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Emission Line Emission Line Galaxy Targeting for Galaxy Targeting for
BigBOSSBigBOSS
Nick MostekLawrence Berkeley National Lab
BigBOSS Science Meeting
Novemenber 19, 2009
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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
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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
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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)
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[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
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• 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)
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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)
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• 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)
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• 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
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• 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
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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
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grzgrz-selected-selected Redshift DistributionRedshift Distribution
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• 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
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• 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
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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
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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