Bulges and nuclei and AGNs, oh my!
I'm just starting to delve into more of the science of Galaxy Zoo, and I'm having some trouble distinguishing between bulges and just the nucleus of a galaxy. I understand how to tell if an edge-on galaxy has a bulge (since it has a bit of "height" to it), but when looking "down" on a galaxy, I don't understand what constitutes a bulge. All of the reference images for bulgeless galaxies I've seen in the discussions here just appear to be...well, galaxies with bright bulges in the center! Is a galaxy considered to have a bulge when it is just brighter? Fuzzier? Larger?
I feel like if I'm going to start diving into the realm of AGNs I should probably know the difference. 😃 Sorry if this is a dumb question, but I am very eager to science correctly.
by Budgieye moderator
Not a dumb question, I would like to know that too. What is a nucleus , a nuclear bulge and halo. Illustrations are usually of the extreme simple kinds.
I'll look for anything written by our scientist , in the meantime have you browsed through the index?
2.2 Nucleus, nuclear bulge, and bulgeless, X-shaped bulges , nuclear rings http://talk.galaxyzoo.org/#/boards/BGZ0000001/discussions/DGZ0000wrb?page=2&comment_id=53d8b8190d43f776b000107c
I have browsed the index, yes. That was actually part of the reason I posted here, because I was still a bit confused. 😃 Most of the information I've found here in Talk is either very basic guidelines for beginners or highly scientific publications. Unfortunately, I fall somewhere right in the middle-- I've been on Galaxy Zoo for a long time and have a good, basic understanding of astrophysics, but I lack the technical experience to read and understand the numerical data that you might find in Examine/SkyServer/etc. Even though my degree is not in physics, this stuff has always been a fascinating hobby, so I'm slowly trying to creep my way up to being able to use that data to better help the GZ cause!
by vrooje admin, scientist
@Budgieye pointed me to this post -- these are good questions.
All of the reference images for bulgeless galaxies I've seen in the discussions here just appear to be...well, galaxies with bright bulges in the center!
First question: would that be mainly referring to the plot from our 2013 paper on AGN in bulgeless galaxies? i.e.:
I actually get this same question all the time when I show this mosaic in talks to other astronomers -- and the short answer is that the bright point of light in the center of each of those galaxies is at least partially the light from the AGN, not from the stars in the galaxy.
The best illustration of this is the galaxy in the bottom right. That galaxy turned out not to be a disk galaxy at all -- it's an elliptical galaxy in an ongoing interaction, where the tidal tails look a lot like spiral arms. So the bulk of the galaxy light, in the center, is all bulge -- except for that brighter spot right in the middle, which is entirely the AGN. That one's particularly obvious because the AGN is also a different color than the galaxy.
The first thing I do when I'm trying to decide whether a galaxy has a small bulge or just an AGN in the center is to look at the stars around the galaxy. Stars are unresolved, and so are AGN, so the shape of the stars gives you an indication of what the shape of unresolved AGN emission should pop out as in the middle of the galaxy. If the AGN is bright, it might be much brighter than the nearby stars, or it could be fainter. As long-term GZ volunteers know well, brightness changes the apparent size of a star on the image... but regardless of brightness, the point-spread function (PSF) should be a similar shape. The shape can vary from image to image here because this is a ground-based telescope and the atmosphere is variable; but we'd only notice it here over large position and time shifts, so using nearby stars for this is okay.
So, a co-author and I went through a set of 100 galaxy images that GZ volunteers selected as clear spiral disks with small "bulges", which we also already knew were AGN due to their optical emission line ratios. We picked these 15 as the best candidates for "maybe those so-called bulges are actually just AGN". To confirm that, we "fit" the images, i.e. used models of each galaxy component to find the best combination of disk, bulge and unresolved emission that neither over- nor under-subtracts the light from the galaxy. (We used the program GALFIT to do this.) The steps we actually took were to:
- Fit a disk and unresolved source only, then subtract that fit from the original and see if there was anything left over;
- If there was something left over, fit it with a bulge component (and let the model vary to find the best parameters).
The first thing we found was that the rightmost 2 images in the bottom row weren't disk galaxies at all; they were mergers with spiral-looking tidal tails. So we didn't include them in the analysis.
We also found that, if you read from top to bottom and left to right along the images, the first 11 galaxies needed both the AGN and a tiny little bulge, in addition to the disk, to get the best fit. The 2 other galaxies (2nd and 3rd in the bottom row) didn't need the bulge, so maybe they are truly "bulgeless" and that central blip in each is just the AGN. If they do have a bulge it is smaller than what we can resolve in these images.
As an example of the fits, let's take the galaxy in the top right. If we fit the galaxy in just the r-band filter (which makes the green part of these color images and is the deepest exposures of the filters), here's the result (in multiple panels, which I'll explain below):
From left to right, the panels are:
- The original galaxy image (rotated from the color version)
- The model fit. The model includes (i) a disk with spiral modes, (ii) unresolved emission from the AGN, and (iii) a small compact bulge. Some of the stars nearby are also fit, especially where they overlap the disk.
- The "residual", which is the model fit subtracted from the original. There's a little bit of under and oversubtraction but it's not too severe - for a galaxy with so many "features", this is a good fit.
- Another residual, but this time only the disk and the AGN are subtracted, so this is a best guess for an image of just the compact bulge.
Okay, so the compact bulge is really quite small. It's resolved, but only barely -- and it's only a few percent of the total galaxy luminosity, so no wonder it wasn't obvious given that the AGN is there too!
The main point of describing all of this in such detail is that when you suspect an AGN is present it's hard to tell whether there might also be a small bulge lurking too; the brighter the AGN is, the harder it gets and the more bulge you can "hide" in these color images. And if you don't know whether there's an AGN, it's even harder to tell. But, AGN or not, bulgeless or tiny-bulge galaxies are really interesting because they significantly constrain the kind of evolution the galaxy must have had. So if you spot anything like the above galaxies, with what look like these really small bulges, it helps to flag it.
Hope that description helped a bit? Thank you for all your classifications and interest!
Great post, very helpful; thanks vrooje! 😃
I'm curious: did you find any bulgeless spiral galaxies which did not have a (discernible) nucleus?
by vrooje admin, scientist
Yes, the highest-redshift of the 13 that were in fact disks needed no compact galaxy component in the fit. But that just means we have an upper limit on the bulge, not that it's confirmed there is no bulge whatsoever. If we had higher resolution we might spot one!
We still called these bulgeless because of all of the other compact components were consistent with disk-like components, which are often called "pseudobulges" because they look like bulges in many ways, but are actually mostly rotationally supported, unlike a "classical" bulge.
(Note: @JeanTate, I know you already know all this about that paper -- just explaining for other readers.)
Another good example of a properly bulgeless galaxy containing a growing black hole is NGC 4395:
The black hole is quite small and low-luminosity compared to those we measured in our sample.
(post edited to add a photo of NGC 4395)
So it seems that, in general, if you're just looking at the image and not examining the numerical data, a proper bulge has a bit more of a diffuse or "fuzzy" look, while an AGN is likely to be much brighter and have more defined borders, or have a shape similar to surrounding stars. But because of some of the variables, one can look like the other. Seems like the best way to determine whether it has a bulge/AGN or not is still to examine the data, but I do feel like I have a bit of a better understanding of at least what we're looking for now. Very helpful responses, thank you!