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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 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?

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

[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)

[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

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

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)



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



Adelberger (2004)

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

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%

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%

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


  • 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

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