The Sloan Digital Sky Survey

Transcription

1 The Sloan Digital Sky Survey Robert Lupton Xiaohui Fan Jim Gunn Željko Ivezić Jill Knapp Michael Strauss University of Chicago, Fermilab, Institute for Advanced Study, Japanese Participation Group, Johns Hopkins University, Los Alamos National Laboratory, Max-Planck Institute (MPIA), Max-Planck Institute (MPA), New Mexico State University University of Pittsburg Princeton University, United States Naval Observatory, University of Washington Irvine, June

2 Objectives Map the large scale structure of the Universe Survey the Northern Galactic Cap (10 4 square degrees) in five bands (u g r i z ) to PSF magnitude limits of 22.3, 23.3, 23.1, 22.3, and 20.8 Obtain astrometry good to 30mas/coordinate Spectroscopic survey at a resolution R 2000 of 10 6 galaxies: 10 5 QSOs few 10 4 stars 2

3 Objectives Make images and associated catalogs of 1/4 of the entire sky, namely that part away from the Milky Way, and visible from the northern hemisphere. We expect to detect and measure well over 10 8 galaxies, and a similar number of stars. Try to use all photons that pass through the atmosphere, but not through silicon; basically 310nm 1050nm. At peak, we detect about 50% of photons that fall on the primary mirror. Obtain images of stars a few million times fainter than those just visible to the naked eye on a dark night. Obtain spectra for 10 6 galaxies, 10 5 quasars, and a few 10 4 stars. These spectra tell us the radial velocities (and hence distances to the galaxies and quasars), but they also tell us a lot about the objects physical conditions. 3

4 Instrumentation A telescope (with a 2.5m diameter primary mirror) at Apache Point, New Mexico A camera containing photometric CCDs; u g r i z filters astrometric and focus CCDs Lots of Electronics, Quartz, Liquid Nitrogen, and Vacuum Two 320-fibre-fed double spectrographs, each with two CCDs Lots of software Charge Coupled Devices 4

5 Software The software was a difficult technical challenge on this project; probably harder than building the telescope and camera. Moderately large data volumes: 20Gb/hr when imaging Tb over the course of the survey Data is taken under varying conditions, but the great strength of a dedicated survey such as SDSS is producing a uniform dataset. We are sensitive to the (non-gaussian) tails of distributions; for example 4 objects with particular properties out of 15 million. I don t count building a functioning collaboration between scientists and institutions as a technical challenge 5

6 The spiral galaxy NGC428 The image maps the i-r-g filters to RGB, so what appears as a tasteful bluish-cyan is really dominated by the strong emission lines of [OIII] (5007Å) and H α (6563Å). 6

7 A cluster of galaxies 7

8 A piece of sky (0.2s) 8

9 These are all very nice, but: Why Bother? 9

10 Why Bother? Doesn t the Hubble Space Telescope do all this? 10

11 Why Bother? Doesn t the Hubble Space Telescope do all this? No; it produces exquisite images of tiny pieces of the sky. We produce OK images of large chunks of the sky. 11

12 Why Bother? Doesn t the Hubble Space Telescope do all this? No; it produces exquisite images of tiny pieces of the sky. We produce OK images of large chunks of the sky. What about those enormous telescopes in Hawai i and Chile? 12

13 Why Bother? Doesn t the Hubble Space Telescope do all this? No; it produces exquisite images of tiny pieces of the sky. We produce OK images of large chunks of the sky. What about those enormous telescopes in Hawai i and Chile? They re great for catching photons, and some of they are producing images almost as good as HST (maybe better in the IR); but they re still looking in great detail at small patches of the sky. 13

14 Why Bother? Doesn t the Hubble Space Telescope do all this? No; it produces exquisite images of tiny pieces of the sky. We produce OK images of large chunks of the sky. What about those enormous telescopes in Hawai i and Chile? They re great for catching photons, and some of they are producing images almost as good as HST (maybe better in the IR); but they re still looking in great detail at small patches of the sky. But the patches are so far away that they represent a large volume, right? 14

15 Why Bother? Doesn t the Hubble Space Telescope do all this? No; it produces exquisite images of tiny pieces of the sky. We produce OK images of large chunks of the sky. What about those enormous telescopes in Hawai i and Chile? They re great for catching photons, and some of they are producing images almost as good as HST (maybe better in the IR); but they re still looking in great detail at small patches of the sky. But the patches are so far away that they represent a large volume, right? Right; but it s a volume of the Universe that s only a few Gigayears old. SDSS can tell you what the local Universe is doing now and we need to know both. 15

16 OK, so SDSS is unique, but how does it Advance Astronomy? or Are Pretty Pictures Science? 16

17 Stars (and galaxies) lie in quite well-defined parts of colour n space. In many cases, the colours tell us much of what we want to know. For example, blue stars are hot; red stars are cool. All the stellar objects detected in about 2.5 degrees 2 of sky; yellow red colour v. red infrared colour 17

18 Colours are not always good enough... A-type star White Dwarf Both of these stars have a similar temperature, but the radii (and therefore luminosities L R 2 T 4 ) are very different. 18

19 But not all blue point-like objects are stars... A Quasar at a redshift z of about 3 (i.e. λ = (1 + z)λ 0 4λ 0 ) The prominent emission line at 5000Å is Ly α, the fundamental n = 2 1 transition of hydrogen. The spectrum to the blue (i.e. left) of Ly α shows absorption by neutral hydrogen between us and the quasar. 19

20 The yellow represents neutral and the white ionized hydrogen; quasars are magenta and you are sitting at the right-hand-side of the page. Quasars that are further away (higher z; further to the left) pass through more hydrogen and therefore more of the light that they emit to the blueward of Ly α is absorbed. 20

21 And not all quasars are blue (or bright) gri (green red near infrared) riz (red near infrared less near infrared) A quasar at z 6.28; all the visible and near infrared light is absorbed by intervening neutral hydrogen. Hint: Look between the star and the galaxy, and down a little 21

22 P Becker et al. 2001, AJ, 122, 2850!"$#%&(')+*-,. / :9 DC 08 E1 34F"GH I 8 J $0K L8 NM$O RQTSVU 22

23 Not all weird-coloured objects are at cosmological distances; some are asteroids in our solar system. The images map the i-r-g filters to RGB. The data is taken in the order riuzg, i.e. GR B 23

24 The colours of a sample of known asteroids observed by SDSS (a is basically g r) 24

25 The semi-major axis v. (proper) inclination of a sample of known asteroids detected by SDSS 25

26 The semi-major axis v. (proper) inclination of a sample of known asteroids detected by SDSS 26

27 The End or (more accurately) Time for me to make way for Eva Grebel and Daniel Eisenstein 27

28 Field ; ; 7s of SDSS imaging 28

29 SDSS response curves 29

30 In each panel, the bottom-left image is the gri colour-composite of an object from the SDSS imaging data. The other two panels show the decomposition of this composite into objects that are measured and catalogued by the SDSS software. An asteroid passing a star A fast asteroid passing a star The images map the i-r-g filters to RGB. The data is taken in the order riuzg, i.e. GR B 30

31 All the stars detected in about 2.5 square degrees of sky; yellow red colour v. brightness 31

32 COMPARISON OF ASTEROID SIZE DISTR IBUTION: OBSERVATIONS AND MODELS Farinella et al. 92 (1) CUMULATIVE NUMBER > D <---- Farinella et al. 92 (2) Farinella et al. 92 (3) Farinella et al. 92 (4) Galileo team Davis et al. 94 Durda et al 98. Model SAM99 Model SDSS 2001 SMALL SIZE BUMP <----- LARGE SIZE BUMP D (km) The asteroid size distribution (Davis 2002, in Asteroids III). SDSS results: 1) Extended the observed range to 300m 2) Detected the second break at 5 km 32

33 1 10 The impact rate for D>1 km: once in a million years 33