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RickJ
2011-Dec-02, 03:22 AM
Arp 74/UGC 01626 is a pair of galaxies not far from Gamma Andromeda. Arp put it in his class for spirals with high surface brightness companions on their arm. Red shift puts it about 240 thousands of light-years away. There is no redshift data that I could find for its companion Arp 074B. A note at NED reads: "Companion 0.75, 169, 0.20 x 0.15, interaction. The arms appear to join close to the companion." This dates back to a 1973 paper. I don't see the interaction they mention. The fact the two arms happen to appear to meet near the position of the other galaxy seems immaterial as to decide if they are interacting or not. Arp 074B doesn't appear distorted. If interacting the smaller galaxy should show more distortion than the larger yet shows no sign of any distortion.

Arp 74 is classed by NED as SAB(rs)c. While the arms are obviously very different they don't earn it a peculiar designation. Unequal arms are rather common in fact. No interaction appears necessary either for the shape nor the high star formation rate these often have. Arp 074 is classed as IrS. Arp's comment on this one reads: "Broad, diffuse extension of arm leads to companion." Rather different from NED's note the arms appearing to join at the companion. Is Arp hinting that the arm leads to the other because they are related? I think he considered it a possibility that needed further investigation. That still hasn't happened it seems. Most likely Arp 74B is a distant background galaxy. The main galaxy appears to have little to no dust in its outer arms to dim a distant galaxy making it impossible to tell if it lies behind or not.

There is little on the rest of the field. There is a spiral at the bottom of my frame that seems as distorted as Arp 74 but didn't make his list. It is UGC 1615. It is classed as SB(s)dm: at NED. There's no distance estimate for it or any other galaxy in the field.

Arp's image
http://ned.ipac.caltech.edu/level5/Arp/Figures/big_arp74.jpeg

14" LX200R @ f/10, L=4x10' RGB=2x10'x3, STL-11000XM, Paramount ME

Rick

Middenrat
2011-Dec-02, 04:09 AM
Not uncommon, such distortion, as you say Rick, apparently requiring no interaction. What then? The smooth gradient suggests a constant acceleration. That would be quite some mechanism.
I love the way you present these mysteries, thanks for another.

Cougar
2011-Dec-02, 06:03 PM
Most likely Arp 74B is a distant background galaxy.

I am with you on this, Rick. As you are likely aware, Arp was on a crusade that has since been relegated to the dustbin of history. His idea needed galaxies and quasars that appeared on the sky to be in close proximity to actually be close to each other and interacting. Well, some were and some weren't. As you say, "The fact the two arms happen to appear to meet near the position of the other galaxy seems immaterial as to decide if they are interacting or not." Arp and his followers seemed to put too much value on such appearance, as opposed to more reliable confirming observations.

Too bad there's no redshift data for Arp 74B....

RickJ
2011-Dec-03, 03:04 AM
Not uncommon, such distortion, as you say Rick, apparently requiring no interaction. What then? The smooth gradient suggests a constant acceleration. That would be quite some mechanism.
I love the way you present these mysteries, thanks for another.

Many galaxies have very unequal arm structure similar to Arp 74 that have no sign of any possible cause. We now think spirals are due to the merger of many small galaxies. We can't reconstruct what all these did but it appears how they merged has something to do with their current shape. Also we see this only in 2D. The two arms may be warped and really come no where near each other. The galaxy appears mostly dust free so little chance of seeing absorption features that might indicate one is in front of the other.

In any case getting redshift data on the "companion" would go a long way to answering this question.

Rick