NGC 772

Adam Block/Mount Lemmon SkyCenter/University of Arizona

Arp 78, also known as NGC 772, was originally listed in Halton Arp’s Atlas of Peculiar Galaxies because of its possible interaction with nearby high redshift companions via its extensive and diffuse spiral arms.  Its redshift is listed in NED as z = 0.008246.  This places the galaxy at a distance of 115 million light years using a so-called Hubble Constant value of 70 (km/s)/Mpc.

The image on the right is from the University of Arizona’s Mt. Lemmon SkyCenter.  It has been rotated and cropped to match the orientation of the original image used by Dr. Arp in his atlas. The bright object on the right is the likely true companion NGC 770.  In a straight line above NGC 770 are two small yellow objects which are the high redshift companions in question.  In addition to their distinct color compared to NGC 772 and NGC 770, their size and morphology indicate that they are most likely not companions after all but distant background galaxies.

Modern observations may have clarified two possible redshift discordancies in NGC 772 but they have also revealed a new and more compelling one!  In the late 1980’s a quasar labeled [HB89] 0156+187 was discovered near the galactic core of NGC 772.  The exact location of this quasar along with two observed supernovae is shown in the image below generated using the free Aladin interactive sky atlas software.  The light green square marks the QSO.

NGC 772 and [HB89] 0156+187

Close examination of even the highest resolution images available reveal little more than bright knots of gas and stars in this inner most region of the galaxy’s spiral arms.  It is difficult to believe that any background object could be visible through the density of stellar material present in this location near the center of the galaxy.  Yet continued spectral analysis of [HB89] 0156+187 into the early 21st century indicates a redshift of z = 2.61 which would supposedly place it far behind NGC 772 at a distance of well over 11 billion light years!

Obviously, as with most of the examples I have posted to this site, there is a need for higher resolution observations and closer examination of the objects presented.  If it were not for the extremely high redshift of [HB89] 0156+187 it would be easy to assume by most scientists that the object was located either within or in front of the inner spiral arms of NGC 772, based on the data available today.  Better observations may clarify the actual position of this quasar and place it in the background as in the case of the previously described companion objects observed by Arp.  However, what will scientists do if better observations reveal that the quasar is indeed part of NGC 772?  Will they finally be forced to reexamine the true nature of extragalactic redshifts and the validity of universal expansion?  Either way it is important that research be continued and questions continue to be asked, regardless of whether or not we like the answers that result.


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12 Responses to “NGC 772”

  1. NGC 772 system is within SDSS DR9 coverage area. Here’s NGC 772’s SDSS image:

    • sbsims says:

      Thanks for the update! I still don’t see an object but I think this latest imagery clearly shows that the source of the discordant redshift is located very close to the core of the galaxy, at the root of one of the spiral arms. I think the density of stars, gas and dust in that particular location makes it highly unlikely that any radiation from a background QSO would be visible through it.

  2. Actually, SDSS DR9 navigate tool shows some blue object there, although slightly off from the position given by NED. However, this seems to be a case of position error in Hewitt & Burbidge catalog. I checked Hewitt & Burbidge (1993). They do have same coordinate data as NED, but they say that the QSO has angular separation of 5.87 arcmin from NGC 772, while the given coordinates give only 0.348 arcmin. So, clearly a case of position error in Hewitt & Burbidge catalog. I have encountered few of these before in that catalog.

    • sbsims says:

      I do not know what to make of the note in the HB catalog. There is also mention of NGC 770 but 5.87 arcmin would place the object far past even that galaxy. I believe the blue object you describe has already been identified as an H II region, NGC 0772:[HK83] 16, even though the NED coordinates appear to place the object about 5 arcmin northeast of current images which I assume is due to precession. If the NED coordinates for the QSO are adjusted to account for similar precession its position coincides with a faint blue object to the south-west, just at the edge of the core of NGC 772 at coordinates 29.8354, +19.0074. But based on this object’s location so close to the galactic core it cannot possibly be a visible background quasar.

  3. NED search around NGC 772 within 10 arcmin doesn’t reveal any candidate objects for the QSO at 5.87 arcmin separation. H&B catalog gives a reference for the QSO as “425”, which is Arp (1976). Arp doesn’t give any data for the NGC 772 system, but he only gives list of new unpublished QSO-galaxy associations, and there he mentions also NGC 772:

    “New quasars (unpublished), which are simply near galaxies, include (1) one certain and possibly three total quasars northeast of NGC 772,…”

    If this QSO would be almost in the nucleus of NGC 772, Arp would have mentioned it. At least he gives direction of the QSO. The QSO was used as calibration object in Arp et al. (1979). Unfortunately they don’t give coordinates and NGC 772 system is only mentioned briefly in section 2.3.

    I digged around in SDSS DR9 SkyServer to find candidates for the QSO, but I didn’t find anything that is at correct angular separation.

    • sbsims says:

      Arp et al. (1979) describes NGC 772 UB2 as “a faint quasar…which was found by its ultraviolet color (apparently fortuitously) on a photographic plate.” Perhaps this is why it is so difficult to locate? This is certainly a case where better observations would be helpful.

  4. Usually these position errors have been such that one digit in coordinates has changed to something else, so if one would like to really dig into this, trying changes in different coordinates and aiming to get 5.87 angular separation might result in finding the actual object.

    Another option would be to ask Arp about it. 🙂

    • sbsims says:

      I actually gave it a shot and sent Dr. Arp an email after your comment but it has been six months now and no reply. I’m not surprised, it has been a few years since I’ve heard anything from him at all. I think he may truly be in retirement now.

  5. Jean Tate says:

    In this page, you give the value of the Hubble constant as 70 (km/s)/Mpc. On others, as 55, and on at least one other, 80. It might be helpful to your readers to either use a single value throughout, or to explain why you use different values.

    “It is difficult to believe that any background object could be visible through the density of stellar material present in this location near the center of the galaxy.” Yet hundreds (thousands?) of distant galaxies are visible in the “Zone of Avoidance” (i.e. the line of sight through our own galactic plane) – there’s even a thread in the Galaxy Zoo forum on this topic: In the recent Hubble image of the famous Horsehead Nebula, you can see lots of background galaxies, even though they’re been seen through a Giant Molecular Cloud (e.g. in this APOD: ). And many hundreds of background galaxies were identified by zooites in the Andromeda Project, being visible through the dense clouds of stars, gas and dust of M31’s disk, which is highly inclined from our POV. In fact, I collected some of these, and gave them unofficial names; e.g. Wright-1:

    • sbsims says:

      Congratulations! You are the first to notice, or at least to comment about my use of the Hubble Constant. The reason I use different values is to demonstrate that the Hubble Constant is not a constant at all. It varies depending on the scientist, see the definition page for examples.

      The Zone of Avoidance refers to the galactic plane of our galaxy, not the galactic center. As far as I know no background objects have been observed through the center of our galaxy or any other galaxies. This object, if it is even really located at the position listed, looks so close to the galactic center of NGC 772 it seems improbable it could be a background object. Also considering that the galaxy is so far away that individual stars cannot be discerned and appear blended together, how could a background object possibly be visible with current observations? As I stated in my post, better observations are needed to clarify the quasar’s existence and true location.

  6. Jean Tate says:

    Thanks, but I can’t possibly be the first reader of your site to have noticed. John Huchra created a great website on the Hubble constant (; sadly, since his death it seems to have not been updated. The chart/plot/graph at the bottom is perhaps the most informative: with few exceptions, the published values of H0 are within ~2σ of each other, strongly implying the differences in estimated value are more apparent than real. Since the first Planck results were published, this has become a rather more interesting topic!

    Not long after I began my own research into spiral galaxies I was astonished to discover that the amount of dust – and so extinction in the optical – in their bulges can vary enormously. Some bulges are, apparently, as transparent as elliptical galaxies (i.e. essentially no dust); others have extinctions of ~>4 mag, even in the I band! A good example is NGC 3314: while the shapes are complicated, you can see the background galaxy through the very center of the foreground galaxy’s bulge (it’s reddened, but still clearly visible).

    In any case, from your chart, “the NGC 772 QSO” seems to be towards the edge of the bulge, and so could easily be in a dust-free, inter-arm region. Oh, and “individual stars cannot be discerned and appear blended together” doesn’t stop light from background objects getting through, as any number of foreground spirals backlit by more distant ellipticals clearly shows.

    • sbsims says:

      I’m sorry but I do not see even the slightest hint of the background galaxy visible through the bright core of NGC 3314 in any of the images that I have closely examined.

      I agree that the QSO could be showing through a dust-free region of the inner spiral arm of NGC 772. But much better observations are needed to establish this fact, as I clearly explained in my post.

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