Dark Galaxies

Jeff Miller here, one of the two astronomers from St. Lawrence University. I’m here with my SLU colleague Aileen O’Donoghue and a group of faculty and students from Union and UPR who are observing during the week of Thanksgiving. Becky Koopmann once again serves as our fearless leader (although why she puts up with me, I’ll never know). There’s nothing better than leaving the cold, icy weather of northern New York behind to bask in the sunshine and radio waves streaming towards Arecibo!

Aileen and I are enjoying our first night of observing, and things are going very well. There was a slight break in the observing while technicians investigated a strange noise in the dome. Fortunately, it turned out that nothing was found, and observing resumed. We’ve had many detections tonight, and consumed many salty snacks. One of the more exciting detections was a “dark” source that Martha suspects is an OH megamaser. A very exciting observation, indeed! We’re here for two more nights before the crew from St. Mary’s arrives, so we’re looking forward to more exciting discoveries.


SLU astronomers Jeffrey Miller & Aileen O’Donoghue


Observing on the third night: Jeff Miller (SLU) and UPR students Jorge Padial, Jonathan Perez and Andres Arrieta

A little more than a year ago, while examining a newly made ALFALFA grid, Riccardo noticed a rather bright source in the constellation of Leo, moving away from us with a velocity of only 264 km/sec. It was not (or only barely) resolved by ALFA’s 4 arcminute beam, and its HI line velocity width was very narrow, indicating either a fully face-on and/or a very low mass object. In fact, it met the criteria of a “ultra compact high velocity cloud”, the targets for which Betsey Adams has been hunting. With such a low redshift, it was not clear whether the signal arose from a cloud in the Milky Way’s halo or a previously unidentified, tiny nearby galaxy. Quick checks of the public Sloan Digital Sky Survey (SDSS) and Digital Sky Survey (DSS) images showed no obvious associated stellar population but a suggestion of some faint, blue-ish emission. Could this really be a very faint, very small Milky Way neighbor, a bona fide (almost) optically-invisible (dark) galaxy? We needed to determine its distance and to look for evidence of rotation (which would suggest the presence of dark matter), so the quest to obtain the required additional observations began.

Optical image of the starlight in Leo P.

Optical image of Leo P showing its starlight

Being able to respond quickly to potentially exciting discoveries is one of the reasons ALFALFA is a team effort. So right away, we contacted ALFALFA team members Kathy Rhode and John Salzer at Indiana University, because IU has access to good imaging instruments on the WIYN (Wisconsin-Indiana-Yale-NOAO) 3.5 meter telescope in Arizona, and John Cannon at Macalester College who has been undertaking the SHIELD (Survey of HI in Extremely Low mass Dwarfs) program with the VLA. We made a special plea to the director of the VLA for “director’s discretionary time” to take a quick peek at the HI source to look for rotation. Knowing us not to ask without good reason, the director approved our last minute request. Although we were sure of the reality of the signal, the (awesome) Undergraduate ALFALFA team, during one of the ALFALFA followup runs took a spectrum centered on the optical object to confirm the position and radio characteristics of the ALFALFA signal. Kathy and John S. were able to take some quick images during an already-scheduled observing run. Indeed there were stars, and even more importantly, not very many! And a single HII region, proving that star formation is taking place. The VLA observations were made a few months later and the map that John C. and his student Elijah Bernstein-Cooper (read Elijah’s comments in an earlier post) resolved and localized the HI gas and confirmed that the object is rotating. Rotation signifies the presence of a significant amount of dark matter proving its extragalactic nature. A truly tiny object, Leo P contains only a few hundred thousand stars, in contrast to the Milky Way’s tens of billions, but Kathy and John were able to tease out an H-R diagram of the stars, yielding a distance of about 1.75 Mpc (or 5 million light years). So, while Leo P meets Betsey’s criteria to be an ultra compact high velocity cloud, it is also a bona fide galaxy, discovered because of its hydrogen gas, not its starlight. In fact, it contains more mass in gas than in stars. Most recent spectroscopic observations made by another ALFALFA team member Evan Skillman of the University of Minnesota confirm its pristine nature as an object that has undergone very little enrichment in heavy elements due to nucleosynthesis in stars, earning it the designation “P” for “pristine”. We believe that Leo P has managed to retain its gas without forming stars because, in contrast to most dwarf galaxies which reside near large ones, it lives virtually isolated in the local universe, just outside the Local Group.

Leo P is the first example of the class of gas-bearing tiny galaxy for which ALFALFA was specifically designed to look. Betsey’s thesis has already produced a catalog of similar “dark galaxy” candidates even though the survey data processing is not yet complete. As in the case of Leo P, we are pursuing the required detailed observations of the very best candidates (see her post on her March 2013 observing run at WIYN with its new pODI camera). The ALFALFA hunt for (almost) dark galaxies continues, but now we have shown that they do exist and that we can find them!

This post was contributed by Elijah Bernstein-Cooper (Macalester College) during his 2nd trip to Arecibo this year. He and Jesse Watson (U. Wisconsin, Stevens Point) first met at the UAT workshop in January and both volunteered to return to Puerto Rico in March for the ALFALFA followup observations.

Elijah and Jesse discuss neutral gas spectra observed at Arecibo in a follow-up ALFALFA observation with the L-Band Wide receiver.

Elijah and Jesse discuss neutral gas spectra observed at Arecibo in a follow-up ALFALFA observation with the L-Band Wide receiver.

We are now operating in a nocturnal fashion: meeting the bedsheets at 6 AM and having pizza for breakfast (if you can call eating at 1 PM breakfast). Elijah has made many detections while Jesse is lagging (not for a lack of trying), and Parker figures it’s about the magic touch. One night of observing consists of about 100 observations of different sources which Martha has been kind enough to fully map out making the observing run relatively easy and stress free. Jesse and Elijah are doing a push-up after slewing to a new source which helps to keep the heart rate up to combat sleepiness, and is a great way to incorporate exercise into science. As usual, the coquis keep everyone plenty of company throughout the night.

We cross our fingers that we’ll discover a starless galaxy, or a dark galaxy. Jesse and Elijah learned plenty about dark galaxies and follow-up objects during the Undergraduate ALFALFA Workshop in January of 2013. They were so excited about the science they accepted Becky’s and Martha’s offer to observe at Arecibo during their spring break. Weather 50 degrees warmer than their homes made the decision even more attractive.

Overall this has been an amazing experience that never would have happened without both Martha and Becky. We can’t thank them enough. While here we have had the joy of using a professional telescope that not many people in the world have the privilege of using. Additionally we have had the chance to meet a wide variety of incredibly nice and helpful people ranging from the scientists, guard staff, kitchen staff, telescope operators, and of course the other students from around the country. This experience made us even more excited to continue astronomy after undergraduate schooling.

I’ve been at Kitt Peak (along with Mike Jones) for the past several days doing optical follow-up on ALFALFA HI detections. Specifically,we’re searching for stellar counterparts to potential nearby gas-rich low mass galaxies. I’ll write more about the science at some point in the future, but for now I want to focus on the observing we’re doing.


The WIYN telescope looking up through the dome slit. (Photo courtesy Mike Jones.)

We’ve been using pODI on the WIYN 3.5m telescope. ODI stands for One Degree Imager, and pODI is the partially filled ODI – rather than a one degree field of view it has a ~24′ field of view, with a few outlying chips sampling the full field of view. The instrument is still being commissioned, meaning it is offered as “shared-risk” observing. This is pretty common for new instruments. Not only does it allow science observations to begin sooner, it also lets the engineering and instrument team know what needs further work and development. There’s nothing like bringing in an outside observer to break your instrument – they don’t know all the ins and outs and will naively try things that don’t work.


Looking at the primary and tertiary mirrors of WIYN. (Photo courtesy Mike Jones.)

We’ve experienced some of the shared-risk with the “penalty box”. Last night we had to cycle power to the detectors two separate times because some of the controllers stopped responding. Unfortunately, every time you cycle the power you have to wait an hour for the system to become stable. Any data you take during that time can’t be calibrated, so you end up twiddling your thumbs a bit. Following the observer previous to me, I’ve been referring to this waiting time as being in the “penalty box”.

There are other aspects that go along with using an instrument that is being commissioned. For example, not all of the control software is complete yet. We have everything we need to run the telescope, but not everything is integrated fully so we have lots of windows open. Some of the windows also have a few quirks that can be confusing, but we’ve developed a routine and it seems to be working well.


All our monitors and windows. (Photo courtesy Mike Jones.)

While there are always issues that go along with using a brand new instrument, it’s also super exciting. And new instruments bring new capabilities.

This contribution was written by Martha Haynes during the 5th annual Undergraduate ALFALFA team workshop at Arecibo Jan 16-19 2012 when we conducted a first set of observations designed to explore the most interesting candidate signals without optical counterparts.

Despite what people might think, our annual trips to Arecibo in January are not spent lying on the beach under a palm tree. We work hard… and sleep little! In addition to full days devoted to lectures and activities centered on radio astronomy and ALFALFA science plus interesting topics presented by Patrick Taylor (“Planetary Radar Astronomy”), Ellen Howell (“The Geology of Puerto Rico”) and Julia Deneva (“Pulsars”) of the Arecibo staff, plus the great tours of the platform (thanks to Dana again!) and the dish itself, we also have conducted 3 nights of observing. Everyone on the UAT has had a chance to participate in the observations for an hour or so on two different nights, so all of us now are a bit sleep deprived.

Cornell ALFALFA grad students Greg and Betsey led the observing runs from midnight until 7am. Unfortunately, there was a power failure before the end of the run on the 3rd morning, so we lost a little time. But, observing on these three nights has given us the change to test and finalize our observing strategy so that we could write up our documentation and develop some routines to schedule, log and undertake quick checks of data quality. And, furthermore, we also have confirmed the reality of some of the “dark galaxy” HI signals, candidate detections found in the main ALFALFA dataset but without any evidence of a stellar counterpart.

These 3 nights are just the first part of a more extensive program we hope to conduct later this spring, and over the next few years. But even with this short run, we have taken a first significant step forward. By making these “follow-up” observations, we have eliminated any possibility of man-made radio interference as the source of the apparent HI signal. Next we will have to go use other telescopes to look harder at the positions where we detect the HI signal for associated starlight, but now we know where to look: the hunt for “dark galaxies” is ready to continue!!!

This post is by Martha Haynes, just before the 5th annual Undergraduate ALFALFA Team workshop at Arecibo, Jan 15-18, 2012.

The UAT is gearing up for our annual workshop in Arecibo (no problem getting those of us from upstate New York to travel to Puerto Rico in January!). This is the 5th workshop, so things ought to be pretty routine, right?

Well, not really. First of all, every year we have lots of new undergraduate participants and also a few new faculty ones. And, this year, as the ALFALFA main survey observations come to completion, we are starting to gear up for the 2nd phase of observing for ALFALFA: conducting longer observations at the positions of very interesting sources or ones which are just marginally detected. Among the most exciting (to me, at least) are the HI sources that seem to have no optical counterparts and are not near any galaxies at similar redshift. These are the candidate “dark galaxies”: are they real HI detections or were the ALFALFA observations corrupted by (insidious) man-made radio frequency interference (RFI)? We have identified the more egregious or expected RFI, but it can sometimes fool us. So before we get too excited, we will make some short (but still longer than ALFALFA observes a given target — about 40 seconds in total — that’s where the “fast” in ALFALFA comes from) observations with the L-band wide receiver just to confirm that the ALFALFA detection is real. This program requires a different receiver, observing strategy and reduction software, all of which we get to try out during the workshop. So, some of us may not get much sleep. But, I know I’m ready for a little astro-excitement: “A sleepy astronomer is a happy astronomer.” So, let the hunt for dark galaxies begin!

Stay tuned for more once we get assembled in Puerto Rico this weekend.