Search results for merger

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!

Galaxy Merger Gallery

I’m Joel Miller, I’m just about to start year 13 at The Marlborough School, Woodstock, and I am here at Oxford University working on mergers from the Galaxy Zoo Hubble data as part of my Nuffield Science Bursary. I have/will be looking at the data and plotting graphs to see how the fraction of galaxies which are mergers changes with other factors therefore determining if there is a correlation between these factors and galaxy mergers. Having looked though many images of merging galaxies I found some really amazing ones.

With some of the images from the SDSS I was able to find high-res HST images of the same galaxy and also find out some more information about them.

Spiral Galaxies NGC 5278 and NGC 5279 (Arp 239) in the Constellation of Ursa Major form an M-51-like interacting pair. This group is sometimes called the “telephone receiver”. The galaxies are not only connected via one spiral arm like M-51, but they also have a dimmer bridge between their disks. Spiral galaxies UGC 8671 and MCG +9-22-94 do not have measured red shifts and therefore there is no data on their distances. They may well be a part of a small cluster of galaxies that includes the “telephone receiver”, but this is not determined at this time.

NGC 5331 is a pair of interacting galaxies beginning to “link arms”. There is a blue trail which appears in the image flowing to the right of the system. NGC 5331 is very bright in the infrared, with about a hundred billion times the luminosity of the Sun. It is located in the constellation Virgo, about 450 million light-years away from Earth.

This pair of Spiral Galaxies in Virgo is known as “The Siamese Twins” or “The Butterfly Galaxies”. Both are classic spiral galaxies with small bright nuclei, several knotty arms, and arm segments. Both also have a hint of an inner ring. The pair is thought to be a member of the Virgo Galaxy Cluster. NGC 4568 is currently the host galaxy of Supernova 2004cc (Type Ic) and was also the host of Supernova 1990B a Type Ic that reached a maximum magnitude of 14.4.

Arp 272 is a collision between two spiral galaxies, NGC 6050 and IC 1179, and is part of the Hercules Galaxy Cluster, located in the constellation of Hercules. The galaxy cluster is part of the Great Wall of clusters and superclusters, the largest known structure in the Universe. The two spiral galaxies are linked by their swirling arms and is located about 450 million light-years away from Earth.

This galaxy pair (Arp 240) is composed of two spiral galaxies of similar mass and size, NGC 5257 and NGC 5258. The galaxies are visibly interacting with each other via a bridge of dim stars connecting the two galaxies. Both galaxies have supermassive black holes in their centres and are actively forming new stars in their discs. Arp 240 is located in the constellation Virgo, approximately 300 million light-years away, and is the 240th galaxy in Arp’s Atlas of Peculiar Galaxies.

With the exception of a few foreground stars from our own Milky Way all the objects in this image are galaxies.

Merger Zoo

Merger Zoo has come to a close.

Since the project started, we have had over 27,000 volunteers contributed their time supporting this project.   Volunteers have reviewed over 3 million simulations.  Out of this huge number of new simulations, we have been able to find the best models for each of 60 different merging galaxy systems using the data you generated in the Merger Wars and Simulation Showdown interfaces.

The two images above show an overlay for two of the best simulation from Merger Zoo.   As the image fades between the simulation and the astronomical image, you can see how closely we matched the shapes of the real Merging Galaxies.   Of course, the underlying purpose of this Merger Zoo was not to make pretty models.   We are now in the process of analyzing the incredibly rich data set that has been generated to address a number of scientific questions.

The first paper we are working on addresses a simple question – how well can the orbit of the mergers be constrained from the shape of the tidal features?   For decades we have been assuming that there is a true “best fit” orbital match for interacting galaxies.    However, Merger Zoo has directly put this to a test.   As an example, take a look at the plot below.    The red line shows the distribution of different disk crossing angles (inclinations) from all the orbits that were viewed by our volunteers.   The green line shows the states that were actually selected and survived the first rounds of the Merger War’s competition on the site.    Even though none of the volunteers ever look at the inclination angle, the states our volunteers have selected are converging toward a single best angle.

The uniqueness of merger orbits is only the first of many of papers that we are working on.   We are also looking how the star formation rates in mergers depend on the orbits between the two galaxies.  We have come a long way on this analysis, and seem to be close to some nice results.   We are also looking at ways to automatically model merging galaxies using computer vision.   The Citizen Science data from Merger Zoo will be used as the training set for the computer vision program.

When Anthony and I look at this Merger Zoo today, we are thrilled with the quality and quantity of the data that you have generated in this project.   I have wanted to have models for a large system of galaxy interactions for decades to test some of these difficult questions.     Without your help, creating this set of models would not have been possible.    With all these data that has generated, the hard work for Anthony and I is really just beginning.   We will be spending our time to make sure we turn your time and effort into scientific knowledge.   Of course, we will keep you informed as this process continues and results are published.

Thank you for all your help in this project!

John and Anthony, The Merger Zoo Team

Chandra X-ray Observations of Mergers found in the Zoo Published

I hope you all had clear skies during the Transit of Venus. If not, it’ll be over a hundred years before you get another chance…. and in Zoo-related news, the Transit of Venus is an example of one way we find planets around other stars. We look for a dip in the brightness of the star as a planet moves across it from our point of view. Want to know more? Head over to the Planethunters blog, or put in some clicks looking for transits yourself!

So, in actual Galaxy Zoo news, I am very happy to report that the latest Galaxy Zoo study has been accepted for publication in the Astrophysical Journal. As we blogged a while back, we got Chandra X-ray time to observe a small sample of major mergers found by the Galaxy Zoo to look for double black holes. The idea is to look for the two black holes presumably brought into the merger by the two galaxies and see if we find both of them feeding by looking for them with an X-ray telescope (i.e. Chandra).

The lead author of the paper is Stacy Teng, a NASA postdoctoral fellow at NASA’s Goddard Space Flight Center and an expert on X-ray data analysis. In a sample of 12 merging galaxies, we find just one double active nucleus.

Image of the one merger with two feeding black holes. The white contours are the optical (SDSS) image while the pixels are X-rays. The red pixels are soft (low energy) X-ray photons, while the blue are hard (high energy) photons. You can see that both nuclei of the merger are visible in X-rays emitted by feeding supermassive black holes.

We submitted the resulting paper to the Astrophysical Journal where it underwent peer review. The reviewer suggested some changes and clarifications and so the paper was accepted for publication.

You can find the full paper in a variety of formats, including PDF, on the arxiv.

So what’s next? We submitted a proposal, led by Stacy, for the current Chandra cycle. To do a bigger, more comprehensive search for double black holes in mergers to put some real constraints on their abundance and properties. We hope to hear about whether the proposal is approved some time later this summer, so stay tuned and follow us on Twitter for breaking news!

The Finale of Merger Zoo

Working toward Fitness – 

Over the last year, we have been pretty quiet at the Merger Wars site.   However, we have been extremely busy analyzing the data that you have created.     So far, the Merger’s Applet has been used to view over 3 million simulations of interacting galaxies.   We have estimated it actually simulated more than 100 million other systems that weren’t viewed by our users.

Of the 3 million simulations viewed, around 60 thousand were selected by the volunteers as interesting. Initially we thought the Evaluate activity within the applet would be sufficient to help us identify the top simulations for each pair of galaxies. However, with millions of simulations to sort through, across tens of thousands of sessions, we discovered that our initial plan was not sufficient. That’s when we decided to add the Merger Wars activity.

In Merger Wars volunteers judge a series of head-to-head competitions to determine which simulation is a better match to the target image of a galaxy merger. Over time, as the simulation competes multiple times, it earns a win/loss record. The percentage of times the simulation has won its competitions can be treated as a fitness value. A value of 1 is a perfect score, all wins, and a value of 0 is a terrible record of all losses. With over 800 thousand Merger Wars competitions judged, our volunteers were able to help us assign a fitness value to each of the 60 thousand selected simulations. These fitness values allowed us to further refine our models for each merger. In total, we identified 290 top simulations for the combined set of 54 pairs of galaxies. However, we need some final help finding the very best model for each system and finding out which collisions have the very best models.

 One Orbit to Rule Them All-

Ideally, there would be a single set of orbit parameters to describe the paths the two galaxies take when flying past each other, and eventually towards their ultimate merging into one galaxy. It is difficult for researchers to know for sure if they have found the single best set of parameters. Is there a better set of orbit parameters? Are their multiple sets of equally good parameters? Are there no good sets? We can call this problem the issue of determining uniqueness. The volunteers for Merger Zoo have achieved an unprecedented level of study for each of these 54 systems. Typically researchers will look at a few dozen to a few hundred simulations of interacting galaxies and pick the best orbit from that sample. Together, we’ve reviewed on average over 50 thousand simulations for each pair and selected over 1000 simulations for further study. We’ve taken the multiple sets of orbit parameters identified for each system and examined them to see how well they’ve identified a single, best-fit orbit. When we look at the entire sample, we don’t see a single orbit. However, if we begin to exclude some of the sample by filtering out the low fitness simulations, we see the range of orbit parameters becomes smaller. If we increase the fitness value used in that filter, we continue to see smaller and smaller ranges of values. In this manner, we can say that we see convergence towards a small range of values for each of the orbit parameters. Arp 82, the image from the top of the post, is a good example of this convergence.    For each population we show a box plot describing the distribution of the parameter. The box represents the range of data from 0.25 to 0.75 of the population. The horizontal line is the median, and the thin whiskers show the outliers. The populations shown are all states viewed by the users, all states selected by the users, and then several populations filtered by fitness to include the to top 50%, 25%, 10%, 5%, 2%, and finally the top 1% of simulations by fitness. The distribution of values describing the time of closest approach demonstrates some convergence. We see that the applet sampled a range of orbits that had times of closest approach ranging between 60 to 600 million years ago. By the time we filter to just the top 1%, we see the range is now only 100 to 250 million years ago with the likely range of 120 to 162 million years ago. Hancock et al find a time since closest approach of around 200 – 250 million years.  The ratio of masses between the two galaxies converges in the same way.

Your Help is Needed-

The big thing we need help with is figuring out THE very best model of each system and comparing models of different systems.    The Merger Wars site has a couple of new interfaces that are now posted.   They include:

  • Pick the Best: find the best model for each system
  • Simulation Showdown: comparing the simulations from two different galaxies to find out which systems have the best models
  • Merger Wars – HST :  the last batch of Merger Wars results from non-Sloan images
  • My Mergers: This is a new update that shows your contributions to the project.  [Note: you need to be logged in for this link to work.]

After we get this final set of data, we will be archiving the site and writing a set of papers based on this work.  However, your help is really needed with this final part of our analysis.

A Zoo of Mergers-

The image below is a combined image of 54 thumbnails showing all of the SDSS galaxy pairs studied in detailed by our Merger Zoo volunteers. Clicking on the image below will take you to our updated Gallery. From that page you can click on each individual thumbnail to see the top simulation results.    You have done an amazing job with this project.   Thanks so much for your help.

Galaxies Modeled in Merger Zoo

John and Anthony, The Merger Zoo guys

The infrared properties of mergers

Another update from Alfredo Carpineti:

Following the previous post, we continue the analysis of galaxy mergers in the infrared.

We want to understand where our galaxies stand with respect to other mergers and other infrared luminous galaxies. Using infrared radiation we can extrapolate the number of stars produced by a galaxy every year, namely the star formation rate(SFR). This number is really important since the star content of a galaxy modifies  both its colours and its intrinsic properties. The average star formation rate is around 15 solar masses per year, which is high, considered that the SFR for a common galaxy is of 1-2 solar masses per year. 

Let’s compare now the SFR with the mass. We can use two parameters to define the mass of a merger: the total mass and the mass ratio. The total mass is the sum of the masses of the two galaxies while the mass ratio is the ratio between the two masses. If a merger had a mass ratio between  1:1 and 1:3 is called a major merger, otherwise it’s a minor merger. 
From the plot you can see that we don’t find any correlation between SFR and mass ratio, while we see a clear trend with the total mass.

Another interesting parameter is the environment density. Density variations give way to difference in the tidal forces, approaching velocities and concentration of intergalactic gas and dust. These could lead to a dependence of the SFR on the environment. When we looked for it we found no such thing. The star formation rate seems independent of environment. 

Seeing mergers in a different light


My name is Alfredo and I’m a Ph.D. student at Imperial College London. I’ve been asked to write a blog about how we take an idea and turn it into a paper, showing exactly what the man behind the scene does.

I’m working with galaxy mergers so the field from which we are going to pluck our idea has to be that one. Merger properties have been described extremely well by the Galaxy Zoo team, which used the Sloan Digital Sky Survey optical data so we thought it might be interesting looking at the GZ merger catalogue in different wavelengths, specifically in the infrared.

You can study pretty much every object in the infrared because what we call heat is simply the emission of infrared light. If you can measure it’s temperature then it radiates in the infrared. In astronomy infrared radiation allow us to see objects that are not too bright in the visible spectrum (cold stars, gas clouds), to probe regions that are obscure in the optical and to explore the early Universe. Our project will use the infrared fluxes to extrapolate interesting characteristics, mostly to do with the star formation process of the galaxies.

In the past, a huge number of papers have shown that galaxies which were very bright in the infrared ( called LIRGs – Luminous infrared galaxies, U(ltra)LIRGs and H(yper)LIRGs) were mostly mergers or post-mergers. We are going in the opposite direction: since we have a strong visually selected merger catalogue, thanks to your hard work, we can now see what’s the real connection between mergers and warm galaxies.

Galaxy Crash Debris: Post-merger Spherodials paper now out!

Today’s post is by Alfredo Carpineti, a Ph.D student at Imperial College:
I’m happy to inform you that a paper on the properties of spheroidal post-mergers (SPMs) has been accepted for publication by the Monthly Notices of the Royal Astronomical Society. The arXiv link to the paper is
We are interested in post-mergers because we want to study in the hierarchical model of galaxy evolution and understanding the evolution of galaxies along a merger sequence is necessary to achieve this. We define post-mergers as single-core galaxies with tidal feature or disruption that can only be explained as merger related.

The specific subset we chose are the likely predecessors of elliptical galaxies, and we compared them to the general merger and an elliptical control sample to see how the properties of galaxies evolve along the merger. The SPMs are part of a sample classified by Galaxy Zoo as post-mergers. We looked at this sample again and we picked the ones which look mostly bulge dominated, a key feature of galaxies that are likely to be precursors of elliptical galaxies. You can see in the figure below how, even though these galaxies are similar in morphology to elliptical galaxies, they appear to be in the process of relaxing into relaxed ellipticals.

In our paper we found that the vast majority of the SPMs inhabit low-density environments and that they sit between mergers and ellipticals in colour space, which indicates that the peak of star formation activity takes place during the merger phase. However the AGN fraction peaks in the post-merger phase (compared to the mergers) suggesting that the AGN phase probably becomes dominant only in the very final stages the merging process.
In general the SPMs are very active, with 84% of the galaxies in our sample showing some emission-line activity compared to the 63% of the mergers and the 27% of the relaxed ellipticals.  The post-merger phase might be less showy than the merger phase, but it’s clear that the dust is yet to set in these galaxies.
Finally we compared the colours of the SPMs to models in which a young stellar population (with an age of 500 million years) is superimposed on an old population that forms 10 billion years in the past (since the bulk of the stars in elliptical galaxies are known to be old). We found that, under these assumptions, the vast majority of the SPMs are likely to have formed more than 5% of their stellar mass in the recent merger-driven burst. Since ellipticals themselves are rather gas-poor objects, our results indicate that ∼55% of the SPMs are products of major mergers in which at least one of the progenitors is a late-type galaxy.

Update on the "Violin Clef" merger: redshifts and Merger Zoo

Hi everyone,

Since I haven’t posted here before, I’d like to introduce myself. My name is Kyle Willett, and I’m a postdoc working at the University of Minnesota in Lucy Fortson’s group. My work for Galaxy Zoo includes development of the next generation of tools that Zooites can use to explore galaxies and conduct their own research. My own scientific focus is on high-energy active galaxies, for which our group is using Sloan and Galaxy Zoo data to try and quantify the environmental properties.

For this post, I’d like to talk about follow-up work we’ve been doing on a recent discovery. About a month ago, Galaxy Zoo contributor Bruno discovered an example of a spectacular merger in the Sloan DR8 data that looked like a triple, or possibly quadruple system. It’s been informally dubbed the “Violin Clef” or the “Integral” based on its shape:

SkyServer image:

This system is scientifically interesting for several reasons. While merging galaxies are common throughout the universe, the merging process is relatively quick compared to the total lifetime of a galaxy. Catching a system with long tails and multiple companions is rarer, and gives us the chance to match our models of galaxy interaction against a system “caught in the act”. This is one of the main drivers of Merger Zoo, and a system like this is a good test to see if we can reproduce the tidal features. If so, then we can start to think about the bigger picture, and predict how often you’d expect a multi-galaxy merger like this to occur.

We’re also interested in the gas and stellar content of the galaxies and their tails. In most merging systems, gas in the galaxies is gravitationally compressed, which leads to a burst of new star formation in the galaxies and their tails. Since this results in more young and hot stars, the colors of these galaxies are typically blue in the Sloan bands. However, all four galaxies and the tidal tails in this system are red. If that’s the case, then we want to estimate the current age of the system. Were the galaxies all red ellipticals to begin with, with very little gas that could form new stars? Or has the starburst already come and gone – and if so, how long-lived are these tidal tails going to be?

After Bruno’s discovery, the team started by looking at what other archived observations could tell us. An ultraviolet image from the GALEX satellite showed no strong UV source in the system. Radio observations showed a point source in the system that might be consistent with weak star formation. This convinced us that we needed an optical spectrum of the system.

Spectra give several crucial pieces of information – first, by measuring redshifts we can determine an accurate distance. This tells us whether all four galaxies genuinely belong to a single interacting group, or whether some appear in projection. Knowing the distance, we can also use the UV and radio flux measurements as diagnostics of the total star formation rate. Finally, with really accurate spectroscopy, we might be able to measure the kinematics of the galaxies, and measure the velocities to get a 3-D picture of how the four members are interacting.

Since Sloan doesn’t have a spectrum of this system, we needed more observations. Danielle Berg, a graduate student at the University of Minnesota, observed the Violin Clef in September using the 6.5-meter Multiple Mirror Telescope in Arizona and obtained two optical spectra.

Raw optical spectra of the Violin Clef

The analysis has shown that all four galaxies lie at the same redshift (z=0.0956 +- 0.002), and are likely all genuine members of the same group. None of the galaxies show evidence of strong star formation, confirming the red colors that we see in the Sloan data.

The next step in the analysis will be working with simulations like the ones in Merger Zoo. Having confirmed that this really is a quadruple merger will significantly constrain the merger models, and hopefully give us well-defined parameters for the age and history of the system. This is a step that Zooites can help with – if you go to, you can identify simulations that resemble the Violin Clef. We need more clicks at this point, so please consider going to Merger Zoo and helping out! We hope that this will result in another scientific publication soon for the Galaxy Zoo team, and it’s been an exciting project to work on.

– cheers,


Citizen Science in Action: the "Violin Clef" merger

Just a few days ago, long-time forum member Bruno posted a curious galaxy as his choice for “Object of the Day” for September 9th. Ahd what a strange merger it is!

(SDSS Skyserver link:

These are some really beautiful tidal tails. They are extremely long and thin and appear curiously poor in terms of star formation (very little blue light from young stars), which is odd since mergers do tend to trigger star formation. There is no spectrum so we do not know the redshift of the object. It is also not clear if the objects at either end are associated or just a projection.

There are photometric redshifts, then the whole system is over 110 kiloparsec across (that’s almost 360,000 light years!) which is big enough to catch even the attention of astronomers.

The “violin clef” merger also has a curious NVSS radio counterpart. What that is all about we don’t know yet – it could be a signal from star formation, or it could be a feeding black hole. The Galaxy Zoo team spent over half an hour discussing this object during a telecon meeting yesterday and we’re all excited.

We still know very little about the system, so if you want to help us figure out what’s going on here, why not head over to the Merger Zoo and simulate the cosmic collision that gave rise to this beautiful and enigmatic object:

Happy galaxy smashing!