“They can’t be real.”

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The uncertainty of science: Astronomers have now detected and measured a new class of large but very dim galaxy that previously was not expected to exist.

‘Ultradiffuse’ galaxies came to attention only last year, after Pieter van Dokkum of Yale University in New Haven, Connecticut, and Roberto Abraham of the University of Toronto in Canada built an array of sensitive telephoto lenses named Dragonfly. The astronomers and their colleagues observed the Coma galaxy cluster 101 megaparsecs (330 million light years) away and detected 47 faint smudges.

“They can’t be real,” van Dokkum recalls thinking when he first saw the galaxies on his laptop computer. But their distribution in space matched that of the cluster’s other galaxies, indicating that they were true members. Since then, hundreds more of these galaxies have turned up in the Coma cluster and elsewhere.

Ultradiffuse galaxies are large like the Milky Way — which is much bigger than most — but they glow as dimly as mere dwarf galaxies. It’s as though a city as big as London emitted as little light as Kalamazoo, Michigan.

More significantly, they have now found that these dim galaxies can be as big and as massive as the biggest bright galaxies, suggesting that, surprise!, there are a lot more stars and mass hidden out there and unseen than anyone had previously predicted.


  • Wayne

    This is very cool!

    “I have no doubt that in reality the future will be vastly more surprising than anything I can imagine. Now, my own suspicion is that the Universe is not only queerer than we suppose, but queerer than we can suppose.”
    J. B. S. Haldane

    I would take this opportunity to shill for Gresham College Astronomy lectures, and in particular this recent talk–

    “The Dark Side of the Universe”
    Prof Joseph Silk
    (April 2016)

    >Wherein he discusses Fritz Zwicky, the Coma Cluster, Dark Matter & Dark Energy.

  • Garry

    Maybe this is covered in Wayne’s link, which I haven’t had the time to look at yet, but my first thought upon reading this post was that maybe this is the alternative explanation to dark matter, which I’ve always thought of as “we can’t explain where all this gravity comes from, so there has to be some mysterious, undetectable stuff out there.” To me, this never seemed too far removed from ancient man explaining everything by attribution to the gods.

    It’s human nature to try to find patterns, and over the ages science has been too ready to fill in the blanks without any real evidence, or worse yet, to doctor data to make it look like something predicted by the theory is really happening (see change, climate).

  • Wayne

    It’s a public talk, but very interesting. He touches upon the “Higgs field,” and The Standard Model, etc.
    (I would propose we are in a major “Paradigm-Shift Mode” right now.)

    Total tangent– I have to ask– do you translate any Japanese Anime? (Totally NOT my thing personally.) You could wind up with a 4th job!

  • Garry wrote: “My first thought upon reading this post was that maybe this is the alternative explanation to dark matter.”

    Nope, these dim galaxies don’t do anything to explain dark matter. It is important to understand the source of the mystery behind dark matter to then understand why it is really a puzzle. Numerous studies of many many galaxies and the movement of the stars within them has consistently found that the stars move much faster in the galaxy’s outer regions than they should, as if there is far more mass out there on the periphery than we can see..

    The easiest way to explain this is to use our solar system. As you move out from the Sun, the orbits of the planets get longer and longer, both because they have farther to travel to complete their circle but also because they are simply moving slower. There is less matter out there to pull on them. However, if the solar system was like most galaxies, the outer planets would not orbit slower, but as fast if not faster than the inner planets.

    Thus, something is causing the outer stars in galaxies to move faster. Scientists have no idea what that is, but all the data they have gathered of that accelerated motion in the last half century consistently points to a cloud of unseen mass that all galaxies appear to sit within.

    These new dim galaxies are not in a position to explain this dark matter mystery. In fact, given time, I am willing to bet that astronomers will find that their outer stars also move faster than they should, given the visible mass that we detect.

  • wayne

    Two links below are tangential to the main Topic, but not all that much, –more of a technical backgrounder on some issues of Cosmology, Gravity, Dark Matter/Energy, and The Standard Model.

    They do however speak directly to what Mr. Z noted above, “…all the data they have gathered of that accelerated motion in the last half century consistently points to a cloud of unseen mass that all galaxies appear to sit within.”

    “Cosmology for Particle Physicist’s”
    Part II – Introduction to Cosmology
    Dr. Edmund Bertschinger
    This is highly understandable, although has some heavy math. (which I just take as given & concentrate on the implications thereof) It’s part of a multi-lecture class aimed at informing Particle Physicist’s of Cosmological Theories.
    (The ultra-small is intrinsically wrapped up with the ultra-large.)

    “Demystifying the Higgs Boson”
    Dr. Leonard Susskind

    Heavy math as well, (lots of Field Theory) but totally within the realm of a large portion of BtB readers. (I just assume that if I can comprehend a bit of this, it’s infinitely easier for folks who are more technically educated than myself.)

    I have stayed at numerous Holiday Inn Express’s but I only play a Physicist/ Cosmologist, on the interweb.

  • Garry

    I should have gotten better informed before commenting; I was under the impression that dark matter related to the total mass in the universe as a whole, not to individual galaxies.

  • Garry: This is an understandable error, compounded by astronomers who have named their other big mystery about the universe’s expansion rate as “dark energy.” Both deal with the total mass of the universe, but deal with significantly different questions.

  • Localfluff

    Another problem with replacing dark matter with normal matter is the Bullet galaxy cluster, and several like it. Two galaxy clusters which have recently collided. One sees that the gas has decelerated from the collision, while most of the mass has continued on its trajectory as if it hasn’t interacted with anything. Btw, now that those galaxies have lost most of its dark matter it would be interesting to see how their rotations and shapes have changed.

  • Edward

    When we learn about orbits in high school, the teachers use the basics: an orbit around a point mass. It makes everything so much easier, and is pretty realistic for what we mostly observe and study at that age, the moon and the planets. Gravity decreases as the square of the distance from the point mass.

    However, for a spherical body with uniform density, gravity decreases as we travel deeper into it, such as in a mine. If we had a mine all the way to the center of the Earth (or the Moon, to avoid Earth’s magma and other nasty interior attributes), we would find that gravity decreases linearly with depth, until it reached zero at the center. This is because as we go deeper, there is more mass above us than when we are on the surface, and that mass pulls upwards, not downwards.

    Issac Newton was once asked what shape orbits would take if gravity decreased with the square of distance, and he replied immediately that they would be elliptical, because he had already done the math. Imagine what orbits would be like if they could exist inside the Earth, with gravity increasing linearly with distance.

    A galaxy has similar gravitational properties, as there are stars farther out than our sun, pulling on our solar system, as well as stars closer to the Milky Way’s center. This all affects the orbit of our solar system (and all stars) around the galaxy’s center. Since the galaxy is not uniformly dense, individual stars affect the motions of other individual stars and their solar systems, but the overall concept is similar to my inside-the-Earth thought.

    Loads of fun, that orbital mechanics.

    Astronomers have found that galaxies generally do not spin the way early models predicted (thus the models were wrong), and to make the models match reality, they have to add mass that is not seen in real galaxies. If this mass were dust and gasses, then they would glow, in infrared, and reflect light in ways that are not observed in the galaxies, so Astronomers have concluded that there is something other than dust in galaxies. Unless gravity does not work the way we understand it, something is causing — or more accurately: something is being — more mass distributed throughout galaxies than we are now able to observe.

    What a mystery!

  • Garry

    Thanks for a concise, thought-provoking introduction to the problem Edward!

  • wayne

    yes, nicely explained.
    quick aside-
    Newton’s 2nd “law”- “entropy always increases.”
    Boltzmann’s contribution to it all– “entropy almost always increase’s.”
    ..but I digress terribly.
    –This is the first time I’ve heard of “ultra diffuse galaxies,” but apparently they are being studied, results are being published, and conundrums encountered.
    (And they are referred to as “UDG’s.”)

    Quick search & ran across a recent talk (April 2016), which included this–

    “Ultra-diffuse Galaxies: the Low Surface Brightness Tail of the Abundant Dwarf Galaxy Population”
    Nicola Amorisco
    (15 minutes)

    “Distribution of Dark Matter versus Luminous Matter”
    Rennan Barkana
    (15 minutes)
    both contained within the video here:

    >>Very technical stuff and I haven’t had a chance to watch them multiple times (and get a grip on the content) but very interesting.

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