Tag Archive | WIYN

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:

M51

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!

tiredtweet

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:

first_science_exposure_zoomout

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:

first_science_exposure

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!