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CN Report: Observing the Arp Peculiar Galaxies

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Observing the Arp Peculiar Galaxies by Alvin H. Huey
Tom Trusock 5/07

Mastered the Messiers? Hit all the Herschels? Wondering what to observe next?

Well, if you've got some decent aperture you might consider investigating the weird and wacky world of Dr. Halton Arp. Several Arp's can be seen in 8-10 inch telescopes but, like always, aperture rules.

The Atlas of Peculiar Galaxies was initially published by the California Institute of Technology using data and observations largely collected through observations at Mount Wilson and Mount Palomar Observatories. Dr. Arp observed several galaxies which did not fall under the Hubble Classification scheme, and thus began the catalog as to ensure a proper appreciation of these oddball galaxies and build a realistic picture of what galaxies are really like. Initially dismissed as statistically insignificant further southern hemisphere observations (not in this catalog) showed a remarkable 8% of the galaxies observed could in some sense, be classed as peculiar.

Dr. Arp's initial catalog can be found online here:
  • Atlas of Peculiar Galaxies - Halton Arp

Ironically, what began as an attempt to catalog or classify abnormal or peculiar galaxies resulted in something that challenged the very foundations of modern cosmology - Hubble's law. Hubble's Law was formulated by Hubble and Humason in 1929, and states that the redshift in light coming from distant galaxies is proportional to the distance of those galaxies. The further an object is, the higher it's recessional velocity, and thus the greater it's redshift. Mathematically, Hubble's law is fairly straight forward, and for simple cases can be expressed as


where v is the recessional velocity, D is the distance from the object to the observer, and Ho is the Hubble constant. Hubble's Law is crucial to the basis of the expanding universe theory, and thus is a critical piece of observational evidence for the Big Bang.

Shortly after publication, Arp found that galaxies between 100 and 150 in his catalog (elliptical galaxies associated with disturbed spirals or disturbed material) seemed to have some increased relation with 3C (3rd Cambridge survey) radio sources - which were newly (at the time) discovered high redshift Quasars. Since the redshift is generally taken as an indicator of distance, and as Quasars are supposedly beacons from the early universe and as such lie far from us, their apparent physical association with bright, relatively nearby galaxies was, and still is disconcerting to many Arp's findings were not to go unchallenged. Arp felt that redshift may not be solely explained through distance, and there may be some other mechanism at work. Thus we see the introduction of the term discordant redshift into cosmology.

From private correspondance with Prof. Roberto Abraham,  Dept of Astronomy & Astrophysics, University of Toronto - 5/29/2007:

Just as an aside: arguably the main significance of the Arp atlas these days, from the point of view of people studying galaxy formation, is that it provides a convenient set of "benchmark" local peculiar galaxies to compare against distant objects seen in deep HST images. A large fraction of  galaxies in HST images (upwards of 30% at modestly deep magnitudes, say I=24 mag, with the fraction increasing sharply as one goes fainter) don't find a natural home in classification schemes that work for local galaxies. At this point we don't know why so many faint galaxies on HST images are morphologically peculiar. Undoubtedly many look strange because they are mergers or at least being tidally distorted by interactions (the case for many objects in the Arp atlas), because the merger rate is rising rapidly with redshift (which is to be expected as the Universe is getting smaller as we look further out and the galaxies are more crammed together).

On the other hand, a fair fraction of morphologically distorted galaxies in HST images don't seem to resemble anything in the Arp atlas, and just what these are (and what they are
evolving into) is not clear at the moment. We should learn a lot more in the next few years as adaptive optics systems with spatially-resolved spectroscopic capability come on-line. These will let us map out the internal motions in these systems and figure out how fast (or even whether) they're rotating, and to say something about their dark matter content. Some (many? most?)  may be collapsing protogalactic clumps, and we should be able to tell if that's the case from their internal motions too. And, of course, when JWST launches, we'll learn a ton more about them. So the next few years should be interesting ones.

--Prof. Roberto Abraham
But for the visual observer, even with the astrophysics of particular members aside, the catalog is fascinating to browse through. If you're used to picturesque ellipticals and symmetrical spirals, then your first view of these Arp's is not unlike visiting a circus freak show; three armed spirals, one armed spirals, colliding galaxies, diffuse counter tails, jets, repelling arms, companion galaxies on the arms of spirals, windblown double galaxies, galaxy chains, and many many other oddball structures. They defy tradition. If you're into the odd or different, then Halton Arp's catalog is just plain cool. And frankly, some of what you find might well surprise you.

A newcomer to the Arp Catalog might be a bit shocked to realize that Arp 85 is M51/NGC 5195. You'll find a few other old favorites in here, as well as some truly odd beasties of which you may have never seen the like.

The Arp catalog is setup along 4 main areas: Spiral Galaxies, Galaxies, Elliptical and Elliptical-Like Galaxies and Double Galaxies, with the (fairly self explanatory) sub-areas shown in the charts below.

(Above images/charts from - Atlas of Peculiar Galaxies, Halton Arp - http://ned.ipac.caltech.edu/level5/Arp/frames.html)

For the visual observer, the Arp Catalog presents a number of interesting challenges. First and foremost, for many will simply be detection of the various objects. The second challenge lies in visually spotting what makes the galaxy peculiar (the jet in Arp 152 / M 87 for example).

As an observing list, the Arp catalog is definitely slanted towards the experienced observer with a large aperture telescope, but there are several good targets for with smaller telescopes. In the table below, I've listed the 21 Arp's under magnitude 11 which can easily be seen in moderately sized telescopes. For comparison, there are 85 brighter than mag 13, and 242 brighter than magnitude 15. (The data set used is from Sky Tools 2.)

Arp Number Other ID Con Mag SBr
153 Centaurus A Cen 7.7 23.3
26 M 101 UMa 8.3 24.4
168 M 32 And 8.8 21.9
85 M 51/Whirlpool CVn 8.9 22.1
337 M 82 UMa 9.2 22
134 M 49 Vir 9.3 22.7
37 M 77 Cet 9.5 22.4
152 M 87 Vir 9.5 22.3
16 M 66 Leo 9.6 22.1
281 NGC 4631 CVn 9.7 22.3
29 NGC 6946 Cyg 9.7 23.5
154 Fornax A radio source For 9.8 23.3
116 M 60 Vir 9.8 22.5
135 NGC 1023 Per 10.2 22.4
77 NGC 1097 For 10.2 23.3
317 M 65 Leo 10.2 22.1
76 M 90 Vir 10.2 23.1
269 NGC 4490 CVn 10.6 22.3
41 NGC 1232 Eri 10.7 23.4
244 NGC 4038/Antennae/Ring Tail Crv 10.9 22.9

Tables like this and finder charts can be generated from most observational planning programs: DeepSky, SkyTools and AstroPlanner are just three examples. But tables and charts can only take you so far – it really helps to have the benefit of an experienced observer to help you identify what you're looking for. Enter Alvin Huey's - Observing the Arp Peculiar Galaxies.

The guide is spiral bound to lie flat on the observing table. The cover is a hard, laminated plastic, obviously waterproof, and the pages are nice and thick so you won't need to worry overmuch about dew. At 10" by 11.5" and just over 1" thick, the guide is large but manageable.

Open Observing the Arp Peculiar Galaxies and you'll immediately find some preliminary material on the origin and setup of the catalog along with the standard bits concerning observing sites and equipment. After this you'll reach the meat; nearly 400 pages devoted to the 338 galaxies in the Arp catalog.

As a good observing guide should be, Observing the Arp Peculiar Galaxies is organized by constellation. It begins in western Pegasus, and proceeds around the sky until you arrive back in Centaurus. Each section is preceded by a regional, naked-eye finder chart where the Arps for that particular area of sky are marked by two rings that represent the inner rings of the Telrad or the two rings of the Rigel Quickfinder. Further on this page, Mr. Huey provides a table listing the Arp number, other ID, coordinates, magnitude, size, specific constellation and Uranometria 2000 2nd edition page number. The charts work well for quickly narrowing down specific sections of the sky, and with the brighter Arp's are all that are really needed. For the dimmer galaxies, one may want an additional resource (such as U2k) to help narrow the field before making use of Mr. Huey's excellent eyepiece finder charts.

Turn the page, and you'll find detailed notes for that particular section of the sky. Typically there is one Arp to a page, and on that page you'll find; the Arp number, it's classification, an eyepiece finder chart, a white-on-black image reversed DSS image with all components, a listing of nearby galaxies, observing notes (the majority appear to be done with a 22" f/4.1) on all items of interest in the field, an eyepiece sketch, and notes on the aperture, magnification, field size, seeing, and conditions for the observations.

If you've seen Mr. Huey's other observing guides (The Hickson Group Observing Guide, The Abell Planetary Observers Guide), you'll spot the absence of an observers "note page" where we were provided a spot to include our observations in the volume itself. When I spoke with Mr. Huey about this he indicated that feedback showed few observers used it, and given the sheer size of the Arp guide he felt removing it would help keep the size of Observing the Arp Peculiar Galaxies a bit more manageable. In effect it cut the size down to one volume from what would have been two.

About the only nit one could pick is the presence of a few typos and minor grammatical errors scattered throughout the volume. But this is the type of thing that you run into in nearly any self-publication (I know these pitfalls all too well ). There's nothing serious, and certainly nothing to detract from the usefulness of the book.

All in all, I found this to be another excellent work from Alvin Huey. Observing the Arp Peculiar Galaxies is a resource that belongs on every serious deep sky observers bookshelf, and is a must have for observers looking to add Dr. Halton Arp's trophies to their collection.

Available From:
Alvin H. Huey

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