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<strong>Matching</strong> <strong>SDSS</strong> <strong>and</strong> <strong>UKIDSS</strong><br />

<strong>Photometry</strong> <strong>for</strong> Luminous Red<br />

Galaxies<br />

Tim Higgs<br />

ICG, Portsmouth<br />

6 th Jan 2011


LRGs<br />

<br />

<br />

<br />

Luminous Red Galaxies<br />

Most massive galaxies in the<br />

universe<br />

– Efficient tracers of large scale<br />

structure<br />

– Small sample of LRGs can<br />

measure most of the mass in<br />

galaxies<br />

Most massive LRGs <strong>for</strong>med in<br />

early universe<br />

– Downsizing<br />

– Interesting from galaxy evolution<br />

point of view<br />

6/1/2011 Creating Matched Aperture <strong>Photometry</strong> 2


Why Match<br />

<br />

Galaxy at redshift of 0.7 emits an r-b<strong>and</strong><br />

(0.6µm) photon<br />

– This gets redshifted to 0.6 x (1+0.7) = 1 micron<br />

• Observers frame Y b<strong>and</strong><br />

– Observed r b<strong>and</strong> is galaxy rest frame u b<strong>and</strong><br />

• Most of the light from old stars<br />

• Most of the flux redshifted into NIR<br />

6/1/2011 Creating Matched Aperture <strong>Photometry</strong> 3


Surveys<br />

<br />

<br />

<strong>SDSS</strong> III BOSS<br />

– 10,000 deg 2<br />

– Redshifts of 1.5<br />

million LRGs out<br />

z=0.7<br />

<strong>UKIDSS</strong> LAS<br />

– 4000 deg 2<br />

– Millions sources in<br />

DR5+<br />

6/1/2011 Creating Matched Aperture <strong>Photometry</strong> 4


Surveys II<br />

• Galaxy photometry<br />

– <strong>SDSS</strong><br />

• Modelmag<br />

• Petrosian<br />

• Fibermag<br />

– <strong>UKIDSS</strong><br />

• Petrosian<br />

• Sersic mag<br />

• Circular Apermags<br />

r-b<strong>and</strong> defined<br />

H-b<strong>and</strong> defined<br />

• 0.5”< r


GAMA Matched Catalogue<br />

• Ideal solution is full re-extraction<br />

• Hill et al 2010 did this <strong>for</strong> GAMA fields<br />

– R-b<strong>and</strong> defined Kron aperture photometry<br />

• “Correct” method, expensive though<br />

– Time<br />

– Computationally<br />

• 4 hours to create 20GB mosaic<br />

• “a few days” to create catalogue <strong>for</strong> each mosaic in<br />

dual image mode<br />

Hill et al MNRAS 2010 (Arxiv 1009.0615)<br />

6/1/2011 Creating Matched Aperture <strong>Photometry</strong> 6


GAMA DR1<br />

Hill et al MNRAS 2010<br />

Arxiv 1009.0615<br />

6/1/2011 Creating Matched Aperture <strong>Photometry</strong> 7


GAMA Matched Catalogue<br />

Hill et al<br />

MNRAS 2010<br />

Arxiv 1009.065<br />

6/1/2011 Creating Matched Aperture <strong>Photometry</strong> 8


Alternative Approach<br />

• Can we take a short cut <strong>for</strong> LRGs<br />

– Small observed sizes<br />

• Average radius ~6 arcsec<br />

– Low ellipticities<br />

• How much can we get from catalogues<br />

6/1/2011 Creating Matched Aperture <strong>Photometry</strong> 9


Method<br />

• Get <strong>SDSS</strong> photoprofile<br />

– Flux within annuli<br />

– Generate curve of growth<br />

<strong>for</strong> galaxy<br />

– truncate curve at chosen<br />

radius<br />

• <strong>UKIDSS</strong> Apermags<br />

– Radius from 0.5” to 12”<br />

– Choose one of these radii<br />

Can produce circular aperture<br />

photometry to same radius<br />

– Matched aperture<br />

6/1/2011 Creating Matched Aperture <strong>Photometry</strong> 10


Data<br />

• Focus on GAMA fields<br />

– <strong>UKIDSS</strong> DR4+<br />

– <strong>SDSS</strong> DR7<br />

– GAMA DR1<br />

• ~300 common LRGs<br />

• 144 deg 2<br />

• Use as test area<br />

– Compare my photometry to GAMA<br />

6/1/2011 Creating Matched Aperture <strong>Photometry</strong> 11


Results<br />

GAMA use kron apertures, leading to slight offset<br />

6/1/2011 Creating Matched Aperture <strong>Photometry</strong> 12


Stellar Mass<br />

• How do we know if<br />

we are doing well<br />

– Use Stellar<br />

masses as<br />

metric<br />

– Compute these<br />

by fitting to M09<br />

LRG template<br />

– Can compare to<br />

known stellar<br />

masses<br />

6/1/2011 Creating Matched Aperture <strong>Photometry</strong> 13


Stellar Masses<br />

• Compare to MPA JHU mass catalog<br />

– Well tested stellar masses <strong>for</strong> <strong>SDSS</strong> galaxies<br />

– Uses Fiber photometry <strong>for</strong> fits<br />

• Assume Stellar Mass proportional to flux<br />

– Scale masses<br />

– Compare derived masses to scaled MPA-JHU<br />

masses<br />

log mass_MPA_JHU scaled =log mass_MPA_JHU −0.4r fiber<br />

−r photo <br />

6/1/2011 Creating Matched Aperture <strong>Photometry</strong> 14


Comparisons<br />

6/1/2011 Creating Matched Aperture <strong>Photometry</strong> 15


Conclusions<br />

• LRGs important <strong>for</strong> Cosmology <strong>and</strong> Galaxy<br />

Evolution<br />

• Can create a simple matched aperture <strong>SDSS</strong>-<br />

<strong>UKIDSS</strong> catalogue<br />

• Early results looking good<br />

– Good <strong>SDSS</strong> colour agreement<br />

– GAMA colours close<br />

– Stellar masses are close<br />

• Improvements to come from degrading <strong>UKIDSS</strong><br />

profiles to the same PSF as <strong>SDSS</strong> galaxies<br />

6/1/2011 Creating Matched Aperture <strong>Photometry</strong> 16


Thank you<br />

6/1/2011 Creating Matched Aperture <strong>Photometry</strong> 17

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