here's my observation from 11/4/2013: ngc 750 and ngc 751 are 12th mag. galaxies in Triangulum ; aka Arp 166, these interacting pair can be perceived as one object in bad seeing, high powers when possible is needed to view both. I used a 6mm Ethos at 304x with my 14.5 Starstructure f/4.3ngc 750.jpg

[Don Pensack]

A nice small pair in the 12.5" at 300x, with NGC739 in the same field of view, skies of 21.45.
Other NGCs are not in the field at that magnification.
I also did not note them as particularly faint, and I noted small bright cores in each of them.
My notes suggested that at first they looked like a bi-lobate planetary, but the brighter cores gave them away as galaxies.

[Uwe Glahn]

I found two sketching results for the described area.

First Arp 166, which shows faint tidal extensions under very good transparency.

sketch: 27", 419x, seeing III, NELM 7m0+
NGC750_NGC751.jpg
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Second the nice galaxy group RSCG 18 only 14' SW. NGC 740 shows some structures within the long axis, while NGC 736 (NGC 737) shows a nice star line just north of the galaxy

sketch: 27", 419x, seeing II-III, NELM 7m0+
NGC736-Gruppe.jpg
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