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Pieter van Dokkum
Astronomer at Yale. Interested in dark matter, galaxy evolution, stellar populations, instrumentation. Also like photography, nature stuff, running, hiking.
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Pieter van Dokkum Feb 18
Amazing images of Betelgeuse from ESO - the extreme AO mapping of changes on its surface, and (perhaps even more spectacular) the wide view of dust plumes from VISIR.
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Pieter van Dokkum Jan 30
Replying to @DokkumPieter
Meanwhile, the long-wavelength MIPS instrument provided the first accurate star formation rates, and MIPS and IRAC together gave us the "star formation main sequence".
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Pieter van Dokkum Jan 30
Replying to @DokkumPieter
All of us working in the field of galaxy formation and evolution owe a debt to this little Great Observatory, and the amazing team who ran it all these years!
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Pieter van Dokkum Jan 30
Replying to @DokkumPieter
Perhaps the most unexpected Spitzer accomplishment was the detection of the earliest galaxies, at z>7, and not just in continuum. Amazingly, very strong rest-frame optical emission lines such as OIII 5007 have been detected because of the excess they produce in IRAC photometry.
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Pieter van Dokkum Jan 30
Replying to @DokkumPieter
The fact that Spitzer measured the rest-frame near-infrared emission of distant galaxies also provided the first good estimates of their stellar masses. The first mass-limited samples appeared in 2006, and all high redshift mass functions that we have today rely on Spitzer data.
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Pieter van Dokkum Jan 30
Replying to @DokkumPieter
Thanks to work by Ivo Labbe, Stijn Wuyts, Kate Whitaker, and others we learned that red galaxies in the early Universe come in two flavors: young/dusty, and old/dust-free. The "UVJ diagram" (with J from Spitzer) has become an indispensable tool for understanding distant galaxies.
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Pieter van Dokkum Jan 30
Farewell Spitzer! In addition to the observations of planets, the Milky Way, and nearby galaxies that are rightly highlighted here, it completely changed our view of the distant Universe.
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Pieter van Dokkum Jan 10
Exciting! GN-Z11 is a very luminous galaxy (relatively speaking) with a WFC3 grism redshift of z=11.1 that we found in 2016 - see . Right now Hubble is taking deeper imaging data of the galaxy and its surroundings.
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Pieter van Dokkum Jan 10
Replying to @CGMundell
Good question! The most widely used measure is the radius containing 50% of the light, as galaxies don’t have an edge (unlike planets). The virial radius is a more meaningful number, but due to dark matter this is vastly larger than the visible (and readily measurable) extent.
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Pieter van Dokkum Jan 10
ha! can't wait to see that!
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Pieter van Dokkum Jan 10
Also, are you and Nacho now convinced about NGC5907? ‘s paper seems to settle things but I haven’t heard a peep!
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Pieter van Dokkum Jan 10
I’m still waiting for a dinner invitation from Nacho! 😉
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Pieter van Dokkum Jan 10
In r80, UDGs are relatively small as they have a very low Sersic index and their light falls off steeply at large radii. On the other hand, in r20 they are even more extreme than in r50, as they have huge nearly constant density cores.
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Pieter van Dokkum Jan 10
Replying to @DokkumPieter
Finally, I'll point to papers by my excellent students Tim Miller and Lamiya Mowla, who discuss the radii containing 20% and 80% of the light: and These measures provide extra information but do not mix luminosity and structure.
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Pieter van Dokkum Jan 10
Replying to @DokkumPieter
A well-known by-product of this definition is that the isophotal extent mostly measures the luminosity of a galaxy (as is clear from the figure). This is why you get a built-in tight correlation between isophotal size and luminosity - as seen in .
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Pieter van Dokkum Jan 10
Replying to @DokkumPieter
Even though the galaxies have the exact same structure, the isophotal or iso-density size of the red galaxy is almost 3x larger than of the green galaxy, and the blue galaxy has no defined size at all.
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Pieter van Dokkum Jan 10
Replying to @DokkumPieter
This figure illustrates the issue. All 3 galaxies have identical exponential disks, with the same scale length. The only difference is the surface density of the disks, the number of stars per unit area. The horizontal line indicates a fixed surface density limit.
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Pieter van Dokkum Jan 10
Replying to @DokkumPieter
For lower surface brightness galaxies than the Coma UDGs, such as Antlia II and most dwarf galaxies in the Local Group, the part of the galaxy that is brighter than their limit is zero! They have a lower surface brightness at all radii - which means they do not have a size.
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Pieter van Dokkum Jan 10
Replying to @DokkumPieter
For bright galaxies that part is big, but for low surface brightness objects such as UDGs it's obviously small, which is why the isophotal sizes of UDGs are small (and that is presented as a result in the companion paper of the Tenerife group, ).
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Pieter van Dokkum Jan 10
Replying to @DokkumPieter
The reason why this measure has been mostly abandoned is that it mixes sizes with luminosities: an isophotal (or iso-density) radius is not really a size, but answers the question "what part of a galaxy is above a given surface brightness".
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