Starship Asterisk*

Anemic Spiral NGC 4921 from Hubble

Explanation: 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. Toward this goal, several images were taken by the Hubble Space Telescope in order to help identify key stellar distance markers known as Cepheid variable stars. Since NGC 4921 is a member of the Coma Cluster of Galaxies, refining its distance would also allow a better distance determination to one of the largest nearby clusters in the local universe. The magnificent spiral NGC 4921 has been informally dubbed anemic because of its low rate of star formation and low surface brightness. Visible in the above image are, from the center, a bright nucleus, a bright central bar, a prominent ring of dark dust, blue clusters of recently formed stars, several smaller companion galaxies, unrelated galaxies in the far distant universe, and unrelated stars in our Milky Way Galaxy.

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Roberto messaged me and asked me to post this for him because he is going to be away and unable to post it himself.

Roberto Colombari wrote:The original frames from the Hubble have a discontinuity in the ACS. For fixing it, I took the 2 parts and make them very near one to each other. Finally I have lowered the differences between their edges on PS.

Roberto

What a fascinating image. I'm intrigued by the streamers of dust trailing from star clusters, especially in the region below centre. What's causing the inward movement of so much dust? Gravity, or something else?

What a fantastic image of a truly fascinating galaxy!!!

Ann

I'm curious about the largish spiral in the upper left corner. If there's no Milky Way star with a chance alignment, it must have a very bright nucleus given the diffraction spikes at its center.

ericchill wrote:I'm curious about the largish spiral in the upper left corner. If there's no Milky Way star with a chance alignment, it must have a very bright nucleus given the diffraction spikes at its center.

That galactic nucleus is also noted as bright in this annotated version from 2009:
http://www.universetoday.com/24684/
(I believe it is a galaxy roughly between NGC 4921 and NGC 4923. In celestial coords, it is in the southwards direction from NGC 4921.

...

In galactic coords, we are looking in a direction ~4° from the Galactic North Pole [of the Milky Way].)

As far as I can tell, the brightest star in today's APOD (biggest diffraction spike, ~2o'clock) has an apparent magnitude of about 13.5. If you look closely, you can just detect a few signs of the blending around the discontinuity in the original frames, within a horizontal band a little above the image centre, and through the central bar.

Incredible image and an amazing way to start the day and the week! Rob Gendler has really started a wonderful revolution of amateurs processing professional data! While people were processing Hubble data before him, the work he has done in the past few years as well as presentations at various conferences has really popularised this and made it more mainstream.

Question
Visible are several "more distant" galaxies. Some galaxies seem to be between the Anemic and Earth. One of those is even (from Earth seen) before the dustring, so I don't think that galaxy is at greater distance than the Anemic. Is that galaxy so very small and the Anemic a giant? Don't believe so.
Strange.

emanueldewitt wrote:Question
Visible are several "more distant" galaxies. Some galaxies seem to be between the Anemic and Earth. One of those is even (from Earth seen) before the dustring, so I don't think that galaxy is at greater distance than the Anemic. Is that galaxy so very small and the Anemic a giant? Don't believe so.
Strange.

Seems to me that the "Anemic " is just more translucent than you would expect and the smaller galaxies are in the background.

Yeah, it does look kinda sickly. Must be having trouble coming out of its Coma.

Serious question here. Isn't it somewhat unusual for a face on spiral galaxy to be showing as much dust as this one does?

I think if you were to look at this edge on, it would look very similar to M104, The Sombrero Galaxy. They don't seem to have a large amount of in fallen gas and dust, and thus have less mass to work with.

:---[===] *

starsurfer wrote:Incredible image and an amazing way to start the day and the week! Rob Gendler has really started a wonderful revolution of amateurs processing professional data! While people were processing Hubble data before him, the work he has done in the past few years as well as presentations at various conferences has really popularised this and made it more mainstream.

Rob Gendler is inspirational but credit where credit is due... the Hidden Treasures contest is what got me going on this and I'm the one who showed Roberto around the HLA. He already had a good handle on processing but now he is addicted to the Hubble archive.

I'd credit the Hidden Treasures contest for both Roberto's and my introduction to Hubble processing.

ozalba wrote: What's causing the inward movement of so much dust? Gravity, or something else?

Is it moving inward? I have seen projections that show some galaxies spiraling outward. Swirling 'Anemic' Spiral NGC 4921 certainly looks to be going down the drain and reminds me of the old W.C*.
*http://en.wikipedia.org/wiki/Flush_toilet#Water_closet

BDanielMayfield wrote:Serious question here. Isn't it somewhat unusual for a face on spiral galaxy to be showing as much dust as this one does?

Why would the orientation affect how much dust we see? Are you suggesting that when we see a face-on spiral, we're looking through a thin sheet of dust, and therefore it might not be very apparent? Seems like there might be some kind of balance between dust regions having a greater optical depth when viewed obliquely, and dust regions not being blocked by intervening arms when viewed face-on.

ozalba wrote:What a fascinating image. I'm intrigued by the streamers of dust trailing from star clusters, especially in the region below centre. What's causing the inward movement of so much dust? Gravity, or something else?

I don't see any overall movement that I could identify as "inward" or "outward". In a spiral galaxy, there is no bulk movement of material in either direction- it merely orbits as some particular radius. That applies to the dust, as well. Tiny streamers of dust are usually being blown by intense winds from new, hot stars. I suspect that's what we see here. Dust often obscures (or partly obscures) regions of new star formation.

Chris Peterson wrote:

BDanielMayfield wrote:Serious question here. Isn't it somewhat unusual for a face on spiral galaxy to be showing as much dust as this one does?

Why would the orientation affect how much dust we see? Are you suggesting that when we see a face-on spiral, we're looking through a thin sheet of dust, and therefore it might not be very apparent? Seems like there might be some kind of balance between dust regions having a greater optical depth when viewed obliquely, and dust regions not being blocked by intervening arms when viewed face-on.

The processing of the image should be noted, as well. The dust has been greatly emphasized.

I'd like to thank very much Judy for helping me in understanding and improving Hubble processing techniques.
She has been very kind everytime I was asking her a help.

Best,
Roberto

geckzilla wrote:

starsurfer wrote:Incredible image and an amazing way to start the day and the week! Rob Gendler has really started a wonderful revolution of amateurs processing professional data! While people were processing Hubble data before him, the work he has done in the past few years as well as presentations at various conferences has really popularised this and made it more mainstream.

Rob Gendler is inspirational but credit where credit is due... the Hidden Treasures contest is what got me going on this and I'm the one who showed Roberto around the HLA. He already had a good handle on processing but now he is addicted to the Hubble archive.

I'd credit the Hidden Treasures contest for both Roberto's and my introduction to Hubble processing.

Thanks for the extra info, I love the origins of journeys! My comment wasn't specifically about Roberto, it was just a generalisation.

I also consider you to be an amazing inspiration to the world as well, you're so nice and helpful! I also love your new image of V1331 Cygni, one of my favourite young stellar objects! You always manage to make me smile with virtually no effort, what a brilliant day it's been!

Haha, you are such a flatterer, starsurfer.

Speaking of the dust ring, are we speaking of actual "dust"-sized particles, or are dust rings and lanes also made up of asteroid-sized chunks such as our own system's asteroid belt?

Also, a bit off-subject, regarding November 23rd's APOD, the updated "video" (see today's Spaceweather site) shows comet ISON barreling down on comet Encke appearing to be on a collision course. I am certain that a collision is not going to occur, but does anyone know how closely these comets will pass by each other?

quigley wrote:Speaking of the dust ring, are we speaking of actual "dust"-sized particles, or are dust rings and lanes also made up of asteroid-sized chunks such as our own system's asteroid belt?

No big pieces. This material is typically sub-micrometer sized particles... actual "dust".

quigley wrote:
Speaking of the dust ring, are we speaking of actual "dust"-sized particles, or are dust rings and lanes also made up of asteroid-sized chunks such as our own system's asteroid belt?

Actual "dust"-sized particles: http://en.wikipedia.org/wiki/Cosmic_dust

quigley wrote:Also, a bit off-subject, regarding November 23rd's APOD, the updated "video" (see today's Spaceweather site) shows comet ISON barreling down on comet Encke appearing to be on a collision course. I am certain that a collision is not going to occur, but does anyone know how closely these comets will pass by each other?

Not close at all. Source: http://www.slate.com/blogs/bad_astronom ... _info.html