Sky Sketcher's Post Mortem 5

SKY SKETCHERS POST MORTEM

By Scott Mellish

THREE DIFFERENT VIRGO GALAXIES

It is always refreshing to do an annual trip into the constellation of Virgo and scratch another couple of galaxies off the observing list (which never seems to end when it concerns this constellation). Renowned as it is as the home to innumerable extra-galactic nebulae of which the vast majority can keep an avid deep sky observer gainfully employed for hours, if not years.

The Virgo galaxy cluster is a massive cluster of galaxies which dominates the Virgo supercluster. There are roughly 2000 galaxies in this cluster (although ninety percent of them are dwarf galaxies). The cluster has a diameter of approximately 15 million light years which is not much larger than our Local Group, but it contains fifty times the number of galaxies.

Because of the Virgo Cluster's enormous mass, its strong gravity accelerates the member galaxies to considerably high peculiar velocities, up to over 1500 km/sec, with respect to the cluster's center of mass. Investigations over a number of years have revealed a quite complex dynamic structure of this huge irregular aggregate of galaxies.

The Virgo cluster is close enough that some of its galaxies, which happen to move fast through the cluster in our direction, exhibit the highest blue-shifts measured for any galaxy, i.e. are moving toward us: The record stands for IC 3258, which is approaching us at 517 km/sec. As the cluster is receding from us at about 1,100 km/sec, this galaxy must move with over 1,600 km/sec through the Virgo Cluster's central region.

Analogously, those galaxies which happen to move fastest away from us through the cluster are receding at more than double redshift than the cluster's center of mass: The record is held by NGC 4388 at 2535 km/sec, so that this galaxy moves peculiarly in the direction away from us at over 1,400 km/sec.

With this brief description of the Virgo Cluster enabling you to get a perspective as to the scale of this vast conglomeration of galaxies, it is now perhaps time to detail three objects of interest which I observed over the new moon period at Wiruna 23/04/09 and 24/04/09.

NGC 5230

To start this Post Mortem I have selected NGC 5230, a visually rather bland but somewhat large galaxy of around 2' x 2' in size. At first glance in the eyepiece of the 56cm f5 dob. this object looks like a diffuse elliptical galaxy of around mag 13.0, but in fact it is a face on spiral. In the eyepiece at 314x mag there is no discernable trace of spiral structure, nor is there any visual trace of a bright core region. This galaxy is very soft and subtle in its appearance and the 15' field is almost void of stars.

NGC 5230 = UGC 08573 = MCG +02-35-009 = CGCG 073-043 = PGC 47932; 13 35 31.8 +13 40 33V = 12.1; Size 2.2x1.9; SB = 13.517.5": fairly faint, moderately large, round, 1.5' diameter, fairly low almost even surface brightness, weak concentration. Brightest in a group with N5221 13' NNW and N5222 9.7' WNW. Located near the Virgo-Bootes border.

- by Steve Gottlieb

NGC 4649 (M60) & NGC 4647

The next object of interest is two rather close galaxies in the form of NGC 4649 (M60) and NGC 4647. Both of these galaxies were quite prominent within the 15' field of the 56cm dob at 314x mag. NGC 4649 is by far the brightest of the pair and also the largest, but its companion NGC 4647 is still quite bright in comparison and both galaxies are quite an impressive sight.

Steve Gottlieb describes the pair as follows:NGC 4649 = M60 = U07898 = MCG +02-33-002 = CGCG 071-016 = Arp 116 = VV 206 = PGC 42831; 12 43 39.8 +11 33 11V = 8.8; Size 7.4x6.0; SB = 12.9; PA = 105d17.5": very bright, fairly large, diffuse halo, slightly elongated ~E-W, very bright core with a bright sub stellar nucleus. Forms a double system with N4647 which is almost tangent to M60 barely off the NW edge.

13": bright, very bright nucleus, slightly elongated ~E-W.

NGC 4649 (M60) is one of the giant elliptical galaxies in the Virgo Cluster of galaxies. As the most following (eastern) Messier galaxy in this cluster, it is the last in a row of three (M58, M59, and M60), which comes into the field of view of a telescope pointed to this region of the sky. At lower magnifications, it lies in the same field of view as M59 (25 arc minutes away).

NGC 4649 was discovered by Johann Gottfried Koehler on April 11, 1779, when he was following the comet of that year, together with neighboring M59. It was independently found one day later by Barnabus Oriani, who missed M59, and four days later, on April 15, 1779, by Charles Messier, who also found nearby M58. Messier describes M60 as "a little more distinct" than M58 and M59.

At its distance of some 60 million light years, this galaxy's apparent diameter of 7' x 6' arc minutes corresponds to a linear diameter of 120,000 light years. Amateur telescopes however only show its bright central region of about 4' x 3' arc minutes diameter. At around mag 10.0 NGC 4649 is conspicuous in telescopes as small as 10cm in size, mostly because of its faint neighbor NGC 4647.

Halton Arp included NGC 4649 as No 116 in his Catalogue of Peculiar Galaxies as an "Elliptical Close To and Perturbing a Spiral". NGC 4649 is at R.A.12 43 40 DEC+11 32 58.

NGC 4606 & NGC 4607

And finally to round off this post mortem we come to a visually charming pair of galaxies that in the 56cm scope view quite prominently in the eyepiece at 314x mag. The 15' field framed these two objects as being very different from each other in their shapes.

The lovely elongated diffuse streak to the left of centre is the edge on galaxy NGC 4607 which was discovered by William Parsons 3rd Earl of Rosse in the year 1854. At around mag 13.0 and some 4.0' x 0.7' in size I found this object contrasted well with its more compact companion enabling one to obtain a field of view spanning 15' which contains two rather impressive galaxies.

NGC 4606 whilst marginally brighter than it’s more subtle neighbor viewed rather less impressively in the eyepiece partly because without NGC 4607 in the same field it would have been just another average Virgo galaxy. However having said that NGC 4606 is still a relatively large galaxy in the 56cm dob, being at around 2.7' x 2.0' in size and just on mag 12.8 in brightness.

Despite using averted vision the overall surface brightness of this object was fairly uniform. Due to the diffuse nature of these galaxies I would recommend a dark sky site well away from light pollution in order to view them at their best.

Friedrich Wilhelm Herschel discovered NGC 4606 in the year 1784. NGC 4606/4607 R.A.12 40 57 DEC+11 54 41.

There is nothing like an observing run into Virgo, the "realm of the galaxies" as one is often heard spoken of this fascinating constellation. No doubt it will not be to long before I will be back there again catching up on a few more of those elusive faint fuzzies.

CLEAR SKIES