Pieter van Dokkum

@DokkumPieter

Astronomer at Yale. Interested in dark matter, galaxy evolution, stellar populations, instrumentation. Also like photography, nature stuff, running, hiking.

Vrijeme pridruživanja: prosinac 2018.

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  1. 30. sij

    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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  2. 30. sij

    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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  3. 30. sij

    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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  4. 30. sij

    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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  5. 30. sij

    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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  6. 30. sij

    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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  7. 11. sij

    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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  8. 10. sij

    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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  9. 10. sij

    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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  10. 10. sij

    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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  11. 10. sij

    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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  12. 10. sij

    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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  13. 10. sij

    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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  14. 10. sij

    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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  15. 10. sij

    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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  16. 10. sij

    A small thread about the sizes of galaxies. The Tenerife group is proposing a different definition of radius, based on a fixed stellar surface density - see . For a given M/L ratio this is identical to the "isophotal" radius, widely used until the 1990s.

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  17. proslijedio/la je Tweet
    8. sij

    Hello participants! Join us for a special session featuring results from the Dragonfly Telescope. We will have a bingo game and awesome prizes!! Tomorrow, Jan 8th, 2 pm, Room 317B. See you there!

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  18. 30. pro 2019.

    It's described here: for instance. The only way to block this route is to set a firmware password - that is then required before you can do anything in recovery mode.

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  19. 30. pro 2019.

    Am I the only one who didn't know that account passwords offer no protection on macs? My 10 year old just told me. "Boot in recovery mode, open a terminal, type 'resetpassword', click 'forgot all passwords', and pick new passwords for all accounts". I tried it and it works - eek!

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  20. proslijedio/la je Tweet
    24. pro 2019.

    Join us for a special session at ! We will discuss recent results from the Dragonfly Telescope. Wednesday, January 8, from 2 to 3:30 pm in Room 317B.

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