Astronomy Without A Telescope – Green Peas


The ground-breaking discovery of a new class of galaxies, Green Peas, in 2009 by a group of Galaxy Zoo volunteers – have recently been followed up by further observations in the radio spectrum.

The Green Peas were first identified from Sloane Digital Sky Survey data – and then in Hubble Space Telescope archive images. Now radio observations of Green Pea galaxies (from GMRT and VLA) have led to some new speculation on the role of magnetic fields in early galaxy formation.

Green Pea galaxies were so named from their appearance as small green blobs in Galaxy Zoo images. They are low mass galaxies, with low metallicity and high star formation rates – but, surprisingly, are not all that far away. This is surprising given that their low metallicity means they are young – and being not very far away means they formed fairly recently (in universal timeframe terms).

Most nearby galaxies reflect the 13.7 billion year old age of the universe and have high metallicity resulting from generations of stars building elements heavier than hydrogen and helium through fusion reactions.

But Green Peas do seem to have formed from largely unsullied clouds of hydrogen and helium that have somehow remained unsullied for much of the universe’s lifetime. And so, Green Peas may represent a close analogue of what the universe’s first galaxies were like.

Their green color comes from strong OIII (ionised oxygen) emission lines (a common consequence of lots of new star formation) within a redshift (z) range around 0.2. A redshift of 0.2 means we see these galaxies as they were when the universe was about 2.4 billion years younger (according to Ned Wright’s cosmology calculator). Equivalent early universe galaxies are most luminous in ultraviolet at a redshift (z) between 2 and 5 – when the universe was between 10 and 12 billion years younger than today.

Spectroscopic data from Green Pea galaxy 587739506616631548 - demonstrating the prominent OIII emission lines which are characteristic of Green Pea galaxies. Credit: Galaxy Zoo.

Anyhow, studying Green Peas in radio has yielded some interesting new features of these galaxies.

With the notable exception of Seyfert galaxies, where the radio output is dominated by emission from supermassive black holes, the bulk radio emission from most galaxies is a result of new star formation, as well as synchrotron radiation arising from magnetic fields within the galaxy.

Based on a number of assumptions, Chakraborti et al are confident they have discovered that Green Peas have relatively powerful magnetic fields. This is surprising given their youth and smaller size – with magnetic field strengths of around 30 microGauss, compared with the Milky Way’s approximately 5 microGauss.

They do not offer a model to explain the development of Green Pea magnetic fields, beyond suggesting that turbulence is a likely underlying factor. Nonetheless, they do suggest that the strong magnetic fields of Green Peas may explain their unusually high rate of star formation – and that this finding suggests that the same processes existed in some of the first galaxies to appear in our 13.7 billion year old universe.

Further reading:
Chakraborti et al Radio Detection of Green Peas: Implications for Magnetic Fields in Young Galaxies
Cardamone et al Galaxy Zoo Green Peas: Discovery of A Class of Compact Extremely Star-Forming Galaxies.

11 Replies to “Astronomy Without A Telescope – Green Peas”

  1. This is new to me. I am curious whether there is dark matter. It is not surprising to suspect that interstellar gas, which is largely the same as it came out of the nucleosynthesis phase of the universe, could form galaxies. What is of interest to know is the role of dark matter.


    1. I think we are all curious whether there is dark matter 😉

      Some people are calling supersymmetry dead in the water, which would rule out neutralinos.

      1. That is interesting.

        – I believe that there is some raising tension between supersymmetry and experiments, some models are constrained, have to infer finetuning or may even be excluded by now.

        That happens for inflation theories (eternal inflation has its simplest models nearly excluded, I believe) and for Higgs’ models as well. One can even say it is good, a witness to the predictivity of the theories.

        – In my bumbling layman way, at face value supersymmetry looked exciting. Who doesn’t want more simple symmetries (laws) in their physics? Physics certainly seems to like them!

        But recently lcrowell made me realize that supersymmetry is a loophole around the Coleman–Mandula theorem.

        So it isn’t exactly simple when you place it in context*, and it isn’t even a broken symmetry (I think). But it explodes a rather elegant symmetry of itself: “external symmetries are external symmetries and internal symmetries are internal symmetries”, and they only meet in dynamics of interactions or possibly as broken symmetries of a cooling universe. (I would describe the latter superficially as some symmetries get stuck in the drawer and introduce “friction”.)

        Supersymmetry thus looks, to me but even with all my layman handwaving aside, like an extraordinary claim. And now it has observational difficulties as opposed to pointing to that overwhelming extraordinary evidence one would like to see.

        So what is the beef on the physicist arena here? The usual “I’ll accept it when I see the evidence” or some heavy hitting from people in the know?

        * Naturally that context is still very superficial. You have to learn to crawl before you learn to walk.

        It is so much easier if you learn this at university. You have to cram it, becomes annoyed and question the most obvious shortcomings, accept the rest “for now” and strive to become technically proficient which at the same time glues over some questions.

        And but years later you may go “waitaminnid!”

      2. I will say that I have far more desire to see supersymmetry found than I do the Higgs particle. Supersymmetry is the only symmetric system that works around the Coleman-Mandula theorem. The CM theorem provides an obstruction for a continuous unification of Yang-Mills gauge physics and the Lorentz transformation. The Lorentz transformation in spinor form will in a graded Lie algebraic system permit this however. The theory says that a Lie algebra G, with elements a that commute as [a, a’] = a”, can be extended to a graded system with elements b that anti-commute {b, b’} = bb’ + b’b = b”. This has a fermionic interpretation, and the elements a and b intertwine as [a, b] = a’. So if we have a field A in the algebra G and the spinor field ? are anticommutative, there is a super field

        ? = ? + c(?-bar A + ? bar-A) + ??-bar F,

        where the “bar” means the multiplication by the ?^0 Dirac matrix and the complex conjugate-transpose. For those familiar with quantum physics and Dirac spinors this should be familiar. Here c = 1/sqrt{2} and F is a constraint variable. So this interwines a Dirac field ? of spin ½ with a vector field A of spin 1 through the application of a Grassmann viariable ?. So if the spinor field ? corresponds to quarks or leptons then the A corresponds to squarks or sleptons. There is of course the converse case where a gauge vector field A is intertwined with a spinor field

        ? = A + c(?-bar ? + ? bar-?) + ??-bar F

        This superfield intertwines a gauge vector field A with a spinor or Dirac field ?. The ? is then fields like photinos or gluinos. So that is SUSY 101.

        The most obvious candidate for dark matter is the neutralino. The quantum state for the photino, the higgsino, the W-ino have the same quantum numbers and can exist in a condenstate. There is another candidate, which is the axion particle. This emerges from string theory in the m = 0 (m = projection of the spin = 2 onto the momentum axis) part of the graviton spectrum, along with the dilaton. However, I will defer from going into that right now.

        Also, having said that I am not as committed to the Higgs particle does not mean that I think there is no symmetry breaking mechanism. The W and Z are massive, and as a result have a longitudinal weak field component. At highly relativistic energies this leads to field components that oscillate so as to give interaction potential that move faster than light. So these longitudinal components can’t be fundamental at higher energy. The two doublet scheme, (H^+, H^0) and (H^-, H^0′) for the basic Higgs field is such that the two charge Higgs get absorbed into the W^{+/-} and one neutral Higgs in the theory is absorbed by the Z. The remaining Higgs is left on its own as the non-Goldstone bosonic part. It might then turn out this theory has another form, but without the remaining H^0 in a Landau-Ginsburg type of description. The theory might in fact be more in line with a BCS theory of superconductivity.


      3. Thanks!

        And you both answered, I like that symmetry at least.

        I can follow up to quantum field respectively gauge theory, both of which I haven’t studied (yet); I have studied classical field respectively group theory.

        That we can’t have a “continuous” unification (a continuous map like space and time unifying as spacetime?) between degrees of freedom doesn’t seem too worrying, discrete d.o.f seems to be what we see. I would be much more worried on that point if spacetime and particle fields behaved exactly alike.

        The converse reason is, I guess, a wish to have a GUT theory (and a step towards a quantum gravity theory, say supergravity). But again there are alternatives AFAIK.

        OK. But this is, to me, not extraordinary evidence for what looks to me like “an extraordinary claim”. I believe I can see why it would take shortcuts to physics beyond the Standard Model though.

        I have bookmarked all the articles. [Seems I need to Grandly Unify my weekend; why is it always the time d.o.f. that is least free? We need a theory here!]

      4. Things might have changed. These results I posted might be signals for partial supersymmetry in a broken phase.

        Supersymmetry is funny, for the bare theory tells us nothing about the energy scale at which it occurs. That entails hanging phenomenology onto SUSY. The earliest models involved “light SUSY pairs” which might have been found in the Fermilab machine — pre tevatron. Nothing was found. Then people worked up models with SUSY at the 100-200 GeV scale; again nothing with the tevatron. So now with the LHC we might be getting some hints of SUSY.

        I hope these results are real, for without SUSY nature is “fractured.” SUSY might “turn on” at the 1000 TeV scale, but we will not get that with the LHC. There must also be some symmetry breaking mechanism as well. The massive gauge bosons W^{+/-} and Z have longitudinal components. This is a field component which is along the direction of motion, rather than the transverse components which are perpendicular to the momentum. At around 1-10 TeV this longitudinal component begins to violate causality, where in effect since it is along the direction of motion it moves ahead of that motion faster than light. Massless particles have no longitudinal degree of freedom, and so if at high energy the W’s and Z are massless this removes this problem, and where these particles pick up that degree of freedom at lower energy by absorbing components of the Higgs doublets. If this is not how it works then something else must play this role.


      5. The situation with respect to supersymmetry (SUSY) is maybe taking a turn. The LHC has not delivered up the Higgs field yet, but it might be giving 3-sigma or better data for supersymmetry. The recent workshop on SUSY searches at the LHC

        may have some data for supersymmetry (broken or partial supersymmetry)

        The three-jet process at 350-425 GeV appears interesting. A lot of this is a bit complicated. A gluon and gluino are color-flavor neutral, but spin 1 and 1/2 respectively. The preferred channel for the gluino is a lepton plus Z. The Z may decay into pairs, but the lepton-antilepton pair is a clearer signature for the gluino.

        I write a bit more below to Larsson,


  2. “Lettuce have green peas.” [I prefer pea soup, myself.]

    It is nice that The Great Laboratory In The Sky runs odds and ends among the more “obvious” experiments!

    I’ll see lcrowell’s dark matter and raise SMBH’s; are they there?

  3. Thats interesting, becasue there have been a lot of ideas that a black hole needs to be in the center of a galaxy. This might not be the case, and could explain why some galaxies don’t have SMBHs (or at least we haven’t detected them.)

    Good stuff!

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