Target Selection and Target Selection and photo-zphoto-z

Filipe B. AbdallaFilipe B. Abdalla

Usual spectroscopic targets:KAOS/WFMOS/BigBOSS/Sumire/DESpec etc…• LRG galaxies out to z~1 with absorption

line spectra• Emission line galaxies out to z~1.7

depending on how red the spectrographs go, with OII mainly

• If IR spectra, gets galaxies around z~2 • High redshift galaxies at z~3 need blue

sensitivity and will get a secure redshift with Lyman alpha in the UV.

• QSO’s will also rely on the blue end of the spectrograph

A DESspec target selection?

• 1)100% spectroscopic completeness of DES galaxies to r=21stmagnitude with resolution 50 km/s.

• 2)Case 1 plus 50% completeness to r=22.5 magnitude evenly distributed over all redshift bins

• 3)~300 km/s redshift precision with 100% completeness to r=22ndmagnitude

• Q: on what parameters give the most improvement on FoM and advice as to what new techniques are opened up.

Largest FOM’s are always with a constant number density.

• 1- Constant density 0.2<z<1.7, 10^7galaxies

• 2- Constant density 0.2<z<0.5, plus I<22.5 for 0.5<z<1.7 @65% eff’y. Total 10^7galaxies. Note redshiftcut-off (right)

• 3- Constant density 0.2<z<0.7, plus emission line galaxies for 0.7<z<1.7. Total 10^7galaxies.

• Is FOM the best way to asses these

• Large difference between different target selection techniques.

• Best is to reach a magic number of n=~2.10^-4Mpc^-2

• This magic number ranges from a few hundred to a few thousand galaxies per sqdeg depending on z range

Photo-z calibration: direct and cross correlation• A spectroscopic survey to

further calibrate photo-z, given depth of DES, most likely a large fraction of DES galaxies will have been calibrated but not all -> small deep field

• Would, possibly, calibrate deeper photo-z surveys with the x-correlation technique. See sims (Matthews and Davis 10).– Problem: degenerate with bias(z)

• Technique not put to the test on data yet! Would be nice to see it actually working. If so, DESpec could calibrate a large fraction of surveys such as LSST.

0.45 < z < 0.5 0.5 < z < 0.55 0.55 < z < 0.6 0.6 < z < 0.65

Combined constraints coming up later plus full cosmological implications in a companion paper

Linear Bias parameter in each bin ~ redshift dependence

Photo-z calibration cross correlation: can we use RSD/CMB?

Thomas, Abdalla & Lahav [0911.5291] PRL

0.28 eV

Red/Infrared selection: (mainly for LRG’s)

• Optical selection not a problem: – BOSS have done it– (Ross et al.) have done it w/ riz.

• Near-IR: VISTA VHS– J 1.2μm AB 21.0– H 1.6μm AB 20.5– Ks 2.5μm AB 20.0

• With several bands photo-z of these galaxies gets significantly improved by a factor of 2. Hence shaped n(z) can be produced.

Other IR selection:

• WISE:– 3.4μm AB ~ 19– 4.6μm AB ~ 19– 12 μm AB ~ 17– 22μm AB ~ 14

• Selecting LRG’s– Higher z LRG’s have the break moving out of the I band,

hence z bands or IR bands are needed.– One way of getting higher redsfhit LRG’s-> use far IR

space data. Use the H minus 1.6 micron bump. (see figure from Sawicki 02)

– BigBoss will probably be able to select these very well. But with Wise only data would probably produce an n(z) which is more peaked.

– Obtaining a flat n(z)=cst will require photo-zs with near-IR data.

Selection of ELGOriginal old strategy…

From K. Glazebrook old KAOS talk

dN/dz

From Laerte Sodre./ WFMOS proposal

Is simpler better?• Should we be doing colour cuts to

be efficient and attempt to have nP=cst?

• BigBoss will use Panstars + Palomar transient (gri) factory for ELG’s -> again possible but not possible to shape n(z) (DEEP (BRI) = mag 24, this is mag 22-23)

• Should we instead be choosing a photo-z selection algorithm to have a constant number density of galaxies? – With good photometry we can get

a sigma_z=0.1 so constant nP over bins of 0.1 in z.

Colour cuts vs photo-z…

Not clear if the scatter here is as good for ELG’s as this is a full population scatter plot

Ideally to obtain a constant n0• Attach emission features in a photo-z catalogue• This catalogue exists: Jouvel et al.09• Attach a necessary exposure time to secure redsfhit

for each galaxy• We want to select targets from the photo-z

estimates and which minimises the necessary exposure time.

• Can fold in any knowledge if objects have strong lines.

• This would lead to objects needing different exposure times, and has implications for survey strategy but would obtain best for FOM and uses the fact that there is deep photometry available.

• Consequences on the Selection function depending how we prioritize high z galaxies

• How done in practice: – build f(m_i)= probability of targeting the galaxy i

The resolution of the spectrograph• OII doublet has wavelengths 3726.1 and 3728.8

angstroms.• Separation is about 3 angstroms.• Absolute minimum to resolve lines is R=1380• As R increases have more read-out noise-> less

efficient observation but better resolution for the line.

• BigBoss R~4000, old WFMOS R~3500• DESspec R~1500?

– With v lines could blend• Do we need to resolve the line?

– Enrique has just shown we can do stuff with 900km/s accuracy (PAU)

• I am not advocating this… a quesiton!

Should we rely on photo-z as a prior for the spec z determination?• The main reason for resolving the OII doublet is to

identify the line correctly.• As an example HETDex cannot separate OII from

Ly\alpha with their instrument.– Their strategy: Separate the populations based on the

rest frame equivalent width…• Requirement on having less than 1%? of objects

with the correct redsfhit. For Wigglez blunder rate ~2-5% (Blake et al. 10)– What would be the problems if such a requirement is not

met?• Could use p(z0)=p(z_spec)*p(z_phot)?*p(EW)*????

– Is there a flaw about this approach?

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