This image of NGC 4921, a large but gas-anemic spiral galaxy in the Coma Cluster ~320 million light years distant, is a recently released composite from the Advanced Camera for Surveys (dead since 2007) on-board the Hubble Space Telescope. The detail within the galaxy is exquisite and the numbers of background galaxies through cosmic space and time is jaw dropping. Get the large image or use the zoom facility.

Enjoy!

p.s. The Spiral galaxy in the lower left corner, labeled here as one "with a very bright nucleus" appears to me to belong to the family of galaxies that harbor actively-feeding supermassive black holes in their centers known as "AGN" (active galactic nuclei), although I don't have any confirmation of that. Do ngc 3314 or parejkoj or Stupendous Man know either way?

Sorry, I was distracted by contemplated color measurements of all those distant background galaxies shining through the disk of NGC 4921. There is a catalogued quasar at z=0.24 which looks as if it might be the one (1259+281, SDSS J 130128.14+275106.6), although that ID depends on my having matched structure in NGC 4921 between the ACS image and Palomar survey to get the orientation right.


Looking at the caption - rats! They didn't get to finish the Cepheid search in this (practically the only) spiral in the Coma Cluster before loss of the ACS WFC. I hope they get to complete that with the repaired camera [always with the proviso that doing so does not interfere with scheduling observations of Hanny's Voorwerp, of course...]

That "galaxy with a very bright nucleus" is SDSS J130128.14+275106.6. If you want detailed information about it, go to

this link within the SDSS on-line database

which will show you a close-up image, provide measured magnitudes, AND display
nice spectrum. The nucleus has strong emission lines and a blue continuum rising to
the short-wavelength end. My guess would be an AGN, rather than a starburst
galaxy. The redshift is about 0.24.

(argh! ngc3314 beat me to it, but I _swear_ that I figured all this out myself :-)

Well, I've been doubly beaten to the punch...

StupendousMan: your link is to a radial search page. Here is the link to the object's explore page (the top link from your link). The width of those emission lines says one thing: quasar, as ngc3314 says. My first thought was that that was a particularly large spiral galaxy, considering its angular size at a redshift of 0.24, but it's only ~35 kpc, which isn't that big.

Definitely a lot of galaxies shining through ngc4921, there. Wow. What an image. Thanks for the post.

For comparison, here's a 1-1 pixel image from SDSS, covering nearly the same field (it looks like the HST image might be rotated slightly). Same size of telescope (~2.4 meters), just inside the Earth's atmosphere, and only a 1 minute exposure instead of 17 hours.

__________________
"What do you care what other people think?" -- Richard Feynman
"For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." -- Feynman, at the conclusion of his Challenger report

Astounding.

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We'll have to outwit the fiend with our superior intelligence.
Yukon Cornelius

Stupendous detective work, guys!

When I saw the star-like diffraction spikes in that galaxy's center, I had a hunch it was an AGN. The spectrum certainly confirmed that, with beautiful broad hydrogen Balmer and FeII emission.

Food for thought: is there a known galaxy with a relatively bright star (say, r<15) at its center, masquerading as an AGN? The colors should give it away, but a spectrum would be better.

My guess is not in the usual areas where galaxy surveys are done, as they are in the directions of low stellar density (e.g. SDSS is out of the plane of the Milky Way). But I'd guess there probably is one in the galactic plane, and certainly toward the galactic center. Problem there is that we can't see the galaxies for the dust.

Any have an example?

__________________
"What do you care what other people think?" -- Richard Feynman
"For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." -- Feynman, at the conclusion of his Challenger report

Yeah, but I have to check a notebook in my office to get the ID (since this dates to a project largely done in the pre-web age...). Spiral galaxy, diffraction spikes, beg for colleagues to get a spectrum - and it's a G star at zero redshift. Not all that close to the galactic plane, either - it was from a jet search that started at |b|=20. I was sort of hoping for another object like the Einstein Cross 2237+030, even though two such alignments nearby would strain credibility.

Nice! That's just the kind of story I was looking for. I love "confusing coincidence" stories. No hurry to get the ID, but if you get a chance, I'd like to see it.

__________________
"What do you care what other people think?" -- Richard Feynman
"For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." -- Feynman, at the conclusion of his Challenger report

I really love this image.

How many stars do you think we're seeing in this image? I figure if each galaxy has 100 billion stars (conservative estimate), and there's at least 1000 galaxies (again, probably an understatement), then we're seeing at least 100 trillion stars? That's a mindboggling number. How big is the field of view here?

And the funny thing is that given the distances involved (hundreds of milllions or billions of lightyears), I'd suppose that most of the stars that we're actually seeing here are long dead by now.

At the distance of this galaxy (320 Mly), the scale is something like 0.465 kpc/" (1 kpc = 1 kiloparsec = 3262 ly, and 1" = 1 arc second = 1/3600 degrees). If NGC 4921 is 100,000 ly (~30 kpc) across, then it spans some 30 kpc/0.465 kpc/" = ~ 1 arc minute. The full field of view is ~1.5x that of the galaxy, so that takes us to about 1.5 x 1.5 arc minutes. However, somewhere the actual field of view must be given -- but I haven't found it.

So to put it more meaningfully... the FOV is roughly comparable to the size of Tycho crater on the moon (as seen from Earth)? (the moon is 31 arc minutes across, right?)

Here we go. 0128-33, also known as MCG -06-04-41 or ESO 353-G7. I came across it while examining the entire SRC J survey for optical jets (with a magnifier on glass copies - the sort of thing we used to do once in a career). The features on the sky survey plate might have either been symmetric jets or diffraction spikes - after getting the appended image with the CTIO 1.5m, I asked colleagues doing spectroscopy at the 4m for a quick exposure. G star, less interesting than a luminous Seyfert nucleus or Einstein cross. The image (combination of 2 1-minute V exposures) shows diffraction spikes, charge bleeding, and an amusing response of the on-chip amplifier to large amounts of charge in one place (the dark horizontal banding).

Attached Images

MCG-060441V.jpg (26.5 KB, 14 views)

Huh. Here's the object in Google Sky. I can definitely see why you wanted a spectrum of that! That's exactly the sort of thing I was wondering about. A very good alignment there.

What kind of imager was used to get that picture? The weird row artifact doesn't look like a typical CCD problem.

__________________
"What do you care what other people think?" -- Richard Feynman
"For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." -- Feynman, at the conclusion of his Challenger report

A TI 800x800 CCD at CTIO. I've used a couple of systems (on at Lowell for a while also did this) in which there was some kind of depression of the bias level when the total row intensity was too high (there are median tricks that can get rid of it pretty effectively). From that same run in 1988, I have some images of SN 1987A and its early light echos which show the same thing (just to make it harder to put together a pretty color composite - or even an acceptable color composite). From that date, you might guess that I'm slightly compulsive about archiving my own data.

By the way - both Skyview and Google Sky use the same sampling of the old SERC J survey plates. The diffraction spikes are a bit narrow to show up well in that, one of the details downplayed by the digitized versions.

Just bumping this up to say I found this A-star in front of a galaxy I was trying to observe.

Good find, matt.o! I think I can see the galaxy's H-alpha emission line around 7000 Angstroms in that spectrum. Why were you observing it, and how did you find it? If you don't mind my asking.

__________________
"What do you care what other people think?" -- Richard Feynman
"For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." -- Feynman, at the conclusion of his Challenger report

Yes, you can see it in my spectra too, I think.

I'm observing* the galaxy cluster Abell~2142 on the MMT with Hectospec to study the dynamics and galaxy evolution in this cluster.

*technically, it's queue scheduled

Nifty. Clear skies! It looked like there might be a cluster in the vicinity of that object in the SDSS photometry.

Are you re-spectraing SDSS sources, or shooting for dimmer ones? How many of the 300 fibers are you using?

Also, I'm still amused that it is called the MMT, even though the name is now rather misleading.

__________________
"What do you care what other people think?" -- Richard Feynman
"For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." -- Feynman, at the conclusion of his Challenger report

Yes, I am reobserving SDSS spectra - purely for homogeneity in my sample. I'm also going about 3 mag deeper than the SDSS spectra although I chop out objects redder than the cluster red sequence. Generally ~260 fibres are allocated to science objects and ~30-40 to blank skies for sky subtraction.

I refer to it as the "mono-mirror telescope"!

OK, here's one that went the other way. Show of hands, everyone who's heard of NGC 4569. Messier heard of it and labelled it 90. It's what van den Bergh has called an anemic spiral, right in the core of the Virgo cluster, with truncated H I distribution and smooth outer arms. Its nucleus is so compact that Humason, Mayall, and Sandage though it was a star, and picked some emission region that probably belongs to an adjacent galaxy for their redshift measurement in the classic paper. Here's a composite image from some old Lowell BVR data:



In fact, it's almost as blueshifted as galaxies come - something like 800 km/s. And the nuclear spectrum looks a whole lot like an A-type star. This once gave me a moment of hair raising on the back of the neck, noticing that its not even a typical aging starburst because the Balmer absorption lines are too narrow for main-sequence stars. The thing is loaded with supergiants, and the central star cluster is so compact that HST didn't even properly resolve it with WFPC2. I'm not convinced I ever worked out just what history gives such a phase, but I did notice that when people were grabbing Kennicutt's spectral atlas from my ftp site, they would notice NGC 4569 in there as well and casually mention that they thought it was interesting and had grabbed it just for grins.

Attached Images

ngc4569spec.png (10.6 KB, 10 views)

I've heard of it. It has a very high HI deficiency which makes it unreliable to use for the Tully-Fisher relation when hydrogen linewidths are used to measure the rotational velocity. The HI linewidth for NGC 4569 underestimates the rotational velocity and so using the TFR you get a distance of ~10 Mpc. However, the optical rotation curve data gives a larger rotational velocity and therefore a larger TFR distance - more in line with the Cepheid distances to Virgo spirals.

A few years ago I was comparing the HI deficiency measures for Virgo cluster galaxies from a paper by Solanes (I think it was) against the difference between the HI rotational velocity and the optical rotation curve velocity. It was pretty clear that in general the more HI deficient the Virgo spiral was, the more difference there was between the HI and optical rotation velocities such that the HI rotation velocities are smaller.

It is certainly something to bear in mind as a possible source of error in TFR distances based upon HI linewidths.

__________________
"The scientist who asks the right question reconnoiters a new patch of the unknown, and may, with luck, bring it within the constricted but expanding boundaries of the known."

~Timothy Ferris (The Red Limit) 1982

No offense, but those IRAF plots are just so... yuck.

No offense to the upscale nature of the forum, but it's not like I'm minded to take the extra time to edit an IDL procedure just to illustrate a quick post - or did you have something else in mind that's not quite so 1980s? I yield to few in understanding and mumbling about the limitations and annoyances of IRAF, but there sure are a lot of things it does quickly and without complaint.

(For the 95% of posters for whom the above might as well be Old Church Slavonic - move along, nothing to see here...)

Oh! Such Magnificence!

Thank you for sharing it Spaceman Spiff Sire!

I am filing it prominently.

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clear skies

If you want to make an apple pie from scratch, you must first create the universe. CARL SAGAN

Mak: Pass the pepperoni please.
Fazor: "Hail, Bautainia! We pledge our hearts to thee! Science and woo, some babbling too, and astron-oh-meee!"
slang: And it made ash out of yew and tree.

Hehe.. yes, this is no place for *that* debate.

IDL? IRAF?

Man... ya'll really need to get on the Python bandwagon! ;-)

(and if you can't give up IRAF, there's always pyRAF, which is certainly less "1980s")

__________________
"What do you care what other people think?" -- Richard Feynman
"For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." -- Feynman, at the conclusion of his Challenger report

I'm a recent IDL convert after spending my entire PhD using good old F77 and a little bit of C. I love it! So much faster with array structures like .fits files. However, I still like fortran for monte carlo type stuff requiring lots of iterations particularly because of multi-threading programs like OpenMP which make things happen a lot faster!

Anyways, a bit off topic there! ngc3314 - that is a fascinating galaxy.

So if there's one like that in the (core of the) Virgo cluster, can one make an a priori assumption that one (or more) will be found in clusters of similar mass, density, and degree of relaxation (for example)? Or, perhaps, at least one in ~10 of such clusters? In any case, are there implications here for possible difficulties in either estimating bias in (certain) samples, or in possibly under-appreciated selection effects?