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Observing Run: Raw Data versus Finished Product

So let’s say you have a galaxy:

Bulgeless AGN 2: SDSS

And you know this galaxy has a growing black hole, and probably hasn’t had any significant mergers, because it has very little, if any, bulge. Which means you have two questions: 1) what counts as significant? and 2) how little is very little?

To answer the first question, you’d like to look for the faint stellar streams that signify the remnants of a minor merger. The optical images you already have aren’t even close to deep enough to see something like this:

NGC 5907: the Splinter galaxy. Credit: R. Jay Gabany

But if you could see that for your galaxy, you could start to put together its minor merger history and answer that first question.

Of course, that kind of depth is not easy. The group that took that data most likely spent weeks observing that one source, and there are many technical challenges involved. You may be in luck, though: you have a bigger telescope, which means you probably only need one night to get a single-filter optical image at the same depth.

So you go to the telescope, and you take some data. After 5 minutes, this is what you have:


Which … doesn’t look so great, actually, until you clean it up a bit by correcting for the different effects that come with a huge mosaic of CCD chips, like different noise levels and so forth. Luckily, the people who wrote the code to observe with this instrument have provided a “first-look” button that automatically does that pretty well:


That’s better. You can see that even with 5 minutes of observing time, you’re close to the depth you already had. To get what you need, though, you don’t need 5 minutes of exposure. You need 5 hours.

But you don’t want to just set the telescope to observe for 5 hours and hit “go”. In fact, you can’t do that. If you do, those well-behaved little stars near your galaxy will be so bright on the detector that they’ll “saturate”, filling their pixels with electrons that then spill out into nearby pixels. This detector in particular doesn’t handle that very well, so you need to avoid that. And what if something happens in those 5 hours? What if a cosmic ray — or many — hits your detector? What if a satellite passes over? What if the telescope unwraps? While that looks kind of cool:

unwrapping the telescope

When the telescope rotates to +180 degrees, it stops tracking and goes -360 so that it can keep tracking from -180. Otherwise cables plugged in to walls + twirling round and round = unhappy telescope.

It wrecks the whole exposure. Plus, those chip gaps are right where your stellar streams might be. You’d like to get rid of them.

So you solve all of these problems at once by observing multiple exposures and moving the telescope just a little in between exposures.

bulgeless agn 2 dithered

This is called dithering.

You end up getting your 5 hours’ exposure time by doing lots of dithers — about 50 of them, to be exact, mostly between 5 and 10 minutes apiece. This has several advantages and a few disadvantages. You can throw out any weird exposures (like the unwrap above) without losing very much time, but then you have to combine 50 images together. And that, frankly, is kind of a pain.

And this is a new instrument, and the reduction pipeline (the routines you follow to make the beautiful finished product) doesn’t fully exist yet, and what does exist is complex — and, for the moment, completely unknown to you.

So the beautiful finished product will have to wait.

In the meantime, you have a few more galaxies to look at, and that second question to try and answer, on future nights and in a future blog post.

Why I’m at WIYN: Mergers and Bulgeless Galaxies

bulgeless WIYN targets

At the start of this year, our paper on bulgeless galaxies with growing black holes was published. These galaxies are interesting because each is hosting a feeding supermassive black hole at its center, a process typically associated (at least by some) with processes like mergers and interactions that disrupt galaxies — yet these galaxies seem to have evolved for the whole age of the universe without ever undergoing a significant merger with another galaxy. In fact, they must have had a very calm history even among galaxies that haven’t had many mergers. If these galaxies were people, they’d be people who had grown up as only children in a rural town where they always had enough food for the next meal, but never for a feast, who never jaywalked or stayed out in the sun too long, and whose parents never yelled at them — because it was never necessary. Sounds boring, perhaps, until you see the screaming goth tattoo.

merger simulations

Major mergers? Not for these galaxies. (Credit: V. Springel. Except the big, vicious X. That’s all me.)

We see the evidence of the tattoos — rather, the growing black holes — by examining the galaxies’ optical spectra. But how do we know they’ve had such calm histories? You told us. Galaxy Zoo classifications revealed that, once you account for the presence of the bright galactic nucleus, these galaxy images have no indication of a bulge. And bulges are widely considered to be an inevitable byproduct of significant galaxy mergers, so: no bulge, no merger.

Of course, that’s a very general statement and it begs many follow-up questions. For instance: what counts as a “significant” merger? These galaxies had to have grown from the tiny initial fluctuations in the cosmic microwave background to the collections of hundreds of billions of stars we see today, and we know that process was dominated by the smooth aggregation of matter, but just how smooth was it? If two galaxies of the same size crash together, obviously that’s a merger, and that will disrupt both galaxies enough to create a prominent bulge (or even result in an elliptical galaxy). If one galaxy is half the size of the other, that’s still considered a “major” merger and it almost certainly still creates a bulge. But what if one galaxy is one-quarter the size of the other? One tenth? One hundredth? At what level of merger do bulges start to be created? Simulations tend to either not address this question, or come up with conflicting answers. We just don’t know for sure how much mass a disk galaxy can absorb all at once before its stars are disrupted enough to make a detectable bulge.

However, we may be able to constrain this observationally. Galaxy Zoo volunteers are great at finding the tidal features that indicate an ongoing or recent merger, and the more significant the merger, the brighter the features. Mostly the SDSS is only deep enough to detect the signs of major mergers, which are easier to see, but which settle or dissipate relatively quickly. In a more minor merger, on the other hand, the small galaxy tends to take its sweet time fully merging with the larger galaxy, and with each orbital pass it becomes more stretched out, meaning faint tidal features persist. The Milky Way has faint stellar streams that trace back to multiple minor mergers. But if we want to see their analogs in galaxies millions of light-years away, we’re going to need to look much deeper than the SDSS does.

faint tidal features in M63

A very deep image of M63 by Martinez-Delgado et al. (2010), demonstrating that these observations are technically challenging, but possible.

So we were thrilled when we got time on the 3.5-meter WIYN telescope. Of the six nights we got, 2 are set aside for infrared exposures to make sure these galaxies aren’t just hiding bulges behind dust, and the other 4 are for ultra-deep imaging to see what (if any) faint tidal features exist around some of these bulgeless galaxies. If we find tidal streams, we can use their morphologies and brightness to help us figure out the size of merger they indicate (by comparing to simulations). If we don’t find any, then these galaxies really have had no significant mergers, and the growth of supermassive black holes via purely calm evolutionary processes is confirmed. (Long live the vanilla farm kid with the wicked tattoo!)

So how’s it going so far? Reasonably well: conditions haven’t been perfect, but until tonight we hadn’t lost much time to full clouds or dome closures. Tonight, though there’s not a cloud in the sky, there’s so much dust in the air that the domes are closed to prevent damage to the optics. Obviously I’m sad about that — it means we’ll miss one of our targets — but in between various incantations to the gods to clear the air so we can re-open, I’m working on an initial reduction and stacking of all the images I’ve taken over the past couple of days, so that I can (hopefully) give everyone a sneak peek at the results soon!

Observing Run: WIYN, Kitt Peak – First Report

I’ve been both excited and nervous about my trip to Kitt Peak. I’m excited because observing is fun and the science is cool, but the program I have planned is also technically challenging and uses a brand new instrument, which is a little scary.

In addition, although I’m plenty experienced with data, I haven’t done a lot of hands-on observing. My PhD thesis used Hubble data, and Galaxy Zoo uses both Hubble and SDSS data — neither of which you take yourself. Because observing is a useful skill for my profession, I made sure to get some experience while I was in grad school, but this is my first solo run to collect data for my own project. I’m here to get very deep images of some of our bulgeless AGN host galaxies, so if it doesn’t work out I’m probably going to be heartbroken. And clouds or technical issues are one thing, but I’ll be even more upset if I fail because I make a mistake that a seasoned observer wouldn’t have. I don’t want to let the Galaxy Zoo participants down! So I’ve been reading the instrument manuals and scouring papers that have done similar work in the past. The pressure is on.

I arrived the night before my first night so that I could “eavesdrop” and start to learn the new instrument on the 3.5-meter WIYN telescope, called pODI. Eventually it will just be the One Degree Imager, but for now it’s only partially complete — which is fine for me, as I only need a fraction of the total area ODI will eventually cover. But Kathy Rhode, who studies globular clusters in nearby galaxies, has slightly larger targets:


This is just one of many images Kathy took, all of which will eventually be combined to fill in the chip gaps and get rid of the usual artifacts. The instrument is working very well — it’s a good thing instruments don’t get as tired as their observers!


Another good reason to arrive a night early is to give yourself time to get adjusted to the observing schedule.

For my own first night, I was assisted by a startup person, an ODI system scientist who knows the instrument backwards and forwards. He walked me through everything, and stuck around to make sure my science observations were starting off right. He was joined by two others, both software gurus who are either writing code for ODI or for similar instruments. Along with Doug, the veteran telescope operator, there was a lot of expertise in the room. They were very patient as I asked all my questions (and made some suggestions — the software is still in progress), and my first science exposure of the night looked exactly as I had hoped:


Okay, like I said, pODI is a little bit more area than I need at the moment. Here’s a zoom in to the central detector grid:


So. Why am I observing these objects? What am I hoping to learn? More soon… for now it’s the start of my second night, and I have to get started on calibrations!

Green Valley: The Town Too Good To Die

Galaxies of different colors

I swear we are consistently trying to keep our live hangouts to about 15 minutes. We have so far failed at keeping to time, but hopefully also succeeded in the sense that we only run over because there’s so much to discuss.

We had a number of good questions from Twitter, Facebook and the blog about various types of galaxies — from red spirals to green peas and blue ellipticals — and I rather arbitrarily decided this was an indication that our hangout should have a color theme. That is, what exactly does “color” mean in the context of astronomy? What is going on physically when a galaxy is one color versus another, or has multiple colors? Is color information always telling us the same thing? We tried to address all those questions, as well as show some examples of different galaxies in the above queried categories. As a bonus, we learned how galaxy colors are related to the town my grandparents retired to. (This post’s title is a quote from the Green Valley Chamber of Commerce’s official website.) That was as much a surprise to me as it was to the viewers!

We also talked about what’s currently going on in Galaxy Zoo behind the scenes. Earlier today, Kyle sent around a really nice draft of the Galaxy Zoo 2 data paper for the team to read and comment on (you’ll have to watch the video to get a sneak peek at some of the figures).

And it’s that time again: Hubble Space Telescope proposals are due in about a week. We talked about the proposal process from concept to submission to review, discussing both specifics of certain telescopes and the general practices that (we hope) help lead to a successful proposal. Here’s a hint: it may not be what you think!

We covered all this and some other questions, too. No wonder we ran a little over…

And here’s the podcast version:

Download MP3 file

GZ4 merger or overlap set

Is it a triple merger? A double overlap? A hybrid?

Spiral Colors

A blue(ish) and red spiral.

Elliptical colors

A red and blue elliptical.

Green pea

A tiny green pea galaxy, and zoomed-in at right.

An anniversary Voorwerpje

Following on the heels of the 5th anniversary of Galaxy Zoo itself, this week marks five years since Hanny pointed out the Voorwerp.

To help celebrate the occasion, we have new Hubble data on another of
its smallar relatives. This time the telescope pointed toward
SDSS J151004.01+074037.1 (SDSS 1510 for short). This has a type 2 (narrow-emission-line) AGN at z=0.0458. This was (as far as I can tell) first posted on the forum by Zooite Blackprojects, and also identified in the systematic hunt. Here it is in the SDSS:

SDSS 1510+07 from SDSS

SDSS 1510+07 from SDSS

Aa usual, these are minimally processed Hubble data, and in particular using filters where we can’t get the color right for both clouds and starlight at the same time without more work. As usual, green comes from [O III] and red from Hα , so green is more highly ionized gas. This one is cool enough already – I think of a Martian flamenco dancer with some cobwebs, but your view may vary:

SDSS 1510+07 from Hubble

SDSS 1510+07 from Hubble

Looking slightly ahead, next week, Alexei Moiseev, known on the Zoo forum from his wrk on a catalog of polar-ring galaxies based on a clever use of Zoo-1 click data, will be working with us next week, obtaining radial-velocity maps of three of these galaxies using the 6-meter Russian telescope in the Caucasus (the BTA, Bolshoi Teleskop Azimutalnyi or Large Altazimuth Telescope).

And on that note, I turn back to a new book on black-hole astrophysics,
which has me peeking ahead to page 749 for a table of timescales for
accretion phenomena. That, and wish everyone a happy, highly-ionized and just slightly late 5th Voorwerpendag!

Something rich and strange – Hubble eyes NGC 5972

We just got the processed Hubble images for NGC 5972. This is a galaxy with active nucleus, large double radio source, and the most extensive ionized gas we turned up in the Voorwerpje project. We knew from ground-based data that the gas is so extensive that some would fall outside the Hubble field (especially in the [O III] emission lines – for technical reasons that filter has a smaller field of view). We expected from those data that it would be spectacular. Now we have it, and the Universe once again didn’t disappoint. Another nucleus with a loop of ionized gas pushing outward (this time lined up with the giant radio source), twisted braids of gas like a 30,000-light-year double helix, and dramatically twisted filaments of dust suggesting that the galaxy still hasn’t settled down from a strong disturbance.

Here’s a combination of the Hα image (red) and [O III] (green) data, with the caution that neither has been corrected for the contribution of starlight yet. The image is about 40 arcseconds across, which translates to 75,000 light-years at the distance of NGC 5972. This gives the team plenty to mull over – for now I’ll just leave you all with this view. (Click to enlarge – you really want to.)

NGC 5972 from HST in [O III] and H-alpha

NGC 5972 from HST in [O III] and H-alpha

Hubble spies the Teacup, and I spy Hubble

Our Hubble image of Voorwerpje galaxies continue to come in, and it seems each one is stranger than the last. Overnight we got our data on the Teacup system (SDSS J143029.88+133912.0). This one attracted attention through a giant emission-line loop over 16,000 light-years in diameter to one side of the nucleus.

I was worried to get email this morning that there had been a failure to lock on to one of the two needed guide stars so that the telescope might have rolled enough during the observations to compromise data quality. Inspecting the data, it looks like we’re OK. We’re OK and the galaxy is strange. This is a composite of [O III] (green) and Hα (red), right out of the software pipeline without any additional processing:

The Teacup AGN in raw emission-line Hubble images

Another giant hole whose origin is obscure. The loop doesn’t even show much sign of being connected to the galaxy. The strongest [O III] does seem to trace out ionization cones, as in showing from structures near the nucleus, but that seems independent of the distribution of the gas. There are filaments in the gas that are nearly parallel, sort of like waves. Well have our work cut out for us to understand more of what’s going on here. I can hardly wait for the next one!

There was an extra treat for me with these observations. Last night, I interrupted a session with my summer class at the campus observatory to look south with binoculars and catch Hubble passing far to the southeast, no more than 13 degrees from our horizon. This was during the Hα exposure, so I saw it while it was doing these observations (it was pointed just about up in my frame of reference, as it happens). I got a picture through a 125mm telescope, showing the telescope streaking by just north of the star k Lupi. At the time, Hubble was 1600 km away over Cuba. Hubble was watching the Teacup, I was watching Hubble, and a couple of slightly puzzled students were watching me.

Hubble trails across the sky north of the star k Lupi in this telescopic view.

Curiouser and curiouser – Hubble and Mkn 1498

Fresh off the telescope, here’s a first view of the “Voorwerpje” gas clouds around the Seyfert galaxy Markarian 1498. Its nucleus, shown in our Lick and Kitt Peak spectra, is a type 1 Seyfert, meaning that we see the broad-line region of gas very close to the central black hole, moving at high velocity. Those data showed highly-ionized gas to a radius of at least 20 kiloparsecs (65,000 light-years). Its nucleus is too dim to account for the ionization of the extended gas clouds, which landed it a spot in our list of seven objects for the Hubble proposal. Getting these data now was an unexpected treat – they were originally scheduled to be taken next November. As another bonus, the good people at the Space Telescope Science Institute just last week implemented the software to deal with charge-transfer problems in the Advanced Camera CCDs, right in the pipeline, improving the image quality a lot (it took months to get to this point with the Hanny’s Voorwerp data). And here it is, Markarian 1498 in a combination of [O III] emission (green) and Hα (red):

This is… interesting. From the few of these galaxies where we have data so far, loops of ionized gas near the nucleus may be a recurring theme. I could add speculations on what we’re seeing in Mkn 1498 – but for now, I’ll just let everyone enjoy the spectacle.

A first Hubble look at UGC 7342

Overnight, Hubble got our first data on perhaps the most spectacular Voorwerpje host galaxy, the merging system UGC 7342. We have to wait until almost the end of the year for what we really wanted to see, the ionized gas. The telescope has particular time pressure in some parts of the sky (as if it doesn’t have extreme time pressure on everything people want to do with it), so we split the two sets of images to fit the schedule better. This time, we got data in WFC3 for two medium-width filters in the orange and deep red, selected to be essentially blind to emission from the gas. These will be used to subtract the contribution of starlight from the gas images, so we can analyze the gas properties cleanly. The emission-line images use the older ACS camera, which has a set of tunable filters which can isolate any optical wavelength we need. They come at year’s end, because we have to specify a particular range of orientation angle of the telescope to fit all the gas in their 40×80-arcsecond filter field. That, plus the requirement that the solar arrays can face the Sun directly, gives us a restricted time window.

As a reminder, here’s UGC 7342 from the SDSS data.

And here is a first look at the Hubble images, warts and all. With only the two filters in orange and deep red, the color information we get is pretty muted. Here’s the whole galaxy, shrunk 4 times from the original pixel scale to fit:

This show the companion and tidal streamers of stars. UGC 7342 itself shows shells of stars, which can be formed when a lower-mass disk galaxies merges with a more massive elliptical. With some additional velocity information, those might be able to give a time since the merger (with some tailored simulations). The elliptical reflections are from bright foreground stars outside this trimmed view; the emission-line images will at least have these in different places, using a different camera and different telescope orientation.

Zooming in 4 times to the nucleus shows that UGC 7342 has complex dust lanes crossing in front of the core. These are perpendicular to the directions where we see that distant gas is ionized by radiation from the nucleus, which is pretty common. The fact that the dust (and almost certainly associated gas) wraps at right angles to most of the structure in the galaxy is another indication that a merger took place recently enough that the situation hasn’t settled down into a long-lasting remnant.

Next up? The scheduling windows for SDSS 1510+07, NGC 5972, and the Teacup AGN all happen in overlapping spans from June to August. Bring on the bits!

Kitt Peak wrapup – for now

The end of this observing run made up for the unnecessarily interesting variety of weather early on. Calm, clear skies, ran through an object list just as fast as we said in the proposal. Here’s a montage of the red-light images from 9 of the target galaxy pairs done on night 4.

Overlapping galaxy pairs from KPNO 2.1m telescope

That’s it for this run. But we’ll be back there late next month to really clean up the target list.