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View Full Version : Today's APOD (1-24-03) and Arp Seyfert sextet



John Kierein
2003-Jan-24, 01:02 PM
I think Arp disagrees that the face-on galaxy is background. He claims it's physically connected and has a partially intrinsic red shift. Sure looks to be surrounded with the same gas as the bottom 3 or 4.
http://antwrp.gsfc.nasa.gov/apod/astropix.html

JS Princeton
2003-Jan-24, 02:57 PM
How does Arp get away with simply ignoring spectral evidence? These galaxies may have different redshifts, but according to the SIMBAD search's spectra for the background and foreground objects there are z-absorbers between for the background spiral not present in the foreground galaxy. Looking at a picture is not conclusive evidence for a physical connection.

The Bad Astronomer
2003-Jan-24, 05:46 PM
On 2003-01-24 08:02, John Kierein wrote:
I think Arp disagrees that the face-on galaxy is background. He claims it's physically connected and has a partially intrinsic red shift. Sure looks to be surrounded with the same gas as the bottom 3 or 4.


This is precisely why interpreting imagesd can be difficult. Appearances can be deceiving. Think of it this way: two stars in an image may look like they overlap. Do they really, physically overlap? Certainly not. But the image of the star is bigger than the angular size of the star itself, so the images can appear to overlap.

The same is not only true for galaxies, but even more likely, since they are extended objects. This is part of the key to Arp's mistake. Galaxies are big, broad, diffuse objects, and an overlap in an image is inconclusive at best, and misleading at worst. You need the spectral data to make sure. As JSP pointed out (and assuming he is correct), the spectra he found shows that the face-on spiral is farther away. Spectra are worth far more than images in a case like this.

Lexx_Luthor
2003-Jan-24, 11:27 PM
What about the connections, if any? They may not be in this photo.

John Kierein
2003-Jan-25, 01:23 PM
On 2003-01-24 09:57, JS Princeton wrote:
How does Arp get away with simply ignoring spectral evidence? These galaxies may have different redshifts, but according to the SIMBAD search's spectra for the background and foreground objects there are z-absorbers between for the background spiral not present in the foreground galaxy. Looking at a picture is not conclusive evidence for a physical connection.

Arp's general arguments are that the discrepant has at least a partial intrinsic red shift unrelated to distance and that the connections show this. I suppose we should go to his papers to see. I just recognized this at the APOD as one of Arp's original connection measurements and maybe I'm thinking of the wrong object.

Yep, to be fair to Arp, I was confusing this with Stephan's quintet.
http://home.earthlink.net/~dascott2/NGC4319.html
Sorry about that.

<font size=-1>[ This Message was edited by: John Kierein on 2003-01-25 10:09 ]</font>