Object of the Week, February 28, 2016 - NGC 4485 and NGC 4490, Arp 269

[Howard B]

Interacting galaxies
Canes Venatici
RA 12h 30m 36s
DEC +41° 38′

Arp269_DSS.JPG

My one observation of these interacting galaxies was in 2010 and was initially a case of mistaken identity. I was looking for Arp 23 and came across these galaxies first – and once I saw they weren't Arp 23 I thought they weren’t an Arp at all. Despite these two mistakes the view of this bright interacting pair turned out to be delightful, and nearly as detailed as the DSS image above.

N4485_N4490_Arp269_crop.jpg N4485_N4490_Arp269_cropinverted.jpg

My sketch above simulates the view on an average night in western Oregon (21.25 SQM) through my 28 inch f4 Newtonian using 253x.

The larger galaxy, NGC 4490, is evidently called the “Cocoon Galaxy” but the only time I’ve seen it referred to as such was when I was researching this OOTW. Is this a commonly known nickname?

About 25 million light years away, these galaxies are pulling away from each other, which is illustrated well in the HST image of NGC 4485 – you can see the trailing edge being shredded into star forming tufts and clumps as it pulls away from NGC 4490, just outside the lower right corner of the photo below.

HST_NGC 4485.jpg

4490 has a visual magnitude of 9.8 and 4485 is magnitude 11.9, so this bright Arp galaxy pair should be well seen in scopes from 8 inches on up.

I saw a distinct dark lane on the northwest side of 4490, which in photos looks more like space between a detached area of star formation than an actual dust lane. 4490 also had a slender and graceful S shape visually, but is much more ragged looking in photos – it’s clearly been disrupted by 4485.

What do you see?

"GIVE IT A GO AND LET US KNOW"

[KidOrion]

Observed this one for the first time back in 2014, with a six-day-old Moon in Leo and hazy skies of steadily-decreasing quality, from a green zone; used a 12.5" Dob and 24mm 68* (65x) and 14mm 82* eyepieces (112x):

"These are even easier to find than the Whale and the Hockey Stick, as they lie just about 3/4 of a degree NW of Beta Canum. They’re right near the middle of the 24mm as I finish looking through the Telrad. Wow—another stunning sight! I temporarily forget which one is -85 and which is -90, referring to them only as the 'larger one and the smaller one' in my recording: the larger one is about 5’ x 2.5’; the smaller one, separated by about 3’ off to the NW, is a round, unconcentrated spot, like a faint spring globular about 1.5’ in diameter. 4490 is mottled across its central two-thirds, even 'speckled' (?). My notes say that 4490 is 'tear-shaped with averted vision,' but was this a case of knowing how it looks in photographs beforehand? 'Considerably bright' is rather beyond dispute. 'Halo of larger considerably extended beyond the core; almost stellar nucleus in larger.'"

Amazingly (to me), I haven't re-observed these!

[wvreeven]

I actually managed to see NGC 4490 in my 6" telescope back in 2005. I logged it as an elongated smear of light that's brighter in the center. The outer parts were only visible with peripheral vision. I didn't try NGC 4485.


Clear skies, Wouter

[Ivan Maly]

Howard, I've heard the name "Cocoon", actually read it in O'Meara's Caldwell book. In your HST picture the companion is clearly a barred spiral. Especially the fine loop or two on the SW side. Kid, your sighting 4490 in the first place was "a case of knowing" where it is on the chart, so I wound't worry about the possible influences on your observation of its shape.

The pair is one of the best sights in the sky. The sketch below is with a 12" SCT, 125-375x, SQM 21.78.



"Previously with the same telescope I had seen the extension of the main galaxy pointing in the direction of the companion. For some reason, during this new observation that bright inner arm (or an enhanced, star-forming inner edge of the W arm), did not extend past the core. The broad outer arm was seen instead, which extends farther W. The suggestion is that of embracing rather than pointing at the companion. The companion (NGC 4485) featured a visually detached star-forming region, which lies along the connection noticed by Arp, and the main galaxy (NGC 4490) had a similarly “free-floating” bright region near its core."

The pair is VV 30.

[Bill Weir]

My first sketched observation of this pair was on Mar 28/08 using my 6" dobsonian when observing SN2008ax. http://rascvic.zenfolio.com/p5661149...bf45#h3012fc97

My second was 2 days later with my 12.5". http://rascvic.zenfolio.com/p5661149...1bf45#h1247fb4

Bill

[Uwe Glahn]

Nice duo Howard.

4", 90x, NELM 6m5+
4490 - bright, 2:3 elongated, without concentration
4485 - very faint, 2:1 elongated

8", 114x, NELM 6m5+
both bright and direct vision, both elongated, curved form of 4490 weakly recognizable

16", 360x, NELM 6m5+
wonderful pair with lots of detail, three faint HII regions within NGC 4490 (brighter spot N of the core within a elongated part of NGC 4490, another at the end of a brighter elongated part towards NGC 4485, third HII very faint and uncertain S of the core), curved morphology, decentralize core of NGC 4485, brighter HII region in NGC 4485 in direction towards NGC 4485
NGC4490.jpg

[Don Pensack]

This pair is an excellent view in anything >4", but especially comes into its own with 12"+.
A view in a really good 20" was one of the finest sights in the sky in terms of pairs, and up there with 4631/4627 and others.

[ChristianR]

Here is my observation

NGC4490_4485_042015_comp1000.jpg
Location: Bavarian Alps, Germany
Date, Time: April 14 2015, 10:15pm local time
Conditions: seeing II (Antoniadi), SQML=21,4mag/arc sec2, very good transparency
Scope: Dobson Hofheim Instruments 300mm / F5
300x (Nagler 5mm)

Center of 4490 stellar, elongation appr. 3:1, mottled impression around the core, brighter area within the elongated end towards 4485.
4485 less elongated, no stellar center, seems slightly brighter at the end towards 4490.