Searching for Needles in the Sloan Digital Haystack

Transcription

1 Searching for Needles in the Sloan Digital Haystack Robert Lupton Željko Ivezić Jim Gunn 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 Atlanta, October 2002

2 Overview of Talk What is the Sloan Digital Sky Survey?

3 Overview of Talk What is the Sloan Digital Sky Survey? Why is the Sloan Digital Sky Survey?

4 Overview of Talk What is the Sloan Digital Sky Survey? Why is the Sloan Digital Sky Survey? Three case studies

5 What is the Sloan Digital Sky Survey?

6 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

7 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.

8 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

9 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

10 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).

11 A cluster of galaxies

12 Overview of Talk What is the Sloan Digital Sky Survey? Why is the Sloan Digital Sky Survey?

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

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.

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?

16 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.

17 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?

18 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.

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

20 Overview of Talk What is the Sloan Digital Sky Survey? Why is the Sloan Digital Sky Survey? Three case studies

21 A piece of sky (0.2s)

22 Palomar 5

23 The image maps the i-r-g filters to RGB.

24 All the stellar objects detected in the Palomar 5 and eight neighbouring fields (30 40 arcmin)

25 All the stellar objects detected in the Palomar 5 field (10 13 arcmin)

26 Credit: Michael Odenkirchen and Eva Grebel

27 trailing tail orbit Pal 5 leading tail Credit: Michael Odenkirchen and Eva Grebel

28 Pal 5 Sun Credit: Michael Odenkirchen and Eva Grebel

29 Asteroids The images map the i-r-g filters to RGB. The data is taken in the order riuzg, i.e. GR B

30 Distribution of velocities for objects with good velocities (e.g. detections in gri). Green objects are moving at > 4σ.

31 Distribution of velocities for objects with good velocities (e.g. detections in gri). Green objects are moving at > 4σ.

32 Objects shown in green are real; red objects are spurious (objects with large motions weren t checked by eye Objects with v < 0.02 deg/day are omitted.).

33 Objects shown in green are real; red objects are spurious (objects with large motions weren t checked by eye Objects with v < 0.02 deg/day are omitted.).

34 Main SDSS Asteroid Results More than 100,000 asteroids (m<21.5, 10,000 deg 2 ) detected. About 10 times more asteroid colors than all previous surveys together (the largest previous multi-color survey: ECAS, Zellner et al. 1985, with 600 objects).

35 Main SDSS Asteroid Results More than 100,000 asteroids (m<21.5, 10,000 deg 2 ) detected. About 10 times more asteroid colors than all previous surveys together (the largest previous multi-color survey: ECAS, Zellner et al. 1985, with 600 objects). A measurement of the main-belt asteroid size distribution to a significantly smaller size limit (< 1 km) than possible before

36 Main SDSS Asteroid Results More than 100,000 asteroids (m<21.5, 10,000 deg 2 ) detected. About 10 times more asteroid colors than all previous surveys together (the largest previous multi-color survey: ECAS, Zellner et al. 1985, with 600 objects). A measurement of the main-belt asteroid size distribution to a significantly smaller size limit (< 1 km) than possible before A smaller number of asteroids compared to previous work: the number of asteroids with diameters larger than 1 km is about 0.75 million

37 1 10 The impact rate for D>1 km: once in a million years

38 Main SDSS Asteroid Results More than 100,000 asteroids (m<21.5, 10,000 deg 2 ) detected. About 10 times more asteroid colors than all previous surveys together (the largest previous multi-color survey: ECAS, Zellner et al. 1985, with 600 objects). A measurement of the main-belt asteroid size distribution to a significantly smaller size limit (< 1 km) than possible before A smaller number of asteroids compared to previous work: the number of asteroids with diameters larger than 1 km is about 0.75 million The distribution of main-belt asteroids in 4-dimensional SDSS color space is strongly bimodal (rocky S-type and carbonaceous C type asteroids)

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

40 Main SDSS Asteroid Results More than 100,000 asteroids (m<21.5, 10,000 deg 2 ) detected. About 10 times more asteroid colors than all previous surveys together (the largest previous multi-color survey: ECAS, Zellner et al. 1985, with 600 objects). A measurement of the main-belt asteroid size distribution to a significantly smaller size limit (< 1 km) than possible before A smaller number of asteroids compared to previous work: the number of asteroids with diameters larger than 1 km is about 0.75 million The distribution of main-belt asteroids in 4-dimensional SDSS color space is strongly bimodal (rocky S-type and carbonaceous C type asteroids) A bimodality is also seen in the heliocentric distribution of asteroids: the inner belt is dominated by S type asteroids centered at R 2.8 AU, while C type asteroids, centered at R 3.2 AU, dominate the outer belt.

41 How do we know this? We cannot estimate orbits from our data. We cannot estimate orbits, but we can get (crude) distances from the apparent motion of our asteroids We can use catalogues of known asteroids, and find them in SDSS imaging data.

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

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

44 The osculating inclination vs. semi-major axis diagram.

45 Quasars All the point-sources detected in about 2.5 square degrees of sky; yellow-red colour v. red-infrared colour

46 SDSS Spectra An A-type star An ordinary red galaxy

47 SDSS Spectra An A-type star A Quasar with z 3

48 2625 Quasars and stars.

49 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

50 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.

51 Credit: Xiaohui Fan et al.

52 Flux (10-18 erg cm -2 s -1 A -1 ) Redshift Lyman Beta Lyman Alpha Redshift The Ly-α forest for SDSSp J (z 6.28) Becker et al. 2001, AJ, 122,

53 Becker et al. 122, , AJ,

54 Conclusions What is the Sloan Digital Sky Survey? Why is the Sloan Digital Sky Survey? Three case studies : Distinctively coloured objects: Palomar 5 Tidal stripping and the orbit of the cluster Weirdly coloured objects: Asteroids Census and Colour Information More weirdly coloured objects: High-z Quasars Observe the Epoch of Reionization Atlanta, October 2002