Using Space Warps to Discover and Weigh Galaxies
John Wheeler once summarized General Relativity as “Matter tells space how to curve, and space tells matter how to move.” While that is a handy description, and while there have been many textbooks written, lectures given and websites constructed to explain this, the quote itself doesn’t address what happens to the light streaming through the universe as it encounters the warped space curved by matter.
The simple answer is: it curves too, and Einstein’s equations provide predictions for exactly how it works. In fact, observations of the bending of starlight around the Sun were one of the first implemented tests of General Relativity, and it passed with flying colors. On the scale of the Universe, the Sun isn’t that massive, but it’s massive enough to bend the light just a little, and by exactly the amount the equations predicted.
Those equations say that more matter in the same place is more likely to produce a strong lens effect, distorting and magnifying a background source. So what happens when you have a *lot* of matter, say, in a big galaxy or a cluster of galaxies?
From left to right: a) an Einstein cross (credit: NASA/ESA); b) an example from the Space Warps dataset; c) a known lens in CANDELS that Galaxy Zoo users spotted.
Some pretty impressive configurations, which are rare but which humans are best at finding — hence Space Warps, the Zooniverse’s newest project and our astronomical project sibling. Co-lens-experts Phil Marshall and Aprajita Verma joined us during this hangout to describe how they use gravitational lenses to weigh galaxies. In particular, they can tell the difference between Dark Matter and “matter that’s dark” — the former being the exotic particles that are very different from stars and gas and planets and people, and the latter being normal matter that isn’t bright, such as brown dwarf “stars” that never actually ignited.
Note: Google+ was feeling a bit out of sorts, so the first minute or so of the broadcast was cut off, during which time Bill Keel showed us the first known image of a gravitational lens, from 1903. We went on to talk about all of the above, and more besides, including the importance of simulated lenses, why the images Space Warps uses are specially tuned to help us find lenses, and how the science team (which includes citizen scientists from Galaxy Zoo!) plan to turn our clicks into discoveries.
(or download the podcast mp3 here)
Notice my swapping of pronouns to “we” — I’m not on the Space Warps science team, but I’ve done nearly 100 classifications now myself! I can’t wait to see the results start to come in from this project.
Update: Peas are a Mess!
Hi all,
About two weeks ago, a group of astronomers led by Ricardo Amorin posted a new paper on the peas to astro-ph. They used the giant Gran Telescopio Canarias (GranTeCan or GTC) to take really high-quality spectra of some of the peas. What they find is amazing, but not entirely unexpected. We already knew from Carie Cardamone’s paper that the peas are extremely intense starbursts, that is, they form more stars relative to their mass than any other kind of galaxy in the nearby universe. Now, Amorin et al. show that they are a real mess:
A GTC spectrum of a pea from Amorin et al. (arXiv:1207.0509) zoomed in on the Halpha emission line. The Halpha line comes from gas ionised by the powerful radiation from very young stars. The high-resolution spectrum clearly shows that the single Halpha line is actually due to several different components.
The Halpha emission lines of the peas, once studies at high resolution and signal-to-noise, show that they are actually composed of several different lines. The Halpha line is generated by the powerful ionising radiation from young, massive stars hitting the surrounding gas. The multiple lines mean that the peas have several chunks of gas and stars moving at large velocities relative to each other.
This makes sense from what we know from the few peas that have nice Hubble images.
Hubble image of a pea galaxy (Amorin et al. (arXiv:1207.0509)) showing that it actually consists of multiple components; it’s a real mess!
The multiple Halpha lines are almost certainly from these multiple components and suggest that the gas (and stars) in the peas are effectively a turbulent mess. Some of those clumps whiz past each other at over 500 km/sec. Yes, km/sec. Some of the Halpha lines are also broadened suggesting that really energetic events are occurring inside those star-forming clumps, such as multiple supernova remnants or powerful Wolf-Rayet stars.
You can get the full paper as PDF or other formats here on arxiv.
Another Galactic Twin
It seems that finding our Milky Way’s twin has become a bit of an industry these days.
NASA/ESA have got in on the act today, releasing a press release about their favourite twin of the Milky Way, NGC 1073 and the below absolutely gorgeous Hubble Space Telescope image they’ve taken of it: Classic Portrait of a Barred Spiral.
Hubble image of NGC 1073: another possible Milky Way twin. Credit: NASA/ESA
And it does look a lot like what we think the Milky Way looks like – except perhaps for having slightly less tightly wound arms.
An artists impression of our Galaxy. Credit: NASA/JPL-Caltech/Robert Hurt (SSC-Caltech)
You might remember, back in September I posted a guest blog by a Lexington Law A-level student, Tim Buckman, who spent his summer with us at Portsmouth finding the Galaxy Zoo galaxy we thought was most like the Milky Way: “A Summer Spent Finding our Galactic Twin“. His project in turn was inspired in part by an ESO press release about spiral galaxy NGC 6744 which was claimed to be a twin for the Milky Way (A Postcard from Extragalactic Space).
NGC 6744 – the previously proposed clone. Credit: ESO.
NGC 6744 is quite a lot more massive than our Milky Way however, so I thought we could do better with SDSS and Galaxy Zoo. Tim applied some mass cuts, then used your classifications to find a face-on 4 armed spiral which he thought matched the maps of the Milky Way (which has a bar, but perhaps a rather weak one which might not be obvious in the types of images we used for Galaxy Zoo).
Tim’s Milky Way clone based on your Galaxy Zoo classifications
I was interested to notice last month that one of the most popular press releases from the AAS this year was about finding a sample of galaxies like our Milky Way and using them to estimate what the colour of the Milky Way would be (BBC Article: Milky Way’s True Colours; AAS abstract it’s based on: What is the Color of the Milky Way?), especially interesting to me as it turns out the Milky Way might be on it’s way to being a red spiral (as has been suggested before, e.g. by Mutch, Croton, Poole (2011), or see New Scientist article about this paper: Milky Way Faces Midlife Crisis), which you might remember I’ve done a bit of work on! 
Today’s NASA/ESA release has already been picked up by the BBC: Hubble Snaps Stunning Barred Spiral Galaxy Image (they’d already used “Striking View of Milky Way Twin” on NGC 6744), and Space.com covers it as Hubble Telescope Spies Milky Way Galaxy Twin.
For Galaxy Zoo people, it should be of interest that the press release also says:
Some astronomers have suggested that the formation of a central bar-like structure might signal a spiral galaxy’s passage from intense star-formation into adulthood, as the bars turn up more often in galaxies full of older, red stars than younger, blue stars.
Well those astronomers are us – Galaxy Zoo results on bars, based on your classifications have shown that bars are more common in redder discs. Thanks again for the classifications which allowed us to do that work.
GZ1 used for the fractions of early-types in clusters
We’re happy to report that we have once again used your (now public) GZ1 classifications to find an interesting result.
We use the classifications in a study we just submitted to MNRAS (or see the arXiv entry for a copy) looking at the observed fractions of early-type galaxies (and spiral galaxies), in groups and clusters of galaxies.
Recent work (De Lucia et al. (2011), which posted to the arxiv in September), used sophisticated semi
analytic models to determine the properties of galaxies found in massive
clusters in the Millennium Simulation. They identified elliptical galaxies
(or more accurately early-type galaxies) in the simulation, and found that the fraction these
galaxies, remained constant with cluster halo mass, over the range 10^14 to
10^14.8 solar masses. They compared their results with previous
observational studies which each contained less than 100 clusters.
With GZ1 we realised we could put together a much larger sample. We
used galaxies with GZ1 classifications, cross matched with cluster and
group catalogues, to compare the above results with almost 10 thousand
clusters. We found that the fraction of early-type galaxies is indeed
constant with cluster mass (see the included figure), and over a much larger range of 10^13 to 10^15
solar masses (with covers small groups of galaxies to rich clusters), than previously studied. We also found the well known result (to astronomers) that outside of groups and clusters, the fraction of early-type galaxies is
lower than inside of groups and clusters.
Plot showing the fraction of early-type galaxies (red lines) as a function of halo mass. We used two different halo mass catalogues, and the agreement between them is excellent. We also examine the fraction of spiral galaxies with halo mass (blue lines)
This work suggests that galaxies change from spiral to early-type when individual
galaxies join together to form small groups of galaxies, but that going from groups to rich clusters does not significantly change the morphologies of galaxies.
Without the GZ1 results at our finger-tips, this work, which was devised,
implemented, and written up in less than 2 months, would have taken much
longer to complete.
Thanks again for making the Zoo such a wealth of information,
Ben Hoyle (on behalf of Karen Masters, Bob Nichol, Steven Bamford, and Raul Jimenez)
Galaxy Zoo at the Durham Galaxy Evolution Conference
I think I won’t get in too much trouble if I say that in my opinion the event of summer 2011 for extragalactic astronomers was a massive international conference which took place in Durham, July 18th-22nd Galaxy Formation. You’ll be happy to know that Galaxy Zoo scientists were represented, with myself, Kevin, Ramin Skibba (who wrote one of the first Galaxy Zoo papers back in 2009), Vardha Bennert (who has done some HST followup for us, she’s profiled in the “She’s an Astronomer” series from 2009) and Boris Haussleur (see his blog posts about Hubble Zoo) all present.
400 Extragalactic Astronomers in Durham. That's me circled in orange, Ramin in pink and Boris in blue. Kevin and Vardha might be there somewhere but I've yet to spot them!
The moment of the conference for me was the first mention of Galaxy Zoo in the plenary talks – my work on the Galaxy Zoo 2 bars (see many blog posts!) was mentioned in a talk on the influence of internal evolution on galaxies (something we call “secular evolution” which bascially means the slow transformation of galaxies by material being moved around by the bars and/or spirals) which was given by Francoise Combes. I got so excited I took a picture of her slide, which you can also see in her talk pdf.
Francoise Combes talking about Galaxy Zoo results on bars
And here’s the slide so you can actually read it.
Francoise Combes's slide so you can actually read it
The red spirals also got a mention in a talk on gas in galaxies (by Luca Cortese – pdf unfortunately not uploaded at time of writing) where it was shown that at least half of them have very low NUV (near ultra-violet) emission for spiral galaxies. This is expected if as we think they are truly passive spirals with very little current star formation (which created NUV light).
Many of the slides for the talks, as well as the posters are available online (including mine, which for once wasn’t about my Galaxy Zoo work, but work with the new SDSS survey which is imaging 1.5 million galaxies at intermediate redshifts – unfortunately as fuzzy blobs, so no new objects for the Zoo from them!). There is also a plan to make video of the talks available. I’ll post an update about that when it happens.
Unfortunately I had to leave before Kevin and Vardha gave their talks on the Friday. Neither of them have posted their pdfs yet either. 
Boris and Ramin had posters – also like me on their non-Galaxy Zoo work (Boris: Measuring the physical properties of galaxy components in modern multi-wavelength surveys, Ramin: Are Brightest Halo Galaxies Central Galaxies?).
It was a great conference and I had a wonderful time in Durham.
