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Counting Faint Radio Sources

Counting Faint Radio Sources. Neal A. Miller March 1, 2013 at. Outline. Counting radio sources Summary of observation field and the radio observing program Source populations in the counts Getting something out of nothing. Early Radio Surveys.

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Counting Faint Radio Sources

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  1. Counting Faint Radio Sources Neal A. Miller March 1, 2013 at

  2. Outline • Counting radio sources • Summary of observation field and the radio observing program • Source populations in the counts • Getting something out of nothing Stevenson University

  3. Early Radio Surveys • Radio astronomy began in the 1930s with Karl Jansky • Bell Labs, trans-Atlantic communication • Task: What are the sources of noise? • Thunderstorms! Ones you can see, and ones off in the distance. • Steady, periodic hiss of unknown origin Stevenson University

  4. Jansky’s “Merry Go Round” Stevenson University

  5. Jansky’sScientific Deduction • Hiss rises and falls with near 24-hour period • Obvious candidates (Sun, man-made sources) ruled out • With more data, period proves to be a little less than 24 hours • 23 hours, 56 minutes: sidereal period! • Jansky pins source as the Milky Way • May 5, 1933 NYT headline Stevenson University

  6. Early Radio Work • Pre-WWII: Grote Reber • Advancements from WWII • Radar techniques, improved technology • Discovery of link between sunspots and radio interference (Hey, Southworth) • Mapping of “noise” in sky for V2 tracking (general locations of brighter discrete sources) • Interferometry Stevenson University

  7. Sea Cliff Interferometry • Noticed with radar signatures from aircraft over water • Return signal has two paths, they constructively and destructively interfere Stevenson University

  8. Astronomy Implication • Australia group shows sunspots as source of radio static from Sun (implied size smaller than Solar disk) • Diffraction limit for a telescope is roughly wavelength over diameter; for interferometer you use baseline distance instead of diameter Cygnus A data, Dover Cliffs Stevenson University

  9. Radio Galaxies 2.3’ (0.038°) 1.3’ (0.022°) (NRAO Image Gallery) Cygnus A Bright radio sources are found to be distant galaxies! Stevenson University

  10. Radio Galaxies (Close to 1° per side) Fornax A (NRAO Image Gallery) Stevenson University

  11. The Radio Sky These are from a more recent radio survey (Condon et al. 1998) but look how “uniform” the sky is! Most sources are distant radio galaxies! Stevenson University

  12. Source Counts • You can detect only the brighter sources (at least in our historical treatment) • They are distant galaxies • “Typical” about half the age of the Universe • Source counts: number of sources as a function of how bright they are. • Big Bang? Steady State? Stevenson University

  13. What To Expect Stevenson University

  14. What To Expect • If sources are uniformly distributed and of the same brightness (or a distribution of brightness that is always the same): • More distant sources are dimmer (inverse square law) • More distant sources are more numerous (bigger volume in shell) • Cumulative counts should go as 3/2 power: N α S-3/2 (where S is brightness) Euclidean Stevenson University

  15. Early Radio Cosmology • The cumulative source counts do NOT follow a 3/2 power law! (Ryle & Scheuer 1955) • Evidence against Steady State theory • But is the density of sources changing, or is it the function that describes their brightness? • Later shown to be correct, but there is controversy: confusion • Penzias & Wilson CMB (1965) “steals” thunder Stevenson University

  16. Modern Source Counts Euclidean normalized (flat slope is Euclidean) Condon et al. (2012) Stevenson University

  17. Chandra Deep Fields • Chandra is the X-ray “Great Observatory” • Two fields (North, South) selected for deepest observations • “Empty” regions, prime for deep X-ray images • Chandra Deep Field South initially observed for one million seconds (Giacconi et al. 2002) Stevenson University

  18. If you observe it,they will come • Deep X-ray image leads to many other facilities observing the same region • Hubble GOODS (Giavalisco et al. 2004), GEMS (Rix et al. 2004), HUDF (Beckwith et al. 2006), CANDELS (Grogin et al. 2012) • Spitzer GOODS (Dickinson et al.), FIDEL; SIMPLE (Damen et al. 2010) Stevenson University

  19. Hubble Ultra-Deep Field Sort of… Stevenson University

  20. (Observed it,they keep coming) • GALEXUltradeep Imaging Survey and Deep Spectroscopic Survey • Ground-based optical, near IR imaging • GOODS ancillary data • COMBO-17 (Wolf et al. 2004) • Photometric redshifts (distances) • Optical spectroscopy (numerous publications, often VIMOS) • Spectroscopic redshifts (better distances) Stevenson University

  21. And MORE X-ray! • Extended Chandra Deep Field South (Lehmer et al. 2005) • Chandra ACIS FOV is ~17’, do a grid of four to get ~30’ square • Each of four is 250 ksec • Second Ms time (Luo et al. 2009), later upped to 4 Ms total (Xue et al. 2011) Stevenson University

  22. (Coordinate grid very close to size of Full Moon) GOODS HUDF CANDELS Xue et al. (2011) Stevenson University

  23. Very Large Array Observations Stevenson University

  24. Summary Details • VLA is a 27-element interferometer, each element a 25-m diameter dish • VLA “Large” program, for full E-CDF-S • 240 hours awarded (0.86 Msec) • A configuration (~2” resolution; about a dime at 1 mile) • 1.4GHz (20cm) • Full coverage of Extended CDF-S • Observations during Summer 2007 Stevenson University

  25. Tricks • Field is in the South… VLA is at 34°N • Observe only 5 hours per day • VLA field of view at 1.4GHz is about 31’ • “Power pattern” roughly a Gaussian • Can’t simply mosaic “tiles” together • Ring of six pointings in a hexagon • 6 pointings, 5 hours per observation, do each 8 times Stevenson University

  26. Full image, about one degree per side (map is 8k x 8k, pixels are 0.5”) You can see the six pointings Stevenson University

  27. This is the “release” image; 34’ on a side Stevenson University

  28. Uses of Radio Data: What Type of Galaxy? • Star forming galaxy or an active galactic nucleus? • Actually use all wavelengths together! • Radio (VLA survey) • Infrared (Spitzer Space Telescope, Herschel) • Near-IR and Visible (Hubble) • Ultraviolet (GALEX) • X-ray (Chandra) Stevenson University

  29. Far-IR and Star Formation • Percentages do vary by a lot, but in rough terms 2/3 of all the energy output of a star-forming galaxy is in the far-infrared! • UV emission of young massive stars heats dust grains, modified blackbody (T ~35K) from them • Radio? Supernovae of those same massive young stars Stevenson University

  30. Far-Infrared/Radio Correlation Yun, Reddy, & Condon (2001) Stevenson University

  31. NGC 4911 in the Coma cluster Radio contours (below) from Miller et al. 2008 Radio emission traces star formation in the disk. Scale length of synchrotron emission is ~kpc, lifetime a few times 107 years. Stevenson University

  32. Radio Data Applied • Source counts by type of galaxy! • Classify using radio, X-ray, optical, infrared • Examine the flattening of counts for the faintest sources • Upturn not just due to star formation! Radio Quiet AGN Stevenson University

  33. Radio Quiet AGN • Strong X-ray sources indicating presence of an active black hole • Weak radio sources, generally following far-infrared/radio correlation • Importance? Strong observed correlation between mass of black holes at galaxy centers and the total mass of stars around them • Black holes at centers of galaxies “co-evolve” with star formation! What connection? Stevenson University

  34. Signals from the Noise • In shallower E-CDF-S, 117 of 762 X-ray sources have formal radio detections (15%) • In deeper CDF-S (4 Msec), 142 of 740 (19%) • What about the non-detections? Stevenson University

  35. Principle of Stacking • If you think in terms of exposure time (and ignoring things like your detector limitations), increasing your exposure time by X lowers your noise by sqrt(X) • How about if you had X objects instead? • Use object positions as centers of images, stack them all up, and your noise in the output image also decreases by sqrt(X) Stevenson University

  36. Stacking • What is really happening is that each pixel is the combination of Gaussian noise (if you’re lucky!) and real signal, with that signal often being swamped by the noise. • The noise is uncorrelated, but if the objects in your sample do have some weak signal, their combination will be present Stevenson University

  37. N = 1 Noise is 6.5 uJy No source seen N = 9  Noise is 2.2 uJy 4σ source N = 25 Noise is 1.3 uJy 4.7σ source N = 94  Noise is 0.7 uJy 9.6σ source Stevenson University

  38. Radio Quiet AGN? • Stacking recovers “detections” even in restricted bins of X-ray flux • Sources have high X-ray brightness, collective radio properties consistent with origin as radio quiet AGN • Possibly half of faintest radio sources will be radio quiet AGN • Important for next generation of radio interferometers (confusion limits) Stevenson University

  39. Conclusions • The focus has changed, but we are still learning from radio source counts • At fainter limits of the deepest radio surveys, star formation becoming increasingly important • Including star formation co-existing with powerful active black holes Stevenson University

  40. Student Opportunities • Radio observing programs • NRAO student support with successful observing programs (additional page in proposal) • Each program gets an NRAO visit • Analysis programs • Archival programs • Stacking of different samples/populations Stevenson University

  41. “Negative K Correction” Stevenson University

  42. Importance for Submm • Submm surveys detect very distant galaxies that are forming lots of stars • Translate submm flux into a star formation rate • Submm instruments have terrible resolution – detections could be any number of sources • Use radio to pinpoint! • Rough numbers: Submm ~30”; Spitzer 70um ~ 16”, 24um ~6”; Spitzer 3.6um ~2” Stevenson University

  43. Radio Position Example From Coppin et al. (2009). Galaxy is at z = 4.76! MUSYC optical/ Near-IR IRAC near-IR, Radio contours Spitzer 24um, LABOCA contours Stevenson University

  44. Key Assumption • Does the FIR/radio correlation hold at high redshifts? • Had NEVER been demonstrated! • Prior IR observations not deep enough • Prior attempts based on 24um data, which gets dicey at z ~ 1 • You want longer IR wavelengths so you are really sampling that thermal dust peak Stevenson University

  45. No Evolution in FIR/Radio Correlation! From Mao et al. (2011) Stevenson University

  46. Example Galaxy GOODS HST Data Stevenson University

  47. Example Galaxy Stevenson University

  48. Example Galaxy • Redshift is 0.669 (Universe was 7.6 Gyr old, 7 kpc equals 1”) • Star formation rate is about 40 Msun/year • Use fitted spectrum to calculate rest-frame FIR/radio correlation • “q” a logarithm of the FIR/radio • q = 2.30 in local universe, dispersion of 0.2 Stevenson University

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