SDSS-II Photometric Calibration: How well can we do? (and why do we care?) Brian Yanny Fermilab

1. SDSS-II Photometric Calibration: How well can we do? (and why do we care?) Brian Yanny Fermilab Work done by: N. Padmanabhan, D. Schlegel, D. Finkbeiner, H. Lin,Z. Ivezic, D. Tucker, D. Eisenstein, H. Newberg, many others!

2. Types of photometric calibration: 1. Stars vs. Galaxies Stars are easier, but crucially depends on modeling the PSF accurately. Galaxies are hard because of very extended profiles (sky subtraction must be exact in outer parts). Accuracy of Flat fields are an important limiting factor. 2. Relative vs. Absolute Much easier to do relative photometry over small areas than global photometry. Easier to do relative photometry than to put on a global system (i.e. the Vega V=B=R=0.0 system). 3. ugriz standard star system vs. AB 'physical units system'. We are doing pretty well already....well enough to do a lot of good science....

3. ~DR6 data sample F stars (0.2 < g-r_0 < 0.3) with 19.5 < g_0 < 20.5 Good calibration Detection of several streams Several Dwarf Galaxies in halo Overall halo density asymmetries Monoceros Halo Dwarfs Virgo Sagittarius

4. Color-Magnitude degeneracies: one in the blue: Blue Horizontal Branch vs. Blue Straggler/A stars One in the red: K dwarfs vs. K giants (actually runs from G-K-M)

5. Distinguishing David (K-dwarfs) from Goliath (K-giant)

6. Giants have narrow, weak lines (low surface gravity), dwarfs have strong Mg triplets (though watch out for metals) Note that the photometry got it backwards....

7. Since the difference in absolute magnitude between BHB and BS stars is about 2 mags (factor of 2.5 in distance) and can be 3-6 magnitudes in G-K-M (factor of 4-16 in distance) If we wish to use stars as 'standard candles' to trace structure in the halo, we must aware of and be able to resolve these degeneracies. If we can do this with photometry alone, the whole SDSSstellar data set (>100 M stars) becomes available for tracing structure. Precise color photometry (< 2%, required). The u-band especially important for gravity/luminosity discrimination work, to separate giants and dwarfs.

8. With no Separation.... BS or BHB? Yanny et al 2000

9. From Yanny et al. 2000, photometric separation of gravities

10. Blue Horizontal Branch and Blue Stragglers,halo 45 kpc 80 kpc 12 kpc Monoceros Newberg et al 2006 Sag. Trailing tail, Virgo, Leading Tail

11. H. Lin, stripe 82 co-add calibration comparison.

12. 4 PT Patches across 6 2.5m columns Indicates possible issue with camcols.

13. Uber cal uses crossing stripes from SEGUE and Apache Wheel to solve large matrix for flat fields and zeropoints across the whole sky.

14. This is a systematic error which directly affects the magnitudes of all objects, and limits our current errors to 2%. However, it can be removed!! Resulting in errors of 0.5% or so. N. Padmanab. Difference between 'Regular SDSS Flat fields' and 'uber-cal' flats. g-band for the 6 camcols, Note the range of +/- 2%.

15. AB photometry is really hard to absolutely calibrate! SDSS photometry is intended to be on the AB system (Oke and Gunn 1983) by which a magnitude=0 object has a source of F_nu = 3631 Jy. This is not exactly true for the SDSS, but it is close. Our present estimate of how close we are comes from comparing with STIS (HST) standards of Bohlin, Dickinson and Calzetti 2001 and involves spectroscopy of fainter hot white dwarfs. u_AB = u_SDSS -0.04. g,r,i _ AB = g,r,i_SDSS +/- 0.01 z_AB = z_SDSS + 0.02 There is an uncertainty of at least 1% in these numbers, and more in the u band. These conversions are important for SN precision work, and for converting work between ugriz and other filter systems such as UBVRI and physical models of stellar atmospheres and Galaxy population synthesis work.

16. Summary of photometric calibration status: 1. Stars vs. Galaxies Stars are easier, but crucially depends on modeling the PSF accurately. Galaxies are hard because of very extended profiles. In the limit, one expects stellar photometry to be better, and for galaxy photometry to approach that of stars. Accuracy of Flat fields are an important limiting factor, can be improved. Currently 2%, can get down to <<1%. 2. Relative vs. Absolute Much easier to do relative photometry over small areas than global photometry. Easier to do relative photometry than to put on a global system. Can get relative to about 0.5% with ubercal. Absolute may still be at about 1.5-2%, but that's vs. Vega system. 3. ugriz filter system vs. AB 'physical system' 4% offset in u, errors of 1-2%, may be hard to improve.

17. Improving the calibration of the SDSS imaging from about 2% to about 0.5% (relative) and perhaps 1% (absolute) opens the door to new science: 1. Greatly improved Photometric parallax (distances from colors) for over 100M Milky Way (and halo stars) 2. Improved dwarf/giant discrimination (without needing spectroscopy) 3. Photometric metalicity indicators 4. Improved quasar/stars separation via photometry 5. Improved photometric redshifts for galaxies (leading to improved large scale structure results) 6. Note that stripe 82 (equator south), with repeat co-added scans (50 to date and growing) offers oppurtunity for deeper, more accurately calibrated 300 square degree gold mine!