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Target Selection for BigBOSS

Target Selection for BigBOSS. Nick Mostek. Target Selection Study. This study looked at the photometric target selection criteria to identify emission line galaxies with brightest [OII] fluxes from 0.8<z<2

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Target Selection for BigBOSS

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  1. Target Selection for BigBOSS Nick Mostek

  2. Target Selection Study • This study looked at the photometric target selection criteria to identify emission line galaxies with brightest [OII] fluxes from 0.8<z<2 • BigBOSS wants to target these galaxies with a projected sky source density of 2000 dn/(dz·deg2) or a space density roughly 3.4E-4 (Mpc/h)3 • Photometric targeting survey must cover >10k sq. deg in the northern hemisphere by the middle of next decade • Questions to answer: • What is the required [OII] line flux sensitivity to achieve the goal target density? • What do the [OII] lines look like between between the OH sky lines? • What minimal set of photometric bands can target these emission line galaxies? • What is the targeting efficiency?

  3. Data Sources • For this study, I used the [OII] line fluxes provided by DEEP2 (Zhu et. al, 2008) and zCOSMOS (Ilbert et. al, 2008) • Zhu develops an [OII] luminosity function from 14,000 galaxy spectra from 0.7<z<1.5 and I extrapolate to z=2 using the function values of the highest redshift bin • Ilbert uses the best fit galaxy template spectra from 30 bands for the COSMOS field and assigns an [OII] flux using a Kennicutt NUV-[OII] calibration • Ilbert [OII] fluxes have been cross checked against VVDS spectra out to z=1.5, observed a scatter of 0.2dex which is reflected in the catalog values • Template spectra from zCOSMOS are used for synthetic photometry of ~520k galaxies over 1.3 sq. deg. • To suppress cosmic variance, I have resampled the zCOSMOS catalog in z=0.4 bins using the brightest [OII] emitters

  4. [OII] Line Flux Sensitivity • Above graph shows the [OII] line flux limit for a sky source density of 2000 dn/(dz.deg2) • Both Zhu and Ilbert are in reasonable agreement from 0.8<z<2, particularly considering that such luminosity functions can easily vary by a factor of 2 or more • I calculate a conservative [OII] minimum detectable line flux (MDLF) of 2.5E-17 ergs/s/cm2 (F[OII]=1E-16 cgs at z=2, reduced by a factor of 2 for split [OII] and another factor of 2 for margin)

  5. [OII] between OH sky lines • Sky background from BOSS studies (Gemini high resolution spectra scaled in flux to SDSS) • Should be a conservative estimate, will update with DEEP2 DEIMOS spectra • Galaxy spectrum and emission line fluxes are from zCOSMOS catalog (Ilbert, 2008) • Emission lines are normalized to Ilbert fluxes and given 50 km/s Gaussian line width

  6. Target Selection Bands Adelberger (2004) • 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. • For this study, I looked at grz color selection from 1<z<1.5 and ugr from 1.5<z<2 • Additional bands may be useful for selection of objects with distinctively flat SEDs, but needs to be studied

  7. grz Selection Bands • I used the zCOSMOS spectra and performed synthetic photometry in the ugriz bands • Figure at right shows grz color plane with Ilbert split F[OII] >5E-17 cgs for 1<z<1.5 galaxies and a magnitude limit of r <23.3 mag • Selection box is drawn around bulk of bright [OII] emitters while avoiding background galaxies with similar colors • A F[OII]=2xMDLF is used for the lower redshift cut (assumes shorter exposure times for these objects) Reddening Redshift

  8. ugr Selection Bands • Selection box is drawn around split F[OII] >2.5E-17 cgs for 1.5<z<2 galaxies and a magnitude limit of r <24 mag Reddening Redshift Adelberger (2004)

  9. Survey Photometric Errors • Targeting efficiency of the previous color selections will depend on the quality of the survey photometry • Looked at the following surveys: • PanStarrs (grizy, PS1=360s, PS2=1200s, 30k deg2) • Palomar Transient Factory (gr, 3 hr, 12k deg2) • New CFHT MegaCam survey (u, 400s, 14k deg2) • Used available sensitivities, sky brightnesses, and median seeing values

  10. grz Selection Bands • Used gr photometry from PTF and z-band from PS1, mag limit of r<23.3 • Selection delivers a mean of 2300 dn/(dz.deg2) from 0.8<z<1.5 • Overall selection of star-forming galaxies with split F[OII]>5E-17 cgs is >85%

  11. ugr Selection Bands • 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 dn/(dz.deg2) from 0.8<z<2 • Overall selection of star-forming galaxies with split F[OII]>2.5E-17 cgs is >90%

  12. CFHT u-band Survey • CFHT MegaCam is a unique wide-field camera (1 sq. deg) with high u-band efficiency • Upcoming large area surveys (PanSTARRS, PTF) in the Northern Hemisphere will not observe in u-band • ugr-band photometry will be an good color selection plane for star-forming galaxies from 1<z<2 • First-order exposure time calcuation: • Goal is S/N = 5 at u=24 • Zeropoint mag = 25.77 • Seeing = 1.2” at 1.5 airmass • Sky brightness: 21.3 mag/arcsec2 • 40s readout and slew overhead ~400s per exposure x 1 sq. deg x 14000 sq. deg / 8hrs/night = 200 nights of observing time

  13. Summary • At a sky source density of 2000 dn/(dz.deg2), the conservative split [OII] MDLF is ~ 2.5E-17 ergs/s/cm^2 at z=2 • Star-forming, late-type galaxies with strong [OII] emission can be selected from ugrz photometry from 0.8<z<2 • PanStarrs z and PTF gr will deliver deep photometry over sky areas and depths that will be useful for target selection from 0.8<z<1.5 • A u-band survey from CFHT MegaCam would greatly enhance the target selection from 1.5<z<2.0 and possibly cover the entire redshift range of interest • Targeting efficiencies for MDLF galaxies in (synthetic) magnitude-limited surveys from 0.8<z<2 top 85% with simple selection cuts

  14. To do: • Investigate application of these [OII] target selection criteria using available photometric surveys and high-resolution spectrographs out to z=2 • Surveys: DEEP fields, co-added SDSS equatorial fields • Spectrographs: GNIRS, GMOS, others??? • Gauge interest level in a u-band survey with CFHT (or others?) • Increase selection efficiency using additional bands and sliding magnitude cuts • Need to look at photometric selection of LRGs for z<1 using riz colors • Simulate BigBOSS spectra using zCOSMOS catalog, expected instrument parameters, and night sky brightnesses • Develop emission line detection algorithms amid sky lines • Incorporate QSO and LRG spectral simulations as well • Prototype instrument will help with optimization of color selection and improve accuracy of simulated spectra / data pipeline

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