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Gems of the Galaxy Zoos: help pick Radio Galaxy Zoo Gems!

Help vote for Radio Galaxy Zoo Gems!

At the Galaxy Zoos (both at Galaxy Zoo & Radio Galaxy Zoo), we are fizzling with excitement as we prepare for observations using the Advanced Camera for Surveys instrument on board the Hubble Space Telescope. These new Hubble maps will have greater resolution than those that we have from the Sloan Digital Sky Server.

As mentioned by Bill’s blogpost, we have been allocated fewer observing slots than our full list of candidates. Therefore, we invite all of you to help shape the observing priorities of our current target list. You will help determine which host galaxies would gain the most from these Hubble observations (and thus have highest priorities on the target list).

The main science targets specific to these Hubble observations are the host galaxies of Green Double Radio-lobed Active Galactic Nuclei (Green DRAGN — pronounced Green Dragon) and Spiral Double
Radio-lobed Active Galactic Nuclei (S-DRAGN).

Figure 1: Example of a Green DRAGN that is also a Hybrid Morphology Radio Source (HyMoRS) found by Radio Galaxy Zoo (Kapinska, Terentev et al 2017)

Green DRAGN — The prominent green appearance in these DRAGN host galaxies come from the strong [OIII] emission line that dominate the emission in the Sloan r-band. Therefore, these galaxies appear very green in a Sloan 3-colour (g,r,i)  image due the lack of equivalently-strong emission in the Sloan g– and i– bands (the blue- and red- filters, respectively).  The Green Pea galaxies (Cardamone et al 2009) from the original Galaxy Zoo project are a class of green galaxies that appear to be dominated
by star formation. On the other hand, the Green Bean galaxies (Schirmer et al 2013) are thought to consists of quasar light echoes (eg Galaxy Zoo’s Hanny’s Voorwerp).  However, the original Green Bean population show little to no emission at radio wavelengths.

In Radio Galaxy Zoo, we have found a population of Green Bean-like galaxies which host bright radio lobes. Therefore, what sort of feedback are galaxies getting from these “radio-active” Green DRAGNs and how do they relate to the other green galaxies and our understanding of galaxy evolution? Figure 1 shows an example of a Green DRAGN that also happened to be a Hybrid Morphology Radio Galaxy
found by Radio Galaxy Zoo and published by team scientist Anna Kapinska in collaboration with citizen scientist Ivan Terentev (see blogpost on their paper).

Figure 2: An example S-DRAGN that is radio galaxy 0313-192 where VLA observations have been overlayed in red over an HST ACS image. (More details can be found in Bill’s paper: Keel et al 2006)

Spiral DRAGN — Typically, radio galaxies with big radio jets and lobes are hosted by early-type galaxies. Spiral galaxies are often thought to not be “mature” or massive enough to host giant radio lobes.  However, a few S-DRAGNs have been found in the past by our very own Bill Keel (Keel et al 2006, see Figure 2) and Minnie Mao (Mao et al 2015).  To shed light on this rare phenomena,
we seek your help through Radio Galaxy Zoo and this observing programme to assemble a more statistically significant number of this rare class of objects. Figure 2 shows a combined HST and VLA map of the S-DRAGN
published by Bill in 2006.

We have to finish this priority selection by the 16th February 2018. So, please help vote now by clicking hereWe have uploaded the targets in batches of 24 and so please click on all the batches for a view of the full target list.  A handy tip for inspecting these images is to ensure that your screen brightness is adjusted to its maximum because many of the host galaxy features can be very faint. 

We thank Radio Galaxy Zooites, Jean and Victor, for their immense help with assembling the priority selection project interface.  You can track what Hubble is observing by proceding to the Hubble archive link or the Hubble Legacy Archive interface here.

 

Gems of the Galaxy Zoos – help pick Hubble observations!

Galaxy Zoo and Radio Galaxy Zoo participants have an unusual opportunity to help shape a list of galaxies to be observed by the Hubble Space Telescope, as part of the “Gems of the Galaxy Zoos” project.

The project came about when the Space Telescope Science Institute circulated a message in August of 2017, seeking proposals for a new category of observation – gap-fillers. These projects will provide lists of target objects around the sky for brief observations when high-priority projects leave gaps in the telescope schedule, allowing 10-12 minutes of observation at intermediate places in the sky. Read More…

New Hubble+Gemini results – history of fading AGN

Just in time to brighten our holiday season, we got word that the Astrophysical Journal has accepted out next paper on the Voorwerpje clouds around fading active galactic nuclei (AGN). The full paper is now linked on the arXiv preprint server.

This time, we concentrated on the clouds and what they can tell us about the history of these AGN. To do this, we worked pixel-by-pixel with the Hubble images of the clouds in the H-alpha and [O III] emission lines, augmented by a new (and very rich) set of integral-field spectroscopy measurements from the 8-meter Gemini North telescope, velocity maps from the Russian 6-meter telescope, and long-slit spectra from the 3-meter Shane telescope at Lick Observatory.

To examine the history of each AGN, our approach was that the AGN had to be at least bright enough to ionize the hydrogen we see glowing at each point at the time the light reaching that point was given off. Certainly we can’t expect each piece of the cloud to absorb all the deep-UV radiation, so this is a lower limit. Two external checks, on quasars unlikely to have faded greatly and on the Teacup AGN which has had detailed modeling done from spectra, suggests that the very brightest pixels at each radius absorb comparable fractions of the ionizing radiation. This gives confidence that we can track at least the behavior of a single object, underestimating its brightness by a single factor, if we look at the upper envelope of all pixels in the H-alpha images. We hoped this would be feasible all the way back to the original Hubble proposal to look at Hanny’s Voorwerp. Here is a graphic from the new paper comparing our AGN in this way. The distance in light-years at each point corresponds to the time delay between the AGN and cloud, and the curve labelled “Projection” shows how much one of these points would change if we view that location not perpendicular to the light but at angles up to 30 degrees each way. To be conservative, the plot shows the data corresponding to the bottom of this curve (minimum AGN luminosity at each point).agnhistories-sm

The common feature is the rapid brightness drop in the last 20,000 years for each (measured from the light now reaching us from the nuclei). Before that, most of them would not have stood out as having enough of an energy shortfall to enter our sample. Because of smearing due the large size of the clouds, and the long time it takes for electrons to recombine with protons at such low densities, we would not necessarily see the signature of similar low states more than about 40,000 years back.

We could also improve another measure of the AGN history – the WISE satellite’s mid-infrared sky survey gave us more accurate measure of these objects’ infrared output. That way, we can tell whether it is at least possible for the AGN to be bright enough to light up the gas, but so dust-blocked in our direction that we underestimate their brightness. The answer in most cases is “not at all”.

New data brought additional surprises (these objects have been gifts that just keep on giving). The Gemini data were taken with fiber-optic arrays giving us a spectrum for each tiny area 0.2 arcseconds on a side (although limited to 3.5×5 arc second fields), taken under extraordinarily steady atmospheric conditions so we can resolve structures as small as 0.5 arc second. We use these results to see how the gas is ionized and moves; some loops of gas that earlier looked as if they were being blown out from the nuclei are mostly rotating instead. Unlike some well-studied, powerful AGN with giant emission clouds, the Voorwerpje clouds are mostly just orbiting the galaxies (generally as part of tidal tails), being ionized by the AGN radiation but not shoved around by AGN winds. This montage shows the core of NGC 5972 seen by these various instruments, hinting at the level of mapping allowed by the Gemini spectra (and helping explain why it took so long to work finish the latest paper).ngc5972-hst-gmos-bta

Work on the Voorwerpjes continues in many ways. Galaxy Zoo participants still find possible clouds (and the moderators have been excellent about making sure we see them). There is more to be learned from the Gemini data, while X-ray observatories  are gradually bringing the current status of the AGN into sharper focus. A narrowband imaging survey from the ground can pick out fainter (and sometimes older) clouds. Colleagues with expertise in radio interferometry are addressing questions posed by the unexpected misalignments of optical and radio structures in some of our galaxies. Finally, the new DECaLS and Pan-STARRS survey data will eventually bring nearly the whole sky into our examination (for a huge range of projects, not just AGN history).

Once again, thanks to all who have helped us find and unravel these fascinating objects!

Eight years, eight Hubble Voorwerpje targets

It’s a week until the 8th anniversary of the launch of Galaxy Zoo.

The Hubble Space Telescope observations of giant ionized Voorwerpje clouds near galaxies with active nuclei, many found for the first time though the effort of Galaxy Zoo participants gives us another 8 – one at the end of a long road of numbers. 16,000 galaxies with known or possible active nuclei, 200 highly-ranked cloud candidates based on input from 185 participants, 50 spectroscopic observations, 19 giant ionized clouds, among which we found 8 with evidence that the nucleus has faded dramatically (and then observed by one Hubble Space Telescope). (You wondered where the numeral 8 would come in by now… and there is another one hidden below.) The first batch of scientific results from analysis of these images was described here, and the NASA/ESA press release with beautiful visualizations of the multi-filter image data can be seen here. As a visual summary, here are the images, with starlight and emission from [O III] and H-alpha shown in roughly true visual color.

Hubble images of Voorwerpje clouds

This project was an outgrowth of the discovery of Hanny’s Voorwerp, which remains probably the signature discovery of Galaxy Zoo. In astronomy, one is a pet rock, ten is a statistically valid sample – so we wanted to know more about how common such clouds might be, and what they could tell us about quasars more generally. Zoo participants answered this challenge magnificently.

The scientific interest in these objects and their history remains intense, and observations continue. I’ve recently finished processing integral-field spectra from the 8-meter Gemini-North telescope, where we have spectra at every point in a small field of view near the nucleus, and just recently we learned that our proposal for spectra in a few key areas at the high resolution of the Hubble telescope has been approved for the coming year.

Even (or especially) for kinds of objects behind its original statistical goals, Galaxy Zoo has provided an amazing ride these last 8 years. Stay with us – and if you see weirdly colored clouds around galaxies, feel free to flag them in Talk!

Hubble science results on Voorwerpjes – episode 1

After two rounds of comments and questions from the journal referee, the first paper discussing the detailed results of the Hubble observations of the giant ionized clouds we’ve come to call Voorwerpjes has been accepted for publication in the Astronomical Journal. (In the meantime, and freely accessible, the final accepted version is available at http://arxiv.org/abs/1408.5159 ) We pretty much always complain about the refereeing process, but this time the referee did prod us into putting a couple of broad statements on much more quantitively supported bases. Trying to be complete on the properties of the host galaxies of these nuclei and on the origin of the ionized gas, the paper runs to about 35 pages, so I’ll just hit some main points here.

Montage of Hubble images of Voorwerpjes

Montage of Hubble images of Voorwerpjes

These are all in interacting galaxies, including merger remnants. This holds as well for possibly all the “parent” sample including AGN which are clearly powerful enough to light up the surrounding gas. Signs include tidal tails of star as well as gas, and dust lanes which are chaotic and twisted. These twists can be modeled one the assumption that they started in the orbital plane of a former (now assimilated) companion galaxy, which gives merger ages around 1.5 billion years for the two galaxies where there are large enough dust lanes to use this approach. In 6 of 8 galaxies we studied, the central bulge is dominant – one is an S0 with large bulge, and only one is a mostly normal barred spiral (with a tidal tail).<?p>

Numerical model of precessing disk of gas from a disrupted companion of NGC 5972

Numerical model of precessing disk of gas from a disrupted companion of NGC 5972

Incorporating spectroscopic information on both internal Doppler shifts and chemical makeup of the gas we can start to distinguish smaller areas affected by outflow from the active nuclei and the larger surrounding regions where the gas is in orderly orbits around the galaxies (as in tidal tails). We have especially powerful synergy by adding complete velocity maps made by Alexei Moiseev using the 6-meter Russian telescope (BTA). In undisturbed tidal tails, the abundances of heavy elements are typically half or less of what we see in the Sun, while in material transported outward from the nuclei, these fractions may be above what the solar reference level. There is a broad match between disturbed motions indicating outward flows and heavy-element fractions. (By “transported” above, I meant “blasted outwards at hundreds of kilometers per second”). Seeing only a minor role for these outflows puts our sample in contrast to the extended gas around some quasars with strong radio sources, which is dominated by gas blasted out at thousands of kilometers per second. We’re seeing either a different process or a different stage in its development (one which we pretty much didn’t know about before following up this set of Galaxy Zoo finds.) We looked for evidence of recent star formation in these galaxies, using both the emission-line data to look for H-alpha emission from such regions and seeking bright star clusters. Unlike Hanny’s Voorwerp, we see only the most marginal evidence that these galaxies in general trigger starbirth with their outflows. Sometimes the Universe plays tricks. One detail we learned from our new spectra and the mid-infared data from NASA’s WISE survey satellite is that giant Voorwerpje UGC 7342 has been photobombed. A galaxy that originally looked as if it night be an interacting companion is in fact a background starburst galaxy, whose infrared emission was blended with that from the AGN in longer-wavelength IR data. So that means the “real” second galaxy has already merged, and the AGN luminosity has dropped more than we first thought. (The background galaxy has in the meantime also been observed by SDSS, and can be found in DR12).

BTA Doppler maps of Voorwerpjes

BTA Doppler maps of Voorwerpjes


Now we’re on to polishing the next paper analyzing this rich data set, moving on to what some colleagues find more interesting – what the gas properties are telling us about the last 100,000 years of history of these nuclei, and how their radiation correlates (or indeed anti-correlates) with material being blasted outward into the galaxy from the nucleus. Once again, stay tuned!

Fibers and Voorwerpjes

While preparing for more observations of the Galaxy Zoo giant AGN clouds (Voorwerpjes), this is a good time to introduce more complete ways of obtaining astronomical spectra. Traditionally, we’ve put a long slit in front of spectrographs, so we can measure everything along that line without worries about overlapping spectra of different objects or pieces of sky. In some cases, as with the optical fibers used by the Sloan Digital Sky Survey, we get the light summed within a circular aperture on the sky (with Sloan, from hundreds of different objects at each pointing of the telescope). But many of the things we want to understand are large and oddly shaped, so these approaches limit us to a very partial view (or to making many observations to cover everything of interest). Enter the Integral-Field Unit (IFU), which is any kind of device that lets us get the spectrum of every point in some region of the sky. They often use fiber optics to rearrange light from the object, so each small region of it comes out at a different place on what would otherwise be the spectrograph slit. After that it all becomes a software problem.

IFUs are becoming more common on large telescopes. We’ve gotten excellent data on some Voorwerpje systems with the unit on the 8-meter Gemini North telescope. Here’s a sample of raw data on UGC 11185. Each horizontal streak is the spectrum of an area 0.2 arcseconds square. The sampling, sensitivity, and image quality are superb, revealing multiple clouds of gas moving within a total span of almost 1000 km/s.

ugc11185obs17quickblue

On the other hand, if we want to use its whole wavelength range, the Gemini device covers only 3.5×5 arcseconds of sky at once. I’m headed to the 3.5m WIYN telescope on Kitt Peak to use a complementary device called Hexpak, newly commissioned by instrument designer Matt Bershady of the University of Wisconsin (who I’ve been emailing about this since I learned of the project three years ago). This fiber bundle plugs into the multipurpose spectrograph kept in a climate-controlled room below the telescope, and combines small and densely-packed fibers in the middle (for things like galactic nuclei, small and bright with lots of structure) and large fibers near the edges (collecting a lot of signal from large diffuse surrounding material – sound familiar?). Matt and his team were able to get a short exposure through thin clouds of UGC 11185 as a feasibility test – here’s a piece of that raw data frame, showing the small central fiber and the larger surrounding ones (which show brighter night-sky airglow lines as well as more object signal; the bright [O III] lines and H-beta are near the middle, with wavelength increasing to the right for each spectrum). I hope to get a lot more data like this shortly.
UGC11185hextest

Elsewhere, the European Southern Observatory has commissioned an enormous IFU, and the Sloan team has rebuilt their fiber bundles so that each one now makes multiple IFUs which can be placed on many galaxies at a time – this part of the Sloan survey extension is known as MANGA. Then there is the Spanish-led CALIFA project for hundreds of galaxies, which has publicly released data for their first two subsets. Then there are SAURON (whose data ca be tamed in software by GANDALF) and the upgrade of SCORPIO-2 and more… Swimming in data as we sift for knowledge, I am reminded of this anonymous computer error message in haiku form:

Out of memory.
We wish to hold the whole sky
but we never will.

Reflections on Voorwerpjes

We’re in the middle of an observing run at the Lick 3m Shane telescope, with the first part devoted to polarization measurements of the Voorwerpje clouds (which is to say, giant clouds of ionized gas around active galactic nuclei found in the Galaxy Zoo serendipitous and targeted searches), and just now switching to measure spectra to examine a few new candidate Voorwerpjes, and further AGN/companion systems that may shed light on similar issues of how long AGN episodes last.

Lick 3m Shane telescope in 2010

Polarization measurements can be pretty abstruse, but can also provide unique information. In particular, when light is scattered, its spectra lines are preserved with high fidelity, but light whose direction of polarization (direction of oscillation of its electric field when considered as a wave) is perpendicular to the angle it makes during this operation is more likely to reach us instead of being absorbed. This is why polarized sunglasses are so useful – glare from such scattering light can be reduced by appropriate orientation of the polarizing filter.

In our context, polarization measurements tell us something about how much of the light we see is secondhand emission from the AGN rather than produced on the spot in the clouds (admittedly as a side effect of the intense UV radiation from the nucleus), and will show us whether we’re fortunate enough that there might be a dust cloud reflecting so much light that we could look there to measure the spectrum of the nucleus when it was a full-fledged quasar. (This trick has worked for supernovae in our galaxy, which is how we know just what kind of supernova was seen in 1572 despite not having spectrographs yet).

Polarization wizard Sebastian Hoenig (now at the Dark Cosmology Center in Copenhagen) has already produced preliminary calibrations and maps from these new data. Here are some visualizations. In each case, the lines show the direction of polarization. Their length and color show the fraction of light which is polarized at points where there is enough to measure. This fractional polarization tells about the mix of light arising on the spot (even if secondhand due to UV radiation ionizing the gas) and that reflected from dust particles. There is a telltale annular or bull’s-eye pattern when the scattered light originates in a central source, which we see over and over (as if we hadn’t figured out to blame the galaxy nuclei anyway).

First up is a personal favorite, UGC 7342 (the last one to have its Hubble images obtained, and among the largest and brightest of the Galaxy Zoo sample).

UGC7342_20140131_halpha

The next one, Markarian 78, is less familiar, oddly because it makes perfect sense (so it has not figured much in the followup observations). In this case, we see a bright and obvious active nucleus, one which is powerful enough to light up the giant gas clouds without having changed over the past 60,000 years or so.

MRK78_20140131_halpha_2

For comparison, here is a polarization map of IC 2497 and Hanny’s Voorwerp itself, from data obtained last year (the first time the weather let us get useful results). Sometimes we can hear the Universe laughing – a quick simulation shows that the reflected light from the nucleus, when it was a quasar, is just a bit too faint for us to have seen its signature broad emission lines in any of the Voorwerp spectra.

HV_20130116_comb

As we switch into measuring spectra for the next few nights, the aim changes to a combination of looking at a few new Voorwerpje candidates from the Galaxy Zoo forom, and a set of newly-identified AGN/galaxy pairs which may let us study the same issues of AGN lifetimes. We can sort of settle into a routine – Anna Pancoast does calibrations and setup during the day and hands over to Vardha Bennert to finish observations during the night. I typically get to work before Vardha finishes the last galaxy observation (thanks to the time-zone difference) and transfer data to start analysis, so we can change the next night’s priorities if something interesting shows up. It takes a (global) vllage, but then if there’s been any single meta-lesson from Galaxy Zoo, that would be it.

We got (some) observing time!

Great news everybody!

We applied for radio observations with the e-MERLIN network of radio telescopes in the UK. The e-MERLIN network can link up radio dishes across the UK to form a really, really large radio telescope using the interferometry technique. Linking all these radio dishes means you get the resolution equivalent to a country-sized telescope. You don’t alas get the sensitivity, as the collecting area is still just that of the sum of the dishes you are using.

The e-MERLIN network (from www.e-merlin.ac.uk) of radio telescopes.

The e-MERLIN network (from http://www.e-merlin.ac.uk) of radio telescopes.

Our proposal was to observe the Voorwerpjes. We wanted to take a really high resolution look at what the black holes are doing right now by looking for nuclear radio jets. The Voorwerpjes, like their larger cousin, Hanny’s Voorwerp, tell us that black holes can go from a feeding frenzy to a starvation diet in a short time scale (for a galaxy, that is). We really want to see what happens to the central engine of the black hole as that happens. There’s a suspicion that as the black hole stops gobbling matter as fast as it can, it starts “switching state” and launches a radio jet that starts putting a lot of kinetic energy (think hitting the galaxy with a hammer).

So, we want to look for such radio jets in the Voorwerpjes. We asked for a LOT of time, and the e-MERLIN time allocation committee approved our request…

… partially. Rather than giving us the entire time, they gave us time for just one source to prove that we can do the observations, and that they are as interesting as we claimed. So, we’re trying to decide which target to pick (argh! so hard).

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:

bulgeless_agn_2_reallyraw

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:

bulgeless_agn_2_raw

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!