For example, Borchkhadze et al (1977) studied a sample of interacting galaxies that exhibited emission lines (another sign of interaction) and said the following about eso 234-49:

Quote:

On the ESO Schmidt plate these galaxies appeared to be physically connected and they were therefore included in the present programme. The reproduction (figure 1i) showsa diffuse object between them. However, the radial velocities differ with about 2200 km s-1 so that a connection is probably excluded; …

and in their conclusion:

Quote:

From the measured radial velocities, all systems in this paper, with the exception of one doubtful case (234-49), are confirmed as physical, close interacting systems.

So here we see that the observed redshifts are a mainstream criteria for objects to be interacting. Redshifts the same with a bridge means interaction whereas redshifts differing even with a bridge means no interaction.

From the Lopez-Sanchez et al paper on Mkn 1087 noted above there is this statement on page 6 of the preprint. Make sure to look at figure 1 to see what they are referring to:

Quote:

The HST image suggests that object #2 could be a background object that is coincident with the eastern bridge of Mkn 1087, although this cannot be confirmed without a spectrum of the object. However, this knot shows very blue colors (U-B = -0.78, similar to other knots around Mkn 1087, so we prefer to consider knot#2 in our study.

In this case we see that Lopez-sanchez et al believe that the presence of a bridge and the observed U-B color is enough to include object #2 as part of their analysis of this interacting system. However, the caution is that the spectra must be taken to determine the redshifts – and the implication is that if the redshift indicates “background” then the other evidence for interaction will be dismissed.

This leads to a very important example discussed by de Ruiter et al (1998).

The topic of this paper is the radio galaxy B2 1637+29 which they note has a twin bent jet and two radio tails about which they say:

Quote:

The structure suggests motion of the radio galaxy through an intergalactic medium, which causes the classical tailed radio structure, and dynamical interaction of the radio galaxy with a companion causing the oscillations in the radio tails. It appeared significant that the radio galaxy indeed has a close companion (separated by ~ 8 arcsec), clearly visible in the CCD image shown in De Ruiter et al (1988). However, spectroscopy revealed a surprisingly high difference in radial velocity (~4100 km s-1), which leaves us with two equally improbable alternatives: either the “dumbbell” is a chance superposition of two unrelated galaxies, in which case the radio structure remains unexplained, or B2 1637+29 represents an interacting galaxy system, but with an extremely high velocity difference.

Notice that a significant redshift differential alone is enough to force a mainstream researcher to abandon all other evidence for interaction. The 1998 de Ruiter paper examines this system in more detail than his earlier 1988 study finding that there seem to be two velocity groups present – but the conclusion that we’re seeing the superposition of two galaxy groups separated by a sizeable difference in distance is not so easily concluded:

Quote:

If we assume that A1 and A2 are at the distances indicated by their redshifts and therefore exclude physical interaction, the signs of interaction may be caused instead by A3, which indeed belongs to the same cluster as A1. A3 is much fainter (2-3 magnitudes), which makes it hard to understand why such a rather common interaction should have produced a strong flux asymmetry in the lobes and such conspicuously oscillating tails.

In fact this system was included in the Colina & de Juan paper cited at the beginning. It meets 3 of the 4 criteria for interaction and thus is considered an interacting system by them.

This is of course the crux of the problem for the mainstream and for Arp. In the context of apparently interacting systems with large redshift differentials, normally sound evidence for interaction – tidal tails, bridges, colors, star formation bursts and others – must either be accepted at the expense of the Hubble relation, or that evidence must be rejected at the expense of everything currently understood about galaxy interactions.

I think this last example above with the B2 1637+29 system is particularly profound because the authors clearly note that they are faced with this uncomfortable decision. Apparently not one other mainstream researcher is interested in discussing this difficult situation because the de Ruiter paper has only been cited twice – both times by Lopez-Corredoira & Gutierrez.

To this point I’ve not even touched upon any papers written by Arp et al. I’ve decided I’m going to refer to two papers for now because of the extreme length of this post. In this paper Lopez-Corredoira and Guitierrez expand upon their earlier observations of the NGC 7603 system. The system has the bridge apparently connecting NGC 7603 to NGC 7603B of course. In the context of what I’ve linked to and discussed above, it should be clear that in the absence of a large redshift differential, this bridge and the close proximity of the galaxies in question would be taken as evidence for interaction. So the only reason the mainstream disputes the interaction at this point is the ~8000 km s-1 difference in the redshifts of the two objects. In fact I did not come across a single instance where bridges and tails in the presence of small redshift differences were not interpreted in the context of some form of interaction.

Now in this newer NGC 7603 paper they obtained better spectra and confirmed the classification of the z=0.24 and z=0.39 objects as HII galaxies. The Mendez, Esteban, & Balcells paper linked to earlier say this about HII galaxies in the introduction of their paper:

Quote:

HII galaxies are characterized by a high star formation rate (SFR) and the presence of a large population of massive stars that ionize a significant part of the surrounding interstellar gas, resulting in emission-line spectrum similar to that of typical HII regions in the Galaxy and Local Group galaxies.

The point of course is that we don’t just have any two random galaxies in the bridge between NGC 7603 and NGC 7603B, - we have two HII galaxies – galaxies that are forming stars at a high rate. Star formation bursts are another form of evidence for interaction and the fact that the z=0.24 and z=0.39 objects are of that type strengthens the interaction scenario - despite the redshift differential. In fact if the objects were at the same redshift as NGC 7603 they would be evaluated as potential tidal dwarf galaxies.

Lopez-Corredoira&Guitierrez strengthen the case for interaction by noting the intense H-alpha equivalent widths. They note that if at their redshift distances these two galaxies are very luminous and the observed H-alpha widths don’t fit the expected relation between luminosity and H-alpha linewidths. But at the NGC 7603 distance they would be dwarf HII galaxies and the H-alpha widths would fit what is expected for dwarf HII galaxies.

So we have a system for which a number of lines of evidence are brought together that strongly suggest interaction – yet there is the large redshift differential. It should be noted that in the NEQ3 system there is an HII galaxy again and NGC 1232B is another HII galaxy. These high z companions around low redshift galaxies that are “Arpian” are not just normal background galaxies.

Edited to Add: In the NGC 7603 preprint you must click on the image links to see some of the images. If you do that you will see that the z=0.39 HII galaxy is strongly elongated back toward NGC 7603. This sort of aligned elongation is another form of evidence in interaction scenarios.

There are quite a few Arp papers that I will eventually point to with additional examples of discordant redshift companions. But it was important in this post to outline the mainstream approach to identifying interaction in galaxies. The length of this post was necessary to illustrate that bridges are in fact strong evidence for interaction and the tough decision the mainstream is faced with when they must choose between sticking to the prevailing view of a tight Hubble relationship or accepting the evidence for interaction that is so readily accepted when redshift differentials are small.


I have invested considerable time in preparing this post and considerable time this morning typing it all up, so I hope that the points and papers linked to will be given careful, thoughtful consideration.

__________________
"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