Spirals NGC 4639 and 4654

NGC 4639 and NGC 4654 are a pair of spiral galaxies on the edge of the Virgo Cluster. They are located in northern Virgo. In fact the top 18% of the image is in Coma Berenices. Large spirals like these usually reside on the edge of major clusters. I suppose this is because they escape mergers that create all the elliptical galaxies in the center by being in the sparsely settled outer reaches of the cluster. While their redshifts are about the same indicating a distance of about 60 million light-years, in line with the average distance of the cluster their non redshift measurements vary. Especially for NGC 4654. Thus I don't know for sure if they really are close together and possibly an interacting pair or not. I'm going to say not. If they are at the same distance their center to center separation is only about 300 thousand light-years. I'd expect some indication of interaction if they were really that close though if just approaching for the first time I suppose the lack of obvious interaction isn't too surprising.

NGC 4639 is a barred spiral whose inner arm structure forms a ring like structure. NED classifies it as SAB(rs)bc with a Seyfert 1.8 core (most sources say the Seyfert signature is weak). Seligman classifies it as SBbc? with no mention of the ringlike arms. I measure a diameter of about 57,000 light-years making it a typical sized spiral. It was discovered by William Herschel on April 12, 1784 and is in the second Herschel 400 observing program. I have no visual notes on it that survived my move to Minnesota.

NGC 4654 is more interesting. It looks as if it could have been included in Arp's atlas under the "wind effects" category as it does appear to be wind swept. That may in fact be rather close to the truth. It is thought to be traveling through the intergalactic medium at high speed the pressure is creating the star plumes giving it the wind blown look.

A note at NED reads: "NGC 4654 This galaxy is known for its lopsided structure in the optical as well as HI disks, most likely due to the pressure effect of the intracluster gas, blowing from the northwest (Phookun and Mundy 1995). The CO distribution is also lopsided, in the same direction as that of the HI and optical image tail. The lopsided CO distribution suggests that the pressure effect is not negligible even in such a central molecular disk. Moreover, the CO distribution is more elongated than the optical/HI disks. The velocity structure is rigid body-like with mildly increasing rotation velocity with the radius. Such rotation characteristics is exceptional among the presently observed PVDs."

PVD: The pairwise velocity dispersion is a measure of the temperature of the "gas" of galaxies. https://ned.ipac.caltech.edu/level5/.../Jones5_4.html
It seems only their advertising links work. I can't make the above link work. Maybe it will work if you paste this in ned.ipac.caltech.edu/level5/March04/Jones/Jones5_4.html


NED classifies it as SAB(rs)cd with HII emission that shows as blue blobs across the face of the galaxy. Seligman says SBcd?. I can't say I see the ring structure that several sources mention and is included in NED's classification. It was discovered by William Herschel the same night as NGC 4639 and is in the first Herschel 400 observing program. My notes on it taken with a crescent moon under only fair to poor seeing at 50x with my 10" f/5 on April 23, 1985 reads: "Interesting, fat, oval galaxy with sharp pointed ends. Seems to be near edge on. In same field with NGC 4639". I can't reconcile that description to my image. Guess moon and conditions were to blame. Thanks to the plumes apparently due to pressure of it ramming into the interstellar medium it measures a bit over 100,000 light-years in size. This makes it one of the 15 largest spirals in the cluster.

The image contains other interesting galaxies. NGC 4639 appears to have a nearby irregular galaxy for a companion if the redshift is to be believed. The image has an apparent close pair of galaxies that may be interacting. Above NGC 4639 is ASK 408994.0 at about 1.27 billion light-years. The identification points to the bright lower portion. There is no identification given for the northern part. So while it appears as two galaxies in my image NED apparently considers it one with a very off center bright core. I see it as two overlapping galaxies.

Above NGC 4639 is an orange galaxy PGC 42731 at 1.17 billion light-years. It is classified by NED as a dwarf E1 galaxy with a question mark. I measure it at 85,000 light-years in size. How this can be a dwarf I can't understand. Above NGC 4654 is PGC 042870 also listed as a dwarf E1 galaxy but without the question mark. I measure it at 4,100 light-years in size. Now that is a dwarf.

Inside the upper right end of NGC 4654 is an object NED lists separately but with the same coordinates as ASK 408988.0 and LEDA 4106392. It has a redshift of a bit less than 60 million light-years. The LEDA identification is listed as being part of the galaxy while the ASK designation says it is a separate object. Fortunately the HST has taken 1 short image of this part of the galaxy and shows it to be a star_cluster, or maybe a tight group of several clusters. It is certainly part of the galaxy. I've attached an image from the HST Legacy site of the object in green light.

One asteroid came through. Details are in the annotated image.

14" LX200R @ f/10, L=4x10' RGB=2x10', STL-11000XM, Paramount ME

Rick