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

Brian YannyFermilab

Work done by: N. Padmanabhan,D. Schlegel, D. Finkbeiner, H. Lin,Z. Ivezic,D. Tucker, D. Eisenstein, H. Newberg,many others!

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 lotof good science....

F stars(0.2 < g-r_0 < 0.3)with19.5 < g_0 < 20.5

Goodcalibration

Detectionof severalstreams

SeveralDwarfGalaxiesin halo

Overallhalo densityasymmetries Sagittarius

Monoceros

Virgo

HaloDwarfs

~DR6 data sample

Color-Magnitudedegeneracies:

one in the blue:

Blue HorizontalBranch vs.Blue Straggler/A stars

One in the red:

K dwarfs vs.K giants

(actually runs from G-K-M)

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

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....

Since the difference in absolute magnitude betweenBHB 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 tracestructure 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 fortracing structure.

Precise color photometry (< 2%, required). The u-band especiallyimportant for gravity/luminosity discrimination work, to separategiants and dwarfs.

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

From Yanny et al. 2000, photometric separation of gravities

Blue Horizontal Branch and Blue Stragglers,halo

80 kpc45 kpc

12 kpc

Sag. Trailing tail, Virgo, Leading Tail

Monoceros

Newberg et al2006

H. Lin, stripe 82co-add calibrationcomparison.

4 PTPatchesacross6 2.5mcolumns

Indicates possible issue with camcols.

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

N. Padmanab.Differencebetween'RegularSDSSFlat fields'and 'uber-cal'flats.

g-band forthe 6 camcols,Note therange of +/- 2%.

Thisis a systematicerrorwhich directlyaffects themagnitudesof allobjects, andlimits ourcurrent errorsto 2%.

However,it can beremoved!!Resultingin errorsof 0.5% or so.

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 comparingwith STIS (HST) standards of Bohlin, Dickinson and Calzetti 2001and involves spectroscopy of fainter hot white dwarfs.

u_AB = u_SDSS -0.04.g,r,i _ AB = g,r,i_SDSS +/- 0.01z_AB = z_SDSS + 0.02

There is an uncertainty of at least 1% in these numbers, andmore in the u band. These conversions are important forSN precision work, and for converting work between ugriz and otherfilter systems such as UBVRI and physical models of stellaratmospheres and Galaxy population synthesis work.

AB photometry is really hard to absolutely calibrate!

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.

Improving the calibration of the SDSS imaging from about 2%to about 0.5% (relative) and perhaps 1% (absolute) opens thedoor 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 improvedlarge scale structure results)

6. Note that stripe 82 (equator south), with repeat co-addedscans (50 to date and growing) offers oppurtunity for deeper,more accurately calibrated 300 square degree gold mine!