For the last few weeks, the main Galaxy Zoo has been waiting for new images, but we’re now back with a new site – and new galaxies.
Galaxies first. There are a few extra images from the Sloan Digital Sky Survey to work through, but we’ve also lined up images from the latest Dark Energy Camera Legacy Survey (DECaLS) data release. We’ve looked at DeCALS data before; its images come from a four meter telescope, larger than any we’ve used for Galaxy Zoo before, and so we get to see fainter details and finer structures than would otherwise be possible. The galaxies in the site now have never been inspected before, so do get clicking – who knows what might be there to find?
You’ll also notice the site is different. We’ve moved Galaxy Zoo over to Panoptes, the system that powers almost all of the Zooniverse projects. Moving to this software makes it much, much easier for us to upload new images, and will mean that Galaxy Zoo will continue to be supported for a long time to come. We don’t, as a team, have any funding ourselves for development, so it’s important we use the up-to-date software maintained by Zooniverse. The old site will continue to be accessible at zoo4.galaxyzoo.org.
There are a few downsides. It will take us a little while to translate the new site – those who translated the old one should have received an email but let us know if not. Most importantly, it means a new instance of Talk, the software that hosts discussion and conversation. Our wonderful moderators have been moving important threads across, and the old site will continue to be available at talk.galaxyzoo.org. We’re also still working on the best way to color our new galaxies. For the first few days, you may notice some galaxies with unusually bright colors. These are rare duplicates created using a different color palette – let us know what you think on the new Talk!
With new galaxies and a stable software platform, Galaxy Zoo is in good shape to carry on producing excellent science. Do come and explore!
It’s away! The final observation plan for the Gems of the Galaxy Zoos Hubble program was submitted earlier this week (24 hours before our deadline, I want you all to know).
We collected votes for over 2 weeks, separately for Galaxy Zoo and Radio Galaxy Zoo objects since they needed distinct image layouts. About 18,000 votes were cast. The Talk interfaces turned out to be very useful for immediate practical matters – some users seeing images twice, some cases of the wrong coordinates being used for images that had been uploaded and needed to be replaced, and finding duplicates both within the candidates and versus older Hubble observations. (A major “thank you” to the volunteers who contributed in these ways). I was strongly impressed at the level of discussions on the Talk sites that went into some of these decisions.
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).
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).
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 here. We 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.
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…
We’re excited to announce the publication of another scientific study. that wouldn’t have been possible without the hard work of the Galaxy Zoo volunteers. The paper:
“Galaxy Zoo: Morphological classification of galaxy images from the Illustris simulation”
is the first Galaxy Zoo publication that examines visual morphological classifications of computer-generated galaxy images. The images were produced in collaboration with the international team of scientists who implemented and analyzed the highly sophisticated Illustris cosmological simulation (you can find many more details about Illustris on the main Illustris project website and about the Galaxy Zoo: Illustris project in this previous blog post). Illustris is designed to accurately model the evolution of our Universe from a time shortly after its birth until the present day. In the process, simulated particles of dark matter, gas, and stars aggregate and condense to form galaxy clusters that contain seemingly realistic galaxies. In our paper we wanted to test the realism of those simulated galaxies by inviting Galaxy Zoo volunteers to evaluate their morphological appearance. We wanted to know whether Illustris galaxies look like real galaxies.
But where to start looking? Well, if you’ve ever classified a galaxy on Galaxy Zoo then you must have answered a question worded something like:
Is the galaxy simply smooth and rounded, or does it have features?
This question represents one of the simplest ways to distinguish between different groups of galaxies, but its answer can reveal a lot of information about a galaxy’s history, as well as its current activity. Visible features and substructure like discs, spiral arms and bars in galaxy images often indicate sites of ongoing star formation and can provide evidence for complex dynamical processes within a galaxy. On the other hand, apparently featureless galaxies may have formed in dense environments where galaxy-galaxy interactions are more common and might act to destroy features or even prevent them from forming in the first place.
In our paper, we compared the prevalence of visible features in galaxy images that were produced using Illustris against an equivalent sample of real galaxy images that were derived from Sloan Digital Sky Survey (SDSS) observations. Some of the differences we found were surprising but quite illuminating!
Each image in Galaxy Zoo is classified by about forty volunteers and their votes for each question are aggregated to obtain a consensus. The level of agreement between volunteers can be quantified using the vote fraction for a particular response. For a particular image and question the vote fraction for a possible response is just the number of volunteers who voted for that response, divided by the total number of votes cast for that question, for that image. A concrete example that applies here is the “featured” vote fraction: the number of volunteers who classified a galaxy image as exhibiting visible features divided by the total number of votes cast for the simple question that was quoted above. Vote fractions close to zero indicate that most volunteers thought the galaxy was smooth and rounded, while vote fractions around one imply almost unanimous consensus that a galaxy has visible features.
The filled green bars in Figure 1 illustrate the distribution of this “featured” vote fraction for real galaxy images. The distribution is dominated by a peak close to zero, which means that most volunteers thought that most galaxies looked smooth and featureless. There is also a smaller peak close to one, corresponding to a population of obviously featured galaxies. In contrast, the blue line shows the “featured” vote fraction for Illustris galaxy images. The bulk of the distribution is now peaked around 0.6, which means that Illustris galaxies were generally perceived to be predominantly featured. However, there are very few Illustris galaxies that were unanimously labeled as exhibiting visible features and a substantial population of visibly smooth galaxies is also present. Overall, the Illustris galaxy images seem more feature rich, but perhaps slightly more ambiguous than their SDSS counterparts.
To try to understand the origin of the mismatch between Illustris galaxies and those in the real Universe, we separated both of the image samples into three sub-groups based on the total mass of the stars that the galaxies contain (more succinctly described as their “stellar mass”). Each of the panels in Figure 2 can be interpreted in the same way as Figure 1, except that they correspond only to the galaxies for each of the three stellar mass sub-groups. The two panels to the left are for galaxies with stellar masses less than the mass of 1000 billion suns. They look remarkably similar to Figure 1 with the SDSS and Illustris distributions matching very poorly. However, the situation changes markedly in the right-hand panel. For these extremely massive galaxies, it appears that the Illustris simulation reproduces the observed proportion of visibly featured galaxies much better, although the population of unambiguously featured galaxies is still absent.
The change in behavior with stellar-mass that we have identified might simply be an artifact of the finite resolution at which Illustris is able to simulate the Universe. Computational power is limited, so Illustris cannot accurately model the positions, interactions and evolution of every star in its simulation volume (and of course tracking individual gas atoms or dark matter particles is completely impossible!). Instead, Illustris models large groups of stars, and large accumulations of gas and dark matter as single “particles” and models the way that they interact with each other. The features that volunteers perceive in Illustris galaxy images manifest substructures formed by groups of many such particles. Simulated galaxies with larger stellar masses contain more stellar particles that enable the simulation to model finer structural details which may be necessary to emulate the appearance of real galaxies.
Studies involving automatic morphological classification of Illustris galaxy images (e.g. Bottrell et al 2017, Snyder et al 2015) have also identified a marked divergence with galaxies in the real Universe below the same 1000 billion solar mass limit that we have found. Confirmation that the visual appearance of galaxies also changes perceptibly complements a growing body of knowledge on this subject.
Dust is another constituent of galaxies that can substantially modify their appearance by absorbing bluer light that typically indicates star formation and re-emitting it at redder wavelengths. This dust reddening effect is not accounted for by the Illustris simulations and could obscure the visibility of features that are actually present in real galaxies. This means that Illustris might be modeling real galaxies better than it seems, and coupling of a dust reddening model to the simulation output might improve the correspondence between the mismatched vote fraction distributions at lower stellar masses.
As is often the case in scientific research, an unanticipated result has provided valuable insight. The results from Galaxy Zoo: Illustris will help cosmologists to improve their models as they develop the next generation of large-scale simulations of our Universe. The results also underline the ongoing potential utility for visual morphological classification of simulated galaxies. The most recent cosmological simulations, including a next-generation Illustris Simulation, address many of the shortcomings that this and other studies have revealed. Comparing their outputs with SDSS galaxy images, as well as observational data produced by other surveys, will undoubtedly yield more insights into the processes that govern the formation and evolution of galaxies. Watch this space!
A preprint of the new paper, which has been accepted by the Astrophysical Journal, can be downloaded from the arXiv.
The following blogpost is from Anna Kapinska about the Radio Galaxy Zoo paper that she published recently with Radio Galaxy Zooite, Ivan Terentev on the first sample of candidate Hybrid Morphology Radio Sources (HyMoRS) from the 1st year of Radio Galaxy Zoo results.
Radio Galaxy Zoo scores another scientific publication! The paper ‘Radio Galaxy Zoo: A search for hybrid morphology radio galaxies’ has been published today in the Astronomical Journal. First of all congratulations to everyone, and what wonderful work from all our citizen scientists and the science team! Special thanks go to Ivan Terentev, one of our very active citizen scientists, whose persistent work on finding and collecting HyMoRS in a discussion thread on RadioTalk (link) without doubt earned the second place in the author list of this paper. But of course the publication wouldn’t be possible without all our volunteers, and special thanks are noted in the paper (check out the Acknowledgements on page 14):
“This publication has been made possible by the participation of more than 11,000 volunteers in the Radio Galaxy Zoo Project. Their contributions are acknowledged at http:// rgzauthors.galaxyzoo.org. We thank the following volunteers, in particular, for their comments on the manuscript or active search for candidate RGZ HyMoRS on RadioTalk: Jean Tate, Tsimafei Matorny, Victor Linares Pagán, Christine Sunjoto, Leonie van Vliet, Claude Cornen, Sam Deen, K.T. Wraight, Chris Molloy, and Philip Dwyer.”
But what are HyMoRS? HYbrid MOrphology Radio Sources, HyMoRS or hybrids for short, are peculiar radio galaxies that show atypical radio morphologies. That is, radio galaxies which we can resolve in our observations come in two principal flavours: 1) FRI – type; and 2) FRII-type — named after two scientists who introduced this classification back in 1974, Berney Fanaroff and Julia Riley [link to paper].
Traditionally, FRIs and FRIIs are distinguished by different morphologies observed in radio images, where on the one hand we have archetypal FRIIs showing powerful jets that terminate in so-called hotspots (can be spotted in right panel of Figure 1 as two white bright spots at the ends of the jets), while on the other there are FRIs with their jets often turbulent and brightest close to the host galaxy and its supermassive black hole (left panel of Figure 1). HyMoRS are hybrids, they show both morphologies at the same time, that is they look like FRI on one side and FRII on the other side. Figure 2 shows two examples of the new HyMoRS candidates that Radio Galaxy Zoo identified in this latest paper.
How are HyMoRS formed? We still don’t have a very clear answer to this question. The thing is that there may be many reasons why one radio galaxy would have so radically different looking jets. One possibility is that the medium in which the jets travel through (the space around) is different on each side of the galaxy. In this case the FRI morphology could form if the medium is dense or clumpy for one jet, while FRII morphology could form if the medium is smoother or less dense on the other side for the second jet (but watch this space for more work from our science team). But there are also other options. For example, we may simply see the radio galaxy in projection, or we are observing rare events of a radio galaxy switching off, or switching off and on again. The more HyMoRS we know of, the better we can study them and pinpoint the scenarios of how they form.
For example, the science team at the University of Tasmania has produced a simulation of jets from an FRII-type radio galaxy located in the outer regions of a cluster (~550 kpc from the centre) and expanding in a non-uniform cluster environment. The jet on one side propagates into a much denser medium than the jet on the other side. The jets are very powerful (10^38 Watts) and the total simulation time is 310 Myr. The movies display the density changes associated with the jet expansion. Credit goes to Katie Vandorou, Patrick Yates and Stas Shabala for this simulation (link to simulation).
How rare are HyMoRS? We actually don’t really know, and this is because so far there are very few complete surveys of these radio galaxies. Current estimates indicate that they may be comprising less than 1% of the whole radio galaxy population. We are hoping that with Radio Galaxy Zoo and the new-generation telescopes we will be able to finally pin down the HyMoRS population. And our paper is definitely one big step towards that aim. It’s very exciting as with the fantastic efforts of RGZ we now have 25 new HyMoRS candidates — this could possibly double the numbers on known hybrids!”
So well done everyone and let’s keep up the fantastic work! We couldn’t have done it without you 🙂
Anna, Ivan & the coauthors on this latest paper
The official open access refereed paper can now be found at http://iopscience.iop.org/article/10.3847/1538-3881/aa90b7
The article can also be downloaded from: http://arxiv.org/abs/1711.09611
A CAASTRO story with embedded animation is now available at: http://www.caastro.org/news/2017-hymors
The following blogpost is from Omar Contigiani about the Radio Galaxy Zoo paper that he published recently on the cosmic alignment of radio sources.
In the Radio Galaxy Zoo an incredible variety of creatures can be found — as our citizen scientists might know by now, radio sources in the sky can have all sorts of shape and sizes. The most powerful among them are plasma-filled jets emitted by the some of the largest elliptical galaxies in existence. Because of their precise structure, anyone can associate orientations to these sources by simply looking at the directions the jets point at.
Recently, our scientists have been looking at the directional properties of these fascinating beasts. If a particular source points in a direction, is it possible that its neighbours also tend to do the same? Because the distances between adjacent objects are (quite literally) astronomical, it seems intuitive to assume that the relative orientations should be random. However, nature always finds subtle ways to mess with our intuition and it turns out that this is currently an open question in astronomy. Thanks to Radio Galaxy Zoo’s numerous (almost two million) image classifications, the team was able to report the most precise measurement of this effect to date. The results are available in a scientific article published in Monthly Notices of the Royal Astronomical Society this November.
The analysis performed in the study suggests that relative alignment of radio sources is present on distances which are dubbed as cosmological. This is because only phenomena related to the history of the Universe as a whole are known to be connected to such large scales.
While this is an exciting step towards an answer, formulating any conclusive statement about this alignment and the reasons behind it appears to be difficult. What drives this effect? Is it related to a shared history or environment? More science needs to be done and more galvanising discoveries are waiting for us just around the corner.
Once again, without the contributions made by our volunteers all over the world, we would not have been so successful in our endeavours. A big thank you to all our Radio Galaxy Zooites!
However, we have only reached 74% of our classification target. Head to Radio Galaxy Zoo to become involved and you will be contributing to real science being done today and may be co-authoring another great discovery with us!
Meet Francesco de Gasperin, an associate science team member (since 2015) who is very interested in the classifications resulting from Radio Galaxy Zoo
I am a VENI fellow at the Leiden University in the Netherlands. My research is mainly based on developing and exploiting new technologies in radio-astronomy to study active galactic nuclei (AGN), galaxy clusters, galaxies and ultimately everything which emits radio waves. I am part of the LOFAR collaboration and most of my time is invested in the commissioning of this new radio-telescope. I am now leading the effort to calibrate the low band antennas of LOFAR to observe the sky at decameter wavelength. Our plan is to ultimately produce the lowest frequency radio survey ever done.
I did my PhD at the Max Planck Institute for Astrophysics in Munich on a thesis titled: “The impact of radio-emitting supermassive black holes on their environment: the LOFAR view of the Virgo cluster”. During my master I also worked for the Planck mission, a satellite designed to study the Cosmic Microwave Background (CMB) – the relic radiation from the Big Bang.
What has Francesco and his student, Omar Contigiani done lately ?
Is the orientation of radio galaxies totally random? Or is it driven by the large scale structure where galaxies are embedded in? Recently, some works on small regions of the sky claimed an intrinsic alignment of radio sources. This is in line with the observed alignment between quasar jets and the surrounding large scale structure at higher redshifts.
With the help of the RGZ we are now able to identify the orientation of a very large number of radio galaxies. This allows us to expand these studies to unprecedented scales, moving from regions of few square degrees to around 10 thousands.
Thanks to everyone who has helped with the Galaxy Zoo Literature Search so far. It’s not too late to join in (for full details see the first blog post), but I thought I would write a followup summarising some of the findings so far. Hopefully it’ll help clarify what we’re looking for, and you won’t have to worry about repeating any of the papers we already have.
Academic life is full of lots of different tasks which occupy our time more or less at different parts of the year. It’s one of those things which I enjoy about it so much, but is is also basically my excuse for why work on the paper has ground to a halt in the last month. I have been occupied with teaching undergraduate Computational Physics, SDSS Spokesperson duties, and keeping on top of lots of remote observing with the Green Bank Radio Telescope. All good fun, but not paper writing! However this paper is making it back up to the top of my to do list now (hence this blog post!), so I hope to have more news for you on it soon.
Anyway here’s my summary of the new papers you have found for us in the different categories. We had no submissions yet of new papers defining early-types as E/S0/Sa, or late-types as being only types Sb/Sc/Sd yet, so keep looking for those.
Also as a reminder that when we’re talking about galaxies red = shorthand for not forming stars (or quenched), so sometimes astronomers use red and quenched (or passive) interchangeably. And spectra can also be used to find galaxies which are not forming stars, so spectral type and colour are highly related.
We also have a ton of different ways to talk about galaxy bulge size, from B/T (bulge-total ratio), to various model fits to light profiles (e.g. the Sersic profile, where Sersic n>2.5 is generally take to mean there’s a bulge, and n=1 means a pure disc). Concentration is also talked about – a more concentrated galaxy is more likely to have a bulge, than a less concentrated galaxy. And in SDSS there is a “fracDeV” parameter, which is the fraction of the model light curve made up from a “De Vaucouleur profile”, which is the same as a Sersic profile with n=4. We don’t like to make life too easy for newcomers in astronomy! 😉
The list of papers that:
- Claim that colour and morphology are equivalent
Strateva et al. 2001: https://ui.adsabs.harvard.edu/#abs/2001AJ….122.1861S/abstract
A classic here, and possibly the root of this idea: “We use visual morphology and spectral classification of subsamples of 287 and 500 galaxies, respectively, to show that the two peaks correspond roughly to early- (E, S0, and Sa) and late-type (Sb, Sc, and Irr) galaxies, as expected from their different stellar populations.”
Note that they are defining early- and late-types in the way I would like to find more examples of.
Fang et al. 2013: https://ui.adsabs.harvard.edu/#abs/2013ApJ…776…63F/abstract
This one is a bit complicated. They state in the Introduction that “galaxies above a Sersic value of n ≈ 2.5 tend to be quiescent. “ and go on to claim that having a large bulge is necessary but not sufficient to be quenched (ie. red). Which means they acknowledge the existence of blue ellipticals (or blue galaxies with large bulges), but not red spirals with small bulges.
In their conclusions they state: “our results imply that galaxy quenching requires the presence of a bulge, but that a bulge alone is not enough to ensure the complete cessation of SF. “
This was not exactly what we were looking for, but definitely related, and interesting.
Huerta-Company et al. 2011 http://adsabs.harvard.edu/abs/2011A%26A…525A.157H
Good try, but I’m not sure this quite counts in this category – they use colour as part of their classification scheme (which we would definitely not recommend), but it’s not the only part, so they’re not really saying it’s the same as morphology. “Three types of parameters used to estimate morphology probabilities; “we decided to include three types of parameters: (1) color (g − r, r − i) k-corrected with Blanton et al. (2005) code; (2) shape (isoB/isoA in the i-band and deVAB_i); and (3) light concentration (R90/R50 in the i-band). For color measurements we use modelmagnitudes corrected for galactic extinction. isoB and isoA are the isophotal minor and major axes respectively, and deVAB_i is the DeVaucouleurs fit b/a. R90 and R50 are the radii containing 90% and 50% of the petrosian flux, respectively”
Cooper et al. 2010: https://ui.adsabs.harvard.edu/#abs/2010MNRAS.402.1942C/abstract
In the introduction they say: “Various observational studies have investigated the formation of early-type or red-sequence galaxies “
They then go on to use colour to define their sample: sec. 2 p. 1943:”To isolate the red- sequence population, we use the following magnitude-dependent cut: g − r = −0.02667 · M r + 0.113”
Good find. This definitely just assumes that colour=morphology.
Tal & van Dokkum 2011: http://adsabs.harvard.edu/abs/2011ApJ…731…89T
In the words of our volunteer (Jean Tate):
“Throughout this paper, the authors conflate “massive red” with “elliptical”. This formally begins with the selection criteria (“We selected galaxy images for this study from the Sloan Digital Sky Survey (SDSS, Abazajian et al. 2009) including all objects classified as Luminous Red Galaxies (LRG) that have a spectroscopic redshift measurement. LRGs are intrinsically red and luminous objects that were identified as such from their central surface brightness and location on a rotated color-color diagram (for full details see Eisenstein et al. 2001). This selection is aimed at finding the most luminous red galaxies in the nearby Universe (L 3L⋆) out to a redshift of z = 0.5.”)”) which does not distinguish between red ellipticals and red spirals (nor red S0s). However, the first sentence in the Discussion section strongly implies that massive red S0s and massive red spirals are irrelevant (“The first and foremost result that arises from this study is that faint, gravitationally bound stellar light can be traced in massive elliptical galaxies out to a radius of 100 kpc.”), to take just one example.”
Another good find, and very common that LRG=elliptical is assumed.
2. Define “early-type” galaxy as any galaxy without visible spiral arms (e.g. our “smooth” category, which can include elliptical galaxies, and smooth disks), rather than as a galaxy that isn’t a disk.
So here I basically meant they were defining early-type as E and S0 only, and we got one submission like that:
Stanford et al. 1998: http://adsabs.harvard.edu/abs/1998ApJ…492..461S. “We present results from an optical-infrared photometric study of early-type (E+S0) galaxies in 19 galaxy clusters out to z = 0.9.”
“In this paper, we use the term ““early-type” galaxy to refer to those galaxies classified morphologically as having Hubble classes E, E/S0, or S0. “
3. Define “early-type” galaxy as including Sa spirals as well as lenticular and ellipticals.
No submissions yet – although note that Strateva et al. 2001 (above) do this.
4. Define “late-type” galaxy as only late-type spirals (e.g. excluding Sa spirals)
No submissions yet – although note that Strateva et al. 2001 (above) do this.
5. Use colour or spectral type to split galaxies into “early-“ or “late-“ types (or “elliptical” and “spiral”)
van den Bergh 2007: http://cdsads.u-strasbg.fr/cgi-bin/nph-bib_query?2007AJ….134.1508V&db_key=AST&nosetcookie=1 actually has a contrary view, claiming that “there appears to be no obvious dichotomy between the morphologies of galaxies that are situated on the broad blue and on the narrow red sequences in the galaxian color-magnitude diagram.”
Bell et al. 2004: http://adsabs.harvard.edu/abs/2004ApJ…608..752B
Exactly what we were looking for. They use colour to split their sample into “late” and “early” types, saying:
“”We find that the rest-frame color distribution of these galaxies is bimodal at all redshifts out to z ~ 1. This bimodality permits a model-independent definition of red early-type galaxies and blue late-type galaxies at any given redshift. ”
Cooper et al. 2010: https://ui.adsabs.harvard.edu/#abs/2010MNRAS.402.1942C/abstract
Another good example of using colour to define an early-type sample:
sec. 2 p. 1943:”To isolate the red- sequence population, we use the following magnitude-dependent cut:
g − r = −0.02667 · M r + 0.113”
In doing this they are assuming that red = early-type. In the introduction they say: “Various observational studies have investigated the formation of early-type or red-sequence galaxies “
Almedia et al. 2011: http://adsabs.harvard.edu/abs/2011ApJ…735..125S
Actually seem to be looking into how well the correlation between spectral type and morphology works, saying: “Given a spectral class, the morphological type wavers with a standard deviation between 2 and 3 T types, and the same large dispersion characterizes the variability of spectral classes given a morphological type.”
So this is actually evidence that morphology and spectral type are not completely equivalent.
Ascasibar & Almeida 2011: http://adsabs.harvard.edu/abs/2011MNRAS.415.2417A
But now in this one, the claim is that the correlation is good enough to use spectral classifications to place galaxies into a morphological sequence: “Considering galaxy spectra as multidimensional vectors, the majority of the spectral classes are distributed along a well-defined curve going from the earliest to the latest types, suggesting that the optical spectra of normal galaxies can be described in terms of a single affine parameter.”
6. Use the bulge-to-total ratio (or some proxy for it like concentration, or the SDSS “fracDeV” parameter) to place spiral galaxies in a sequence.
Goto et al. 2003: http://adsabs.harvard.edu/abs/2003MNRAS.346..601G
p. 604 : “Based on the Tauto parameter, we divide galaxies into four subsamples…”
Looking into this, “Tauto” is an attempt to parameters both the bulge size, and the amount of structure not captured in a smooth light profile (some form of clumpiness), so it does indeed neglect the degree of windiness of the spiral arm in placing them in a sequence by bulge size and clumsiness only. Nice find.
Drory & Fisher 2007: http://adsabs.harvard.edu/abs/2007ApJ…664..640D
This paper actually provides evidence that B/T is not enough to classify galaxies into a sequence, pointing out that there are different types of bulge (pseudobulges, which are thought to be build up slowly from discs, and classical bulges, general thought to form in mergers) which correlate with galaxy colour even at the same B/T.
“(1) the red-blue dichotomy is a function of bulge type: at the same B/T, pseudobulges are in globally blue galaxies and classical bulges are in globally red galaxies; “
In summary, I hope this was interesting, and there’s definitely some examples you hve found which will make it into the reference list for the paper. Thanks again for your help, and if you are now inspired to join in go read the instructions here. We’d love to have you along.
Here at Galaxy Zoo we know that some of you are looking for ways to be more involved in the entire process of making science from your clicks.
So we had an idea…..
The team are currently in the process of writing a paper which in its introduction discusses some of the current assumptions/errors/approximations common among our fellow astronomers when thinking about galaxy morphology and classification. As such we’d like to collect as many papers as possible which do the following things:
- Claim that colour and morphology are equivalent
- Define “early-type” galaxy as any galaxy without visible spiral arms (e.g. our “smooth” category, which can include elliptical galaxies, and smooth disks), rather than as a galaxy that isn’t a disk.
- Define “early-type” galaxy as including Sa spirals as well as lenticular and ellipticals.
- Define “late-type” galaxy as only late-type spirals (e.g. excluding Sa spirals)
- Use colour or spectral type to split galaxies into “early-“ or “late-“ types (or “elliptical” and “spiral”)
- Use the bulge-to-total ratio (or some proxy for it like concentration, or the SDSS “fracDeV” parameter) to place spiral galaxies in a sequence.
The current draft text in the paper which talks about these assumptions is:
“The morphology of a galaxy encodes information about its formation history and evolution through what it reveals about the orbits of the stars in the galaxy, and is known to correlate remarkably well with other physical properties (e.g. Roberts & Haynes 1994). These correlations, along with the ease of automated measurement of colour or spectral type, have resulted in a recent trend for classification on the basis of these properties rather than morphology per se (e.g. Weinmann et al. 2006, van den Bosch et al. 2008, Zehavi et al. 2011). Indeed the strength of the correlation has led some to authors to claim that the correspondance between colour and morphology is so good that that classification by colour alone can be used to replace morphology (e.g. Park & Choi 2005, Faber et al. 2007). Meanwhile the size of modern data sets (e.g. the Main Galaxy Sample of the Sloan Digital Sky Survey, SDSS, Strauss et al. 2002) made the traditional techniques of morphological classification by small numbers of experts implausible. This problem was solved making use of the technique of crowdsourcing by the Galaxy Zoo project (Lintott et al. 2008, 2011). One of the first results from the Galaxy Zoo morphological classifications was to demonstrate on a firm statistical basis that colour and morphology are not equivalent for all galaxies (Bamford et al. 2009, Schawinski et al. 2009, Masters et al. 2010) and that morphology provides complementary information on galaxy populations useful to understand the processes of galaxy evolution. “
and later when talking about the spiral sequence:
“Modern automatic galaxy classification has tended to conflate bulge size alone with spiral type (e.g. Laurikainen et al. 2007, Masters et al. 2010a), and automatic classification of galaxies into “early-” and “late-” types, referring to their location on the Hubble Sequence and based on bulge-total luminosity ratio (B/T ) or some proxy for this through a measure of central concentration, or light profile shape (e.g. Sersic index, as reviewed by Graham & Driver 2005) has become common (e.g. van der Wel et al. 2011). Indeed, Sandage (2005) says this is not new, claiming ”the Hubble system for disk galaxies had its roots in an arrange- ment of spirals in a continuous sequence of decreasing bulge size and increasing presence of condensations over the face of the image that had been devised by Reynolds in 1920.””
We’d like to ask for your help in searching for more examples of these behaviours. We have made a simple Google form, and we ask that you submit any examples you find in the next few weeks.
Some of the papers you find might end up cited in the Galaxy Zoo team paper (please be aware there are rules/guidelines about the appropriate number – we don’t want to have too few; it doesn’t make the point about how widespread this is, and we don’t want to single out specific astronomers, but the journal won’t accept too many either). If there are more papers found than we can use, they will be kept in a list on the Galaxy Zoo website (and we can continue to add to them if needed).
I want to reassure you that helping with this does not mean you have to read the entire extragalactic astronomy literature, or even the entirety of a paper! The best place to look for this information in a paper will be the “Sample Selection”, or “Data” sections. Modern online PDF papers also have excellent search facilities – so searching the text for key words (e.g. “spiral”, “early-type”, “colour/color-selected”) may work extremely well.
We’re happy for you to do this however you like (e.g. Google Scholar is fine), but we’d like you to return the NASA ADS (Astrophysics Data System) URL for the paper you find. You can search ADS here: http://adsabs.harvard.edu/abstract_service.html, and I give examples below of the URL I mean. This makes it easy for us to get the full bibliographic data to add the reference to the paper.
One tip – there are some papers in extragalactic astronomy which are cited by most/many results. A good place to start looking through recent papers would be the citation and reference lists of such papers, which can be found in ADS.
Strateva et al. 2001 “Color Separation of Galaxy Types in the Sloan Digital Sky Survey Imaging Data”
http://adsabs.harvard.edu/abs/2001AJ….122.1861S is cited by 926 papers, and references 25 – this would be an excellent starting place, and the more papers you read the more mentions you may find other other papers doing similar things.
Other good starting places:
Strauss et al. 2002: “Spectroscopic Target Selection in the Sloan Digital Sky Survey: The Main Galaxy Sample” http://adsabs.harvard.edu/abs/2002AJ….124.1810S
Ironically the papers which cite some of our Galaxy Zoo papers where we demonstrate there are galaxies which are not in the normal correlation between colour and morphology may also be good starting points (some citations to these are along the lines of saying things like: “most galaxies fall into blue=spiral; red=elliptical, a few don’t (cite Galaxy Zoo here), but we’re going to use this definition anyway”.
The initial papers on colour not being the same as morphology are:
Bamford et al. 2009 (281 citations): http://adsabs.harvard.edu/abs/2009MNRAS.393.1324B
Schawinski et al. 2009 (81 citations): http://adsabs.harvard.edu/abs/2009MNRAS.396..818S
Masters et al. 2010 (125 citations): http://adsabs.harvard.edu/abs/2010MNRAS.405..783M
We hope that if several of you take up the challenge, you’ll find different paths through the literature and find lots of different examples for us. Again here’s the link to our Galaxy Zoo Literature Survey.
Thanks for your help!
Karen Masters (Galaxy Zoo Project Scientist)