Click to Enlarge The Arp Atlas of Peculiar Galaxies – A Chronicle and Observer’s Guide

I had been curious about this volume for quite some time and was delighted to win it with some other treasures in a door prize at Border Stargaze in 2008. I have now had a year to explore it as an observer’s guide and thought it worth writing a review.

As the sub-title indicates, this is both an atlas/observer’s guide and the story of the guide and its author. So to me it has two merits : i) a very interesting story of science, discovery, observation, dissent and its consequences and ii) a guide for some interesting and at times challenging observing.

This handsomely presented 384 page volume starts with a concise background to the various systems of classifying galactic morphology and some of the interesting challenges of interpreting what’s going on when galaxies don’t conform to these systems, when they look peculiar. A brief overview of Halton Arp’s Atlas of Peculiar Galaxies follows, with monochrome negative plates of all 338 Arp galaxies, showing the various classes of peculiarity that Arp noted. To me, although these sections are well written, lucid and necessary, they would more logically come after the next section “The Peculiar Universe of Halton Arp”.

“The Peculiar Universe of Halton Arp” starts with the story of Edwin Hubble’s discovery that “spiral nebulae” are not, as previously thought part of the Milky Way, but separate galaxies of enormous scale at vast distances. It traces Arps’ entry into these heady times of concepts of an expanding universe and expanding concepts of the universe, his association with Bok, Hubble, Sandage and his residency at Mt Wilson observatory and his research with its 200” telescope.

The plot thickens with Arp’s cataloguing of peculiar galaxies and his assessment that they have an unusual relationship with quasars, his ponderings about discordant red shifts and his conclusions which are greatly at odds with currently accepted notions of our universe. The extraordinary implications of Arp’s thesis for the nature of matter are explored and his resulting painful estrangement from mainstream astronomy is thoughtfully charted. I will not spoil the story by going into details.

I found this section of the book to be absolutely fascinating. The telling of the tale is well supported by the authors’ even handed, careful and thoughtful approach to an issue that has clearly been very divisive and difficult. They appear to have researched this very well and spent considerable time interviewing a number of protagonists. They write with an empathy for both sides of the dispute and they have not tried to simplify the difficult question of allocation of scarce resources to research that does not have the backing of mainstream science.

A wide divergence of scientific ideas with one model having considerably more support than its rival does not necessarily call for even-handedness, but does require an openness to considering evidence and theoretical integrity of both. In the astronomical world, there is much theory, modelling and congruent observations to support the mainstream notions of red shift, quasars and fundamental particle physics. There seems much less to support Arp’s conclusions (not that this means they are incorrect). The book does not present in proportion the weight of evidence for both arguments, but as it does not attempt to conclude one way or another, but really presents the story of Arp’s conclusions and the scientific community’s reaction to them, this seems to me to be quite appropriate. I very much enjoyed the respect with which these chapters are written.

As a very good read about some important astronomical history, to me the “book about the book” is of itself worth the investment.

And now to the atlas and observing guide…

In the observer’s guide and atlas, I again found much to enjoy. The authors have included a well-crafted outline of how to get the most out of observing some often faint galaxies and subtle features thereof. It must be borne in mind that Arp based his atlas on photographic plates taken with a 200” telescope in the Northern Hemisphere. Observing these galaxies with an amateur telescope in the Southern Hemisphere does impose some limitations. The authors have general notes on how to observe Arps, the benefits of sketching the galaxies and also on imaging them.

The introduction to the Observer’s Guide includes tables on distribution of Arps by observable declination, where bright Arps are found, fields of brighter Arps (NGC objects, mainly) and Arps suitable for 6-8” telescopes (of which 11/19 fields are observable from southern climes – 17 if you can get a good look at Canis Venatici). It also includes helpful and clear suggestions about how to use the guide.

My rough census of the Arps with declinations south of +40 shows 17 with magnitudes brighter than or equal to 10, 130 between magnitudes 11 and 14 inclusive and 113 with magnitudes dimmer than 14. I include these figures as they will clearly influence the extent to which you might use the observing guide.  A large telescope and dark skies clearly put a lot more Arps within range.

The observers guide has 26 key charts showing the location of each Arp. It is worth noting that “an Arp” may be more than one galaxy, as they often include interacting galaxies and chains of galaxies. I have found these charts to be very useful. Many of the Arp galaxies are not included in my Sky Atlas 2000 and I have depended on the observer’s guide to find them and have found this to be quite straightforward in most cases. Next to each key chart is a list of the galaxies for the chart, their non-Arp designations, dimensions, magnitude, RA/Dec, some of Arp’s notes and suggested highlights, like bright Arps or groups. The authors also list characteristics of the charts including latitudes from which the chart will be visible and the midnight culmination of the charts. Very useful!

Click to Enlarge Figure 1: Chart X

Following each chart are black and white negative plates of each Arp accompanied by visual observation notes, some of Arp’s notes and suggested observing challenges.

Click to Enlarge Figure 2: Plates for Arps 22 and 34

Do I have any gripes about the observer’s guide? Not a one. This is a very well put together work. I do think that I would get quite frustrated with this if I had a small scope or light polluted skies, as so many of the galaxies are faint. If this were my circumstance, I would find the main benefit of the book to be the narrative about the Atlas itself. But if your scope will show a magnitude 11 galaxy reasonably and you can get to dark skies, there is a plethora of observing in this book.

My experience of this book is as a strictly visual observer and most of my comments so far are from this perspective. For those of an imaging bent and with an appropriate set up, the observer’s guide offers a wonderful list of interesting targets and challenges. Enough to keep you busy for quite a long time.

I don’t think the Arps would be a great start for an absolute beginner, although I started my Arp observing only 2 years into my astronomy obsession. Observing Arp galaxies has had considerable benefit in training me to observe more closely and I look forward to seeing more in each Arp as my observing skills progress. However, enough knowledge to successfully star hop is necessary, unless a go-to scope is used, and it is worth having at least a little experience observing DSOs, galaxies in particular.

In summary, I can highly recommend this well written and well-presented book as both a fascinating story and observer’s guide. I would suggest that it is more useful as an observer’s guide for those with a larger telescope and access to dark skies. Whilst experienced observers will see more in each Arp, as a relative newcomer I have found it a valuable aid to enhancing my observing skills.

Review by Patrick Kavanagh (Paddy). Discuss this Review on the IceInSpace Forum.