Starship Asterisk*

Lovely picture.

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. My first thought was that the galaxy may be remarkably dusty and the dust somehow obscures the visible red light of hydrogen alpha radiation. But the wikipedia article on NGC 4921 discusses radio observations of this galaxy. Radio waves pass through dust more easily than visible light does (e.g., my bedside radio still receives a signal even if I haven't dusted recently).

wikipedia wrote:When examined at the 21 cm wavelength Hydrogen line, NGC 4921 was found to be strongly H I deficient, which means it is low in non-ionized hydrogen gas. The distribution of the gas has also been deeply perturbed toward the SE spiral arm and is less extended than the optical disk of the galaxy. This may have been caused by interaction with the intergalactic medium, which is stripping off the gas.[7]

So if there is remarkably little hydrogen in this galaxy, how did these stars form? And if there was abundant hydrogen a few tens of millions of years ago and very little now, where did it go? If there was some sort of interaction with the intergalactic medium how might this work?

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.

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.

Thanks Chris for the clarification about the colors in this image. There still seems to be evidence from radio astronomy that this galaxy is deficient in hydrogen, which doesn't make sense given the swirl of bright young star clusters.

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.

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.

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.

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.

It doesn't have a lot of bright, young stars. The galaxy is deficient in new star production. What little there is seems concentrated in just a few areas- consistent with the observation in the article you quote that there has been some kind of interaction with the ISM.

Low amounts of total hydrogen certainly doesn't preclude local concentrations high enough to allow star formation.

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.

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.

I imagine the metric expansion rate and acceleration of the universe must always be measured between two points in spacetime. I understand at this point in the Earth's spacetime, we observe the universe expanding at a rate which is increasing. But at an earlier point (possibly before the Earth was formed?) one would have observed the universe expanding at a rate which was decreasing. I don't quite know where I'm going with this, but before I go anywhere, can anyone poke any holes in my rudimentary understanding of the metric expansion of the universe? I feel l don't have a fundamental grasp of the topic.

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.

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.

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

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:

Well, you never know. This forum attracts people with a wide range of skills. I'm often surprised how many people don't understand the difference between what Wikipedia presents and primary sources. It's one of the first things I teach in the classroom. The middle school classroom!

I didn't read the paper, so I don't know anything about the mechanism being discussed. But I'll look at it later and post back if anything makes sense.

That said, I hope the original question has been reasonably answered: this galaxy shows a low level of new star formation, and star forming regions aren't unreasonable even in galaxies with low levels of gaseous hydrogen.

For those looking for more colour information, there is a beautiful large image from the HST. Here's a small part of it near the center of the galaxy. Obviously there's little of the usual pink fluffy clouds you see in star-forming regions, while there are definite bits of blue where new stars are. But mostly the galaxy is white, always remembering that we're looking at some variation of the Hubble palette.
Rob

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.

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.

The Hubble palette is a very specific false color scheme. It maps singly ionized sulfur to red, doubly ionized oxygen to blue, and hydrogen alpha to green. AFAIK that is the only standard usage of the Hubble palette.

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

Thanks Ann, this sequence of events makes sense. The article secondarily referenced in the wikipedia article on NCG 4921 (H. Bravo-Alfaro, V. Cayatte, J. H. van Gorkom, and C. Balkowski, "VLA H i Imaging of the Brightest Spiral Galaxies in Coma", Astronomical Journal, February 2000) said that the most H I poor galaxies are toward the center of the cluster, and the sides of galaxies toward the center of the cluster are more H I poor than the sides toward the edges. I don't remember reading anything about black hole jets, but I can comprehend them acting like firehoses that blow all the gas out of galaxies in their paths.

Oddly, I was able to read (skim) the article online for free earlier today, but now I can only find the abstract and a link to buy the article.

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 Butcher-Oemler effect may play a role, but the effect may or may not be real, and if it were real I couldn't understand it. So, powerful jets from supermassive black holes are the best explanation yet for why there are relatively young blue stars in NGC 4921 but no hydrogen for forming new stars!

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.

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.

Maybe this lovely picture of NGC 4921 will make it into the 2015 APOD wall calendar.

It's a beautiful picture. Looks a lot like a spider web with a captured moth wrapped up in the middle.

Since Hubble is in space, I guess we don't need to go check the lens, just in case.

You know, I think that's actually an egg sac. A moth would not receive such a careful weave.

Looks more like a cotton ball to me.

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.

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.

Who's NED?

Anthony Barreiro wrote:Who's NED?

Meet NED.