I’ve looked at numerous papers relevant to the topic of galaxy interaction and bridges. Since some of Arp’s examples with discordant redshifts have optical bridges between the apparently interacting galaxies, my goal was to examine how important bridges are in establishing interaction between galaxies from the mainstream point of view. I also took a look at other forms of evidence that are examined in interaction cases. I’m not sure how much of what I read should be included in this post, but this will be long and so with no further prologue …

A very important paper is the 1972 paper of Toomre&Toomre . This is a heavily cited paper that was very important in establishing that gravitational interactions could cause bridges and tails. Prior to this paper it was believed that many of the tidal features accepted today could not be caused by tidal interaction. I won’t say any more about this paper now, but there are aspects of it that may become relevant as people respond to the points I will make in this post.

Before considering specific examples of interaction as treated in mainstream literature, I think it is important to discuss the criteria typically used for identifying interacting galaxies. Johansson&Bergvall 1990 identified galaxies as interacting if they met just one of the following criteria:

1. Both components have a disorted morphology; an angular separation < 6x the diameter of the larger galaxy and a difference of less than 2 magnitudes in brightness.
2. At least one component has distorted morphology, the angular separation is less than twice the diameter of the largest and the magnitude difference is less than 3 mags.
3. “The components are physically connected via a well-defined bridge.”
4. Single distorted galaxies (companion not evident) if the diameter of the galaxy was > 1.0 arc min.

Colina & de Juan (1995) identified the following criteria for interaction for FR I radio source elliptical host galaxies:

1. Twists in isophotes which is a change in the position of the major axis as a function of radius. Isophotes may be thought of as contour map lines for light intensity.
2. Non-concentric isophotes which they state are “among the strongest evidence of interaction between members of a pair of group of galaxies.”
3. Departure of the elliptical host from the de Vaucouleurs r1/4 luminosity profile. Specifically the interaction evidence is an excess of luminosity above the r1/4 law.
4. Presence of close companions.

Finally they defined a galaxy as interacting if it met any 2 of these 4 criteria.

Other factors that are often identified as evidence for interaction:

(1) Presence of recent or ongoing bursts of star formation. This may be identified by optical B-V or U-B colors – specifically small values for B-V or U-B are indicative of “blue” colors resulting from the recent burst of star formation.
(2) Spectral features such as the presence of emission lines and/or early stellar type absorption spectra. Early type stellar spectra are related to the presence of large numbers of early type (O and B class stars) which have short lifetimes but give characteristic blue colors to the early stages of bursts of star formation.
(3) The starbursts may also be characterized by high surface brightness.

There are other lines of evidence not mentioned here, but the major forms seem to be among those listed above. What must be kept in mind is that as you read the literature it becomes apparent that any given interaction may only show a fraction of the various forms of evidence. And finally, it is very clear that in the mainstream view a difference in the observed redshifts of the interacting companions must be less than 1000 km s-1 for researchers to accept them as definitely interacting companions – even in the face of other forms of evidence supporting interaction.

Before I get into specific cases and the role of bridges in interaction studies, it is important to realize that any given study may examine a limited scope of the possible forms of evidence for interaction. For example, with regard to the bridges the study may typically look at optical images, neutral hydrogen from radio maps, or X-ray data – but it is not common that a paper will examine all three. And papers that examine those forms of data, may or may not comment on blue colors, spectral features and the other forms of evidence.

What I wish to do now is point to specific papers and select passages which illustrate the reasoning used to establish interaction. I will provide links to all papers I reference so that anybody that wishes can check that I am using the quotes from the papers in proper context. The main focus will be on bridges.

I’ll begin with a general statement made by van der Hulst & Hummel who report the discovery of a radio continuum bridge between a pair of interacting galaxies:

Quote:

Optically and also in neutral hydrogen the most dramatic sign of gravitational interaction is the occurrence of bridges and tails (refs).

The importance of this general statement in their introduction is simply that it illustrates that bridges and tails are taken as dramatic evidence for interaction. Other papers confirm that bridges are taken as evidence for interaction. In fact, as I was reading numerous papers I became aware of a pattern of using the word “clearly” in the context of discussing these bridges.

From Mendez, Esteban, & Balcells (1999)

Quote:

The presence of a sharp and very straight bridge (~65” = 14.7 kpc) connecting both galaxies is clear.

From their conclusion:

Quote:

The interaction caused the formation of a bridge of stellar nature connecting the two galaxies, …

From Irwin (1994)

Quote:

Although no strong optical distortions or tidal tails are evident, we have detected a clear interaction in the form of two, well-defined HI bridges connecting the galaxies, and we suggest that the NGC 5775/NGC 5774 system may be in the early stages of a merger.

It is important to note that in this case the bridges are taken as evidence of interaction even in the absence of any other optical evidence. Bridges are important evidence for interaction.

From Kantharia et al

Quote:

The low resolution image at 330 MHz clearly showing the bridge and the high resolution image at 1280 MHz showing fine structure in the galaxies are shown in Fig 1.

Kantharia et al also note the effect of interaction on star formation:

Quote:

The H-alpha image of NGC 2814 (see Fig 7c) shows enhanced star formation in a small tail which is likely triggered by the tidal interaction. Star formation, triggered by the tidal interaction is also observed in and close to Mrk 108.

Boyce et al examined the M-81 group in neutral hydrogen making the following statements about their maps:

Quote:

The tidal bridge of gas between M81 and M82 (refs) can clearly be seen in the HIJASS data, …

From Bergvall (1981) :

Quote:

At least one of these companions 14 kpc to the south-east is connected to NGC 4441 via a bridge. This is clearly seen in Fig. 2.

From Lopez-Sanchez, Esteban, & Rodriquez

Quote:

In order to obtain a deeper image, we carried out new observations of the system in R broad-band filter. We present this image in Figure 1. The bridges commented before are now clearly detected, especially the ones that end at #11 and #12.

Quote:

Some important structures inside the main galaxies are now clearly distinguished. … Mkn 1087 reveals two spiral arms or structures: an upper arm connecting with the bridge towards knot #2 and a bottom arm that seems to end in knot #7.

They also note the presence of star formation as a result of the interaction:

Quote:

Knot #15 located at the beginning of the long tail to object #1, shows blue colors, (U-B) ~-0.6, indicating star formation activity. The images (see Figures 1 and 3) reveal small knots along this tail, that decrease in number in the direction to #1. They seem to be tiny star-forming regions too, because very weak H-alpha emission is detected in this zone of the WHT spectrum. …

Bridges are also observed in X-rays such as that reported by Colina & Borne (1995)

Quote:

A high surface brightness bridge is seen in the ROSAT image, with a luminosity Lx(0.1-2.4keV) = 1 x 10^40 ers s-1 (labeled as “Bridge” in Table 1).

About these features they conclude:

Quote:

All of these features are believed to be induced by the hydrodynamical tidal forces acting in the strong close collision between NGC 4782 and NGC 4783 and are, most likely, believed to be a general characteristic of strongly interacting ellipticals.

The above passages clearly establish that - from the mainstream point of view - bridges are a strong form of evidence for interaction between galaxies. In fact, if a system has close proximity, disturbed morphology or bridges and bursts of star formation for components of the system – then you have a very strong case for interaction. What I must now turn to is the important role that in the mainstream view – the observed redshifts play in establishing interaction. In fact, it is largely the case that all the forms of evidence for interaction (discussed above and not discussed) will be tossed out if the redshifts are significantly different.

Continued on next post to keep it within the length limit.

__________________
"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