Starship Asterisk*

Chris Peterson wrote:

Anthony Barreiro wrote:Who's NED?

Meet NED.

Anthony Barreiro wrote:Who's NED?

Chris Heinz wrote:Is this maybe a foreground galaxy of the Coma Cluster? Coma Cluster per NED is at ~107 Mpc, NGC 4921 is at ~77 Mpc. In Google Sky, it looks almost as big as the 2 central cD galaxies of the cluster. These are normally much larger than any other cluster galaxy, having been gobbling up smaller galaxies. It also shows (in Google Sky) more detail than the central galaxies of the cluster. it looks like it's 0.37 degrees from the cluster center, which at 1.7 Mpc/degree says 0.63 Mpc, way different from the 30 Mpc difference per red shift.

So foreground? Or proper motion? Or maybe the cluster just extends towards us in the direction of NGC 4921. Nah, too much

Hmmm, NGC 4911, just to the southwest of NGC 4921, also shows well defined spiral arms, but has a distance in NED of 109 Mpc.

neufer wrote:

Anthony Barreiro wrote:
Bravo-Alfaro and colleagues referred to NGC 4921 as a "post-starburst galaxy". A little googling yields these lecture notes by Caryl Gronwall of Penn State University. Apparently a post-starburst galaxy was forming stars "recently" i.e. within the past 1.5 billion years, but is no longer forming new stars, and is transitioning from a spiral to a lenticular galaxy. \

The poster child for a post-starburst galaxy.

Anthony Barreiro wrote:
Bravo-Alfaro and colleagues referred to NGC 4921 as a "post-starburst galaxy". A little googling yields these lecture notes by Caryl Gronwall of Penn State University. Apparently a post-starburst galaxy was forming stars "recently" i.e. within the past 1.5 billion years, but is no longer forming new stars, and is transitioning from a spiral to a lenticular galaxy. \

Ann wrote:

Anthony Barreiro wrote:

Neither the arxiv snippet referenced in the wikipedia article nor the full article from the Astronomical Journal explains how the intergalactic medium strips hydrogen from galaxies in clusters

I'm sure that I have read that the monster black holes in the largest galaxies in clusters spew out enormous jets that heat the intergalactic hydrogen gas in the clusters so much that it can't be captured by the galaxies and form cold molecular clouds, which are (or so I believe) a prerequisite for star formation. The galaxies themselves may have used up much of their own gas supplies in previous bursts of star formation due to a lot of galactic interactions. And because the galaxies can't gain fresh hydrogen from the surrounding galactic medium, they stop forming stars.

Galaxies that are close to the cluster center are usually devoid of any obvious star formation, while galaxies on the outskirts of the cluster may still be forming stars at a healthy rate. There is an intermediate distance from the cluster center at which galaxies may form some stars, but not a lot of them. NGC 4921 may be at this intermediate distance from the center of the Coma Cluster.

Ann

geckzilla wrote:What, exactly is the Hubble palette? I see people use this phrase to describe some colors in their own astrophotography all the time and it implies some kind of standard and there are certainly some filters almost always presented in red, others almost always in green, etc., but I haven't noticed anything I could strictly call The Hubble palette. The only almost universal thing is the logical presentation of longer wavelengths as redder and shorter wavelengths as bluer.

Anthony Barreiro wrote:Thanks Chris, I did that before I posted my original question. Neither the arxiv snippet referenced in the wikipedia article nor the full article from the Astronomical Journal explains how the intergalactic medium strips hydrogen from galaxies in clusters, at least not in terms I can understand. I had hoped that somebody here might be able to explain it in layperson's terms. But advice about basic undergraduate library research is always welcome. :roll:

Anthony Barreiro wrote:

Neither the arxiv snippet referenced in the wikipedia article nor the full article from the Astronomical Journal explains how the intergalactic medium strips hydrogen from galaxies in clusters

Chris Peterson wrote:

Anthony Barreiro wrote:

Chris Peterson wrote:... - consistent with the observation in the article you quote that there has been some kind of interaction with the ISM. ...

The wikipedia article referred to a possible interaction with the intergalactic (not interstellar) medium. I've never heard about such interactions before and a cursory google search doesn't turn up anything immediately relevant. So I'm still curious where this galaxy's hydrogen went.

Wikipedia provides references for claims like this. I'd suggest you follow the reference to the associated paper (publicly available on arXiv) and see if that helps. Of course, you could then follow the references provided by the primary source to explore this in even more detail.

APOD Robot wrote:How far away is spiral galaxy NGC 4921? Although presently estimated to be about 310 million light years distant, a more precise determination could be coupled with its known recession speed to help humanity better calibrate the expansion rate of the entire visible universe.

Anthony Barreiro wrote:

Chris Peterson wrote:... - consistent with the observation in the article you quote that there has been some kind of interaction with the ISM. ...

The wikipedia article referred to a possible interaction with the intergalactic (not interstellar) medium. I've never heard about such interactions before and a cursory google search doesn't turn up anything immediately relevant. So I'm still curious where this galaxy's hydrogen went.

Chris Peterson wrote:... - consistent with the observation in the article you quote that there has been some kind of interaction with the ISM. ...

Anthony Barreiro wrote:Cold, i.e. non-ionized, hydrogen is what makes stars. I still don't understand how this galaxy could have low amounts of hydrogen and a lot of bright young stars.

geckzilla wrote:Being low in non-ionized hydrogen gas does not mean that it's low in hydrogen overall. That paragraph you quoted to me could be rephrased more simply as "H I emission at the 21 cm wavelength was lower than usual." rather than reading it as there somehow being a dearth of hydrogen in the galaxy.

Chris Peterson wrote:

Anthony Barreiro wrote:It seems odd to me to see a spiral galaxy with bright blue clusters of young hot stars but no obvious red areas of the hydrogen gas from which stars form.

How can we even tell from this image? It's essentially monochromatic- a combination of one filter that covers the entire visual range longer than blue, and another that is only infrared. These two channels were manipulated to create some sort of color space, but it bears no resemblance to any true color as our eyes would see it, and there is nothing at all that would make hydrogen look any different from any other visual spectrum narrow band sources, or from continuum sources.

Look closely, and you'll see that there are only two hues in this image, blue and orange, with all the variation created by varying the brightness and saturation.

Anthony Barreiro wrote:It seems odd to me to see a spiral galaxy with bright blue clusters of young hot stars but no obvious red areas of the hydrogen gas from which stars form.