If I understand what you are saying, the current belief is "sort of" that spiral galaxes turn into ellipticals. If spirals are "born" at different times over the ages, wouldn't it be correct to say that spirals are at an "early" phase of the process cycle, and ellipticals at a "late" phase?

From what I understand, thousands of galaxies were examined by the present authors, and the authors of the references cited, to get to the M51 type catalog in ths paper. So I would assume that the intial sample size was large enough to be representative of the whole. So I would be interested in yours, or others, opinions on what would be the correct statistical treatment of this catalog regarding redshifts of the parents and companion galaxies.

TomT

I am not sure what you mean by that. Our purpose was to assemble the most complete catalog possible of apparently-interacting galaxies of the M51 type. You have made reference to the fact that we knew the redshifts of some of the first members of the catalog - you seem to forget that when I undertook that preliminary study in response to antoniseb's question, I had NO idea what I would find, nor (except for a few highly-publicized galaxy associations) did I have a clue what the redshifts of the members were. I selected the galaxy associations by appearance in optical bands ONLY, and when I had mined my initial sources, I looked up their redshifts in NED. It was then that I discovered that most of the companion galaxies had no published spectroscopy. Though there appeared statistically-significant trends in the sample, the sample was too small, so we had to expand it.

With Ari and Dave, the situation was similar. We refined the guidelines by which we would visually select M51 type associations, threw out some of the associations in the initial sample that did not meet the more stringent guidelines, and went to work. I stayed out of NED and HyperLeda and worked with DSS survey images. I would make an initial examination in IRSA using the "reproject" option and study each case in multiple bands (and often in multiple surveys - DSS1 and DSS2). This was slow going, as I could only process about 10 of these associations per hour (perhaps 12 if there were several unambiguous rejects in the list). I would categorize the associations and pass them along to Ari. He would review them and if he disagreed with my assessment or wanted another opinion on some of the associations, we sent them along to Dave (who was often busy, as he has said) and we traded emails until we had come to some sort of agreement about the classification of the objects. When the associations were added to the tables, Ari would then look up the relevant data and populate the data set. All this was designed to be as comprehensive and unbiased as possible. Ari kept a log of our emailed comments about the associations, and when our tables were complete, we went back and reviewed each case again in light of these comments, and looking at the DSS images and others.

About the trends - the trends that we found, including those regarding main galaxy morphology, number of arms, etc were not clear when we were building our tables. It's only after examining the tables that the trends became evident. That's the value of a large catalog compiled under well-defined guidelines. It is our hope that someone in both the Northern and Southern hemispheres will take spectra of the objects in this catalog.

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The ether of general relativity therefore differs from that of classical mechanics or the special theory of relativity respectively, in so far as it is not 'absolute', but is determined in its locally variable properties by ponderable matter.

Albert Einstein, "On the Ether", 1924

(Nit: Hubble tuning-fork diagram - "Hubble diagram" started out as a magnitude-redshift relation and is often co-opted into a distance-redshift relation...)

So whatever terms do you young whippersnappers use when finding it necessary to talk about (elliptical plus lenticular/S0 galaxies) versus (spirals plus irregulars) or compare galaxies at various stages along the tuning fork or various T values in de Vaucoulers' system? (I freely admit the inconvenience of having early-type galaxies comprised mostly of late-type stars and neither actually having anything to do with an evolutionary sequence).

Well, it's an odd situation. The paper is A Catalogue of M51 type Galaxy Associations. M51 and its companion are close to each other and are gravitationally interacting. In the past, researchers have -- and I would guess most researchers would -- throw out candidate pairs from such a catalog whose redshifts are vastly different, concluding that one galaxy is far in the background of the other and therefore they could not be interacting, so they're not really "M51-type". They only appear to be "associated" because they are both close to the same line of sight.

And I recognize that you authors of this paper are not exactly like "most researchers." Further, I don't criticize the inclusion of these disparate redshift pairs in your paper. Having more information is usually better than having less. But to include them as standard entries in your M51-type catalog is rather strange. Putting them in an appendix might have been more appropriate.

Having been a participant in the very lengthy Arp thread, I expect that each of you would have liked your results to have supported the contention that the disparate redshift pairs are actually close and interacting, and the grossly different redshifts are the result of some "intrinsic" effect that we don't know anything about. Unfortunately, as you found, there isn't enough data on these apparent companions, and like many papers, you point to this area where more data is needed. No complaints there!

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Everyone is entitled to his own opinion, but not his own facts.

You have missed the point entirely. We have compiled a catalog of optically-selected galaxy pairs that appear to be interacting, and we designed the process to produce the least amount of bias in the compilation. Afterward, we studied the tables, and discovered that our optically-selected samples had some pretty interesting morphological trends in Table 1, and the trends did not follow consistently to the associations in Table 2.

You may object to the inclusion of associations with discordant redshifts, but as Ari and Dave have explained, we have compelling reasons for doing so, not the least of which is the stunning incompleteness of the spectroscopy. Rejecting members based on some threshold redshift differential would result in an arbitrary bias in the sample, which we need to avoid. The morphological trends in our sample seem to support our decision not to split out candidates based on redshift, though you would know that if you have read the paper carefully.

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The ether of general relativity therefore differs from that of classical mechanics or the special theory of relativity respectively, in so far as it is not 'absolute', but is determined in its locally variable properties by ponderable matter.

Albert Einstein, "On the Ether", 1924

That argument of mine was not very well thought out. Of course your catalog is not a random collection. It has very specific rules for membership. So you compare all the members and see what you can figure out. This was obviously a major effort. And you got a publication out of it. Congratulations.

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Everyone is entitled to his own opinion, but not his own facts.

When you construct a sample, you need to apply a consistent set of selection criteria to the larger candidate sample from which you compile your final sample. For example, part of our selection criteria was that the companion would be 50% or less the size of the main galaxy. When we submitted the paper the referee suggested that we modify our selection criteria by calculating flux ratios from magnitude data. This was a great suggestion. However, we found that only a small fraction of the sample had the necessary magnitude data. Ari would probably know the exact percentage but it was certainly less than 30%. Therefore we could not apply the suggested criteria - so we stuck with the visual criteria described in the paper (see section 2).

As with the magnitude criteria, it would be inappropriate to eliminate the discordant redshift cases from the catalog when 82% of the companions don't have measured redshifts. That would be introducing a selection criterion that cannot be applied to the entire sample. We didn't apply the magnitude criteria for the same reason.

No Cougar, applying a selection criteria to only 18% of the sample would be strange. You don't apply selection criteria in such a careless fashion. The catalog is a classification based upon visual inspection of images. The criteria selected was consistent and appropriate for the available data.

If redshift data for the companions was complete (or even nearly complete) we could have created a table with M51's that have redshift differentials smaller than 1000 km s-1 (or whatever cutoff is most appropriate) and a second M51 sample with discordant redshifts. But you cannot start separating the sample that way when only 18% of the sample that made the visual inspection cut has the necessary redshift data. That would be ridiculous.

Your statements in this post do not show defensibly consistent logic. You say above that there isn't enough data. Yet you claim earlier in the post that the discordant redshift M51 galaxies are not interacting without providing a single shred of independent data to back that up. The redshifts themselves are not proof that the galaxies are not interacting. You need more evidence - evidence that establishes with reasonable certainty the M51 companions are not at the same distance.

Given the sparse nature of the data on most of these M51 companions, you lack that evidence needed to make your assertion that the discordant cases are chance alignments anything more than a re-statement of the standard view of redshifts that everyone understands is the standard view of redshifts.

And if comprehensive data is ever collected on these systems, actual tests that could be applied to the interaction scenario would be possible. For example, if the discordant cases are actually in the background, then the incident of enhanced star formation could be smaller in the discordant cases than in the non-discordant cases. What about direct distance estimates for both the main galaxy and the companion. Many of the companions are ellipticals which could at some point have their distances determined by Fundamental plane or Surface brightness fluctuation methods - when the resolution of telescopes allows the data to be collected. And what about the spectra - are there any peculiarities that contrast the discordant systems from the non-discordant systems?

We need more data to answer these questions. Prematurely ruling out the possibility that the discordant cases are interacting based upon zero data that actually allows one to rule out that possibility only serves to inhibit progress in our understanding of the universe.

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"The scientist who asks the right question reconnoiters a new patch of the unknown, and may, with luck, bring it within the constricted but expanding boundaries of the known."

~Timothy Ferris (The Red Limit) 1982

None of our reference catalogs did that, so in that sense there's nothing odd about it. What works on M51 systems are you referring to?

What you fail to notice is that this is not the paper we set out to do during the Arp thread. This is only a description of our sample, and has next to nothing to do with discordant redshift issues. We have only now begun to look at this sample from that angle for subsequent work.

And kindly stop guessing our motivations and what we would like or hope, it's quite insulting.

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"Stupidity gets denser in a crowd" - Old Finnish saying. [My website] [Nimblebrain forums]

Yes, that's true. There are quite a lot of DR cases both in rejected and possible piles.

Of our 232 Table 1 cases 40 had K-band data for both objects (which referee suggested to be used) = 17 %.

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"Stupidity gets denser in a crowd" - Old Finnish saying. [My website] [Nimblebrain forums]

Oy,

Missed this thread by a week! Sorry Skip, Ari and Dave, but congrats for this huge effort! Hopefully this is the beginning of a fruitful line of investigation in an interesting area of research.

Cheers.

Thanks for the good wishes. We hope that others can find uses for the data, too, and that they will help spur the demand for the missing spectroscopy.

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The ether of general relativity therefore differs from that of classical mechanics or the special theory of relativity respectively, in so far as it is not 'absolute', but is determined in its locally variable properties by ponderable matter.

Albert Einstein, "On the Ether", 1924

That's why I asked about the cases with extremely discordant redshifts, where the redshift difference "cannot be ignored."

But perhaps we'll get lucky - when the additional spectra trickle in - and find intermediate redshift absorption lines in the "companion" spectrum that are absent from the host's, confirming it as a background object.

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Everyone is entitled to his own opinion, but not his own facts.

Hopefully, the spectra will not "trickle in", but someone will take it upon themselves to fill these huge gaps. I hope that you will not be too offended if some of the companion galaxies that have apparently pulled tidal streams from (or triggered accelerated star formation in) their host galaxies have redshifts that place them impossibly distant from the hosts to be in interaction. You'll have a blast explaining how any secondary evidence for interaction can be safely ignored. It'll be fun!

"When you believe in something that you don't understand, then you suffer. Superstition ain't the way." My Man, Stevie.

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The ether of general relativity therefore differs from that of classical mechanics or the special theory of relativity respectively, in so far as it is not 'absolute', but is determined in its locally variable properties by ponderable matter.

Albert Einstein, "On the Ether", 1924

Certainly multiple lines of evidence will be needed to find a resolution to the matter. Absorption lines will be one of them. We're hoping to put together a list of promising candidates for follow-up study.

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"The scientist who asks the right question reconnoiters a new patch of the unknown, and may, with luck, bring it within the constricted but expanding boundaries of the known."

~Timothy Ferris (The Red Limit) 1982

To the three paper authors: one thing you might want to try if you are going to look at other objects is using Sky in Google Earth: it uses data from SDSS and DSS, and you can do a search by object name and it will take you straight to a color image of that object. Might be faster than using NED. Some of the KML plugins from that page include data overlays from other surveys. There are others floating around the web (and STScI is working on a Hubble archival data overlay, which will be great!).

First of all, how do you always manage to find my nits to pick? Yes, I realize you have a license, but still, it's creepy sometimes...

Second of all, I've heard things referred to as elliptical-type, spiral-type and irregular/other (I swear someone used "deformed" or the like for the last grouping) by us whippersnappers. Unfortunately, the best citation I can come up with at the moment is from the first galaxy zoo paper, which you're already familiar with, and which still uses the terms occasionally.

I fully admit that the terms will probably be around until after Hubble's great-great-great-great-etc. grandchildren (does he have any?) are long dead.

But, besides tradition, is there a compelling reason to keep the early/late terms? They are both confusing and misleading and mask a number of subtleties regarding galaxy evolution. I'm not much of a fan of outdated traditions. Guess I shouldn't get in an argument with Tevye...

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"What do you care what other people think?" -- Richard Feynman
"For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." -- Feynman, at the conclusion of his Challenger report