The most distinctive feature of this object is that it is not just one object but in fact two interacting objects. Each is a galaxy unto itself with a specific hubble classification.

There isn't a visually based morphological type for interacting galaxies as the orientations, sizes, shapes and types of galaxies interacting can be so varied. There is however a timescale that can be associated with interactions and a merger sequence once the galaxies are close enough that their interaction will result in a merger. Another factor to consider is the size ratio between the two galaxies involved. As the smaller galaxy (UGC01813) is just that, smaller, this is known as a minor merger. The most common interaction of this type is M51 or the Whirlpool galaxy. The smaller companion galaxy in Arp 273 can be found at the end of a tidal bridge like the small companion in M51 and so this interaction is known as an "M51 type" interaction. (Dahari, O. The Astrophysical Journal supplement series, 57:643-664, 1985 April)

Arp 273 image from the Arp Catalogue	M51 taken on the Palomar 48-inch Schmidt Telescope at 6450Angstroms (Band=103aE)

My objects seem to be at the initial stages of interaction; they are showing visible signs of interacting but are not so distorted that their original form has been completely lost. According to a really nice mpeg movie done by Kathryn V. Johnston of Wesleyan University, Arp 273 seems to be in the first quarter of the interaction sequence. (This only shows the effects of interaction on the smaller galaxy. The effects on the larger will be discussed below)

• Click Here to watch the mpeg movie (not the full interaction, just the first third or so).

To see the whole sequence or, if for whatever reason, the movie can't be run there is an image of the movie broken down into selected frames Here. Arp 273 is just about at the same stage as the third of the selected frames.

The reaction of a larger galaxy to a merging smaller companion is nicely seen in the following image from Chris Mihos' page on "Minor Mergers and Satellite Accretion"

The overall mass of the larger galaxy is distorted into a spiral in the third image in the sequence. I would guess the larger companion (UGC01810) currently is between the third and fourth images, with the extension of the one arm caused by the spiral structure of the overall mass. There is a tidal tail seen in the above image that isn't seen in images of Arp 273. This will have to be explained through other research, which has been started in one of the papers I talk about in the history of Arp 273 called "Sloshing in High-Speed Galaxy Interactions" by Thomas Zeltwager (see the history page for more detail).

UGC01810

has a Mophological Classification: SA(s)b pec (SA(s)b pec is interpreted as follows: S=Spiral A=Not barred (s)=The arms have a 's' like shape b=Moderately wound arms(a less, c more) pec=The fact that it is 'peculiar') From the Arp Catalogue Explanation: the center looks more like an SA(s)b type galaxy but the extended arms look more like the SAB(s)c type galaxy. The Arp Catalogue image shows the SA(s)b features in the center much better than they Digital Sky Survey image which made me question why UGC01810 wasn't given the SAB(s)c designation. The long spiral arm is possibly caused or at least amplified by the interaction and so it seems people thought the center gave more of an indication of it's original morphological type (before interaction) image from Skyview Digital Sky Survey

Other Galaxies with the same classification are NGC 3223: NGC 3673 A Galaxy which looks similar but has a different classification: NGC 4535 [SAB(s)c]:

UGC01813

has a Morphological Classification: SB(s)a pec (SA(s)b pec is interpreted as follows: S=Spiral B=Barred (s)=The arms have a 's' like shape a=Tightly wound arms pec=The fact that it is 'peculiar') From the Arp Catalogue Explanation: These comparison images show what UGC01813 might look like if seen face on. I am still trying to find an image of an SB(s)a type galaxy edge on. If you are reading this and know of any please e-mail me

Other Galaxies with the same classification are NGC 1433: NGC 5850

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