Emission Line Emission Line Galaxy Targeting for Galaxy Targeting for

BigBOSSBigBOSS

Nick MostekLawrence Berkeley National Lab

BigBOSS Science Meeting

Novemenber 19, 2009

Why Emission Line Galaxies?Why Emission Line Galaxies?

• Strong emission line spectral features come from excited ISM gases, most commonly found in galaxies with ongoing star formation

• Lines fall at specific wavelengths which can be used for accurate redshift measurement

Advantages:• ELGs are numerous in the range of 1<z<2 as they do not require much

hierarchical structure formation • Bright, unique identifiers - ideal for measuring redshifts efficiently • Evolution in the [OII] luminosity function is observed up to z=2 due to an

increased SFR . (For a fixed volume density, the [OII] luminosity in distant ELGs is higher than rest-frame ELGs)

Disadvantages:• Accurate luminosity functions are difficult to measure beyond z>1.5• Single emission line detections can be ambiguous• Must have sufficient spectroscopic resolution to achieve z<0.001(1+z)

AND efficiently detect the line over background noise

Target Selection ParametersTarget Selection Parameters

• Photometric depth required to target constant 3.4E-4 (h/Mpc)3 number density

• Select brightest [OII] lines ELGs from 0.7<z<2

• High completeness (>70%) for [OII] line flux

• Optical photometric selection

• 10-15k sq. deg targeting in the Northern Hemisphere within 3 years

COSMOS Mock CatalogCOSMOS Mock Catalog

• Left panel shows template SEDs fit from 30 band photometry in the zCOSMOS survey field (Ilbert et al., 2008)• Right panel shows the [OII] line fluxes from VVDS spectra and M(UV)-[OII] calibration• LePhare photo-z code used to generate synthetic photometry in ugrizJHK bands

Ilbert (2008)Ilbert (2008)

[OII] Luminosity Function[OII] Luminosity Function

Zhu et al., 2008

• Left figure shows the measured luminosity function of DEEP2 [OII]-emitting galaxies for 4 redshift bins. 14,000 [OII] emission line galaxies were used in this sample.

• Right figure shows the predicted redshift distribution for a fixed [OII] flux limit from DEEP2 and Ilbert/VVDS.

• Stage IV BAO experiments are considering densities in the 10-3 -10-4 (Mpc/h)-3 range

• Strong [OII] emission comes from star forming, typically late type galaxies (Sb-Sd and Irregulars)

• Adelberger (2004) showed that such galaxies can be selected using optical bands from 1<z<3

• These galaxies typically have relatively flat SEDs except for 2 regions:

– Hydrogen Balmer absorption bands, rest frame ~4000 Å.

– Ly absorption, rest frame ~912 Å.

Target Selection BandsTarget Selection Bands

Adelberger (2004)

grzgrz Selection Selection

Redde

ning

z=1.0

z=1.5

• Figure at right shows grz color plane with F[OII] >5E-17 cgs for z>0.7 galaxies and a magnitude limit of r <23.5 mag

• Selection box is drawn around bulk of bright [OII] emitters at z<1.5

Adelberger (2004)

• u-band can also provide a redshift color selection, although it works best for z>2

• Synthetic photometry from Ilbert zCOSMOS code shows a selection box is drawn around F[OII] >5E-17 cgs for 1.5<z<2 galaxies and a magnitude limit of r <24 mag

ugrugr Selection Selection

Adelberger (2004)

Redshift

Reddening

Reddyr (2006)

• Looked at the following surveys with available sensitivities, sky brightnesses, and median seeing:– PanStarrs (griz, PS1=360s, 30k deg2)– Palomar Transient Factory (gr, 3 hr, 12k deg2)– Theoretical CFHT survey (u, 400s, 14k deg2)

• Errors are statistical. Only systematic considered is a floor on photometric error.

Note: PTF errors used here are larger than those quoted by Peter yesterday

Survey Photometric ErrorsSurvey Photometric Errors

• Figure at right shows grz color plane (PTF+PS1) with F[OII] >5E-17 cgs for 0.7<z<2 galaxies and a magnitude limit of r <23.5 mag

Low redshift Low redshift grzgrz selection selection

Note: PTF errors used here are larger than those quoted by Peter yesterday

rr magnitude limit for magnitude limit for grzgrz cut cut

• For a redshift distribution ~103 galaxies/deg2, we should expect a magnitude limit of ~23.5 and F[OII]~1E-16

grzgrz-selected-selected Redshift DistributionRedshift Distribution

• Used gr photometry from PTF and u-band from CFHT, mag limit of r<24• Marginal u-band data scatters lower redshift objects into selection box…could ugr alone be

sufficient?• Selection delivers a mean of 2400 N/(dz.deg2) from 0.8<z<2• Overall selection of star-forming galaxies with F[OII]>5E-17 cgs is >90%

ugrugr Selection Bands Selection Bands

• Strong OII line emission and high redshift sample (z>1.5) can be found in the “blue corner” of the grz color plane

• Plot on right shows COSMOS objects with magnitude errors from PS1 (grz)

Life on the Blue CornerLife on the Blue Corner

z=1.0

z=1.5

ugr ugr vs. vs. grz grz blue cornerblue corner

ugr grz

• r<24 for both color cuts, no additional color information used for lower redshifts• ugr averages 90% in selection efficiency of bright [OII] for z>1.5, grz averages 85%• For all practical purposes, the cuts are very similar with grz being more effective at weeding

out lower redshifts but producing more targets beyond z>2

SummarySummary

Proof of Concept Target Selection

• ugr produces a ~flat redshift distribution from 1<z<2 at r<24, minimal number density• grz produces a higher density redshift distribution from 0.7<z<1.5 at r<23.5 with a shot at

extending to higher redshifts• Limiting [OII] line flux ~1E-16 cgs flux

Much to do:

• Alternative color selections• Photometric surveys and real errors (PTF, PAN-STARRS)• Observational tests of selection criteria• Color selection optimization