BigBOSS Survey and Spectral SimulationsBigBOSS Survey and Spectral Simulations

Nick Mostek

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0.2<z<1: Luminous Red Galaxies (extended from BOSS footprint)

Spectroscopic TargetsSpectroscopic Targets

2<z<3.5: Ly forest from QSOs (pioneered from BOSS)0.7<z<2: Emission line galaxies

BigBigBOSSBOSS

Padmanabhan, 2004

REALLY?

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925 950 975 1000 1025 1050 1075 1100λ (nm)

QE

130K140K150K160K170K180K

500 μm thick

z=1.75 for [OII]

Extreme Silicon (Bebek)Extreme Silicon (Bebek)

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• BigBOSS will have 5000 fibers spread over a 4.9 sq. deg. field of view. With ~1000 fibers/deg2, we will need to split up the ELG sample into low and high redshift bins.

• Lower redshift bins will make measurements in half the time as the z>1.5 galaxies and be re-assigned to new objects

• Fibers can be repositioned in less than 2 minutes, or less than that of the LBNL CCD read time at 20kHz.

• LRGs and QSOs require longer exposures as they need to measure the continuum flux levels

• If we change our upper redshift limit, can we:– push the low redshift bin to higher densities?

– survey faster?

White Paper Survey YieldWhite Paper Survey Yield

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• Keep LRG and QSO number densities– LRGs require at least 30 minute exposures– QSOs require the entire exposure time of a fiber

• ELG redshift distribution is defined by minimum desired [OII] flux

• ELG minimum exposure time must scale flux according to previous exposure time estimates (S/N = 8 in 30 minutes at z=2)

• Maximum exposure time on the field must be an integer number of minimum exposure times (coherent readout of all spectrographs)

• Number of available exposures in fiber positioners that of a given diameter that can fit into a field of a given size

– Used 15mm diameter positioners in a 2.8 deg diameter field in this study (6400 fibers total, 20% area overlap)

Survey ConstraintsSurvey Constraints

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grzgrz-selected-selected Redshift DistrubtionRedshift Distrubtion

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• Used “Maximal Pack” of 2.5’ (12mm diameter, 20% overlap) diameter circular positioners into a 2.8 deg diameter field• Distribution of emission line galaxies with [OII] > 8.5E-17 and 75% completeness in grz• Uniform spatial distribution is currently used (no clustering)

Focal Plane Fiber MapFocal Plane Fiber Map

Black Square: Fiber CenterRed Triangle: QSOLt. Green Star: LRGOrange Diamond: Low-z ELG

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• Red line corresponds to a constant volume density of 3.4E-4 (Mpc/h)^-3

• grz selection needs improvement for z>1.2 galaxies

Measured Measured z z DistributionsDistributions

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• Numbers correspond to [OII] flux limit x 10-16 ergs/s/cm^2• Vertical line = 12k sq. deg in 3 years (6.15 deg2 field, 90 nights, 8.5 hrs/night)• Extended field exposures are more efficient at sampling faint target distributions but sacrifice

efficient fiber exposure time usage and longer a longer survey

Target Measurement EfficiencyTarget Measurement Efficiency

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Most of the BAO signal for [OII] will be measured with the Red spectrograph

BigBOSS SpectrographsBigBOSS Spectrographs

White paper

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• 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

• Throughput calculation comes from a spreadsheet calculation from Robert Barkhouser and is based off of WFMOS efficiencies

– *INCLUDES loss at 150micron fiber due to 1” seeing (~32% loss)

• Only considered Red Spectrograph Arm (0.8-1.1 micron) for now

OH sky lines and ThroughputOH sky lines and Throughput

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• Simplified spot model is a circle (fiber) convolved with a Gaussian (optics)

• Since we are using the SDSS III / BOSS spectrograph design, match spots at 9000 Ang, rescale for 150 micron fibers, and put on 18 micron pixels

• Created data cube with spot center shifted along one axis (dispersion direction)– Provides look up table for each position in 2D spectrum, speeds up simulation

BigBOSS spotsBigBOSS spots

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Spot C

ente

r Shi

ft

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λ

Advantage 1: High resolution allows us to work between the night sky lines

Why High Resolution?Why High Resolution?

Advantage 2: High resolution splits the [OII] doublet

– Forbidden transition gives an unambiguous line identification

– Two lines doubles the chance of line measurement among bright sky lines

Sky Lines

[OII][OII]λ3726, λ3729 @ z=1.4

25 sky fibers

Observed Spectrum

Sky-Subtracted Spectrum

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• Collapsed 2D spectra and variance to a 1D Signal-to-Noise spectrum

• Repeat simulation for galaxy spectrum redshifted from 0.7<z<2 in dispersion steps (0.732 Ang/pix) on both CCD and HgCdTe detectors

• Galaxy continuum is interpolated from a zCOSMOS template and [OII] flux draws from the DEEP2 luminosity function at 3.4E-4 (Mpc/h)-3 volume density

• Data are fed to redshift fitting code (see Jonathan Pober’s talk tomorrow morning)

Spectral SimulationSpectral Simulation

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[OII]

[OIII]λ4959[OIII]λ5007

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Effect of Resolution on Line SensitivityEffect of Resolution on Line Sensitivity

• Compute the error per 1-D pixel for read + sky + Poisson noise

• Compute the flux required for S/N=8 in 30min for a single line at the minimum 2 pixel resolution• Higher resolution leads to a smaller redshift coverage for one spectrograph• Lower resolution leads to decreased overall line sensitivity as more sky is present in the line

measurement

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• Seems plausible to do 12k sq. deg in 3 years by limiting z<1.5 at the source densities we have been talking about for BigBOSS

• We can go to much higher densities at lower redshifts, but must sacrifice objects at higher redshifts

• Simulated 2D spectra creation tools exist with some simple assumptions about the design

Need to do:

• Color cut needs refinement for surveying targets at higher redshift

• Improve flux scaling to reflect latest instrumentation

• Refinement of survey distributions and creation of mocks

• Instrumentation studies! (Throughputs, scattered light in PSFs, resolution optimization)

• Different field sizes, fiber densities, fiber overlap fractions, placement logic

• Feed final measured redshift distributions to FoM calculators

Results so far….Results so far….

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Going Forward…Going Forward…

• Checks for Star-Forming Galaxies– Current clustering estimates from small surveys {Sumiyoshi (2009), Geach (2008), Orsi (2009)}– Need wide field survey with mask (CARS?)– Need clustering from appropriate color selection (zCOSMOS)– Stellar Mass vs [OII] ?– LF out to higher redshift (z=1.7) with DEIMOS or alternate? Completeness for faint end (z=0.7)?

• Semi-analytic mock catalogs / HOD N-body– Define goals for each simulation study– Do we focus on lower mass ELGs for 0.7<z<1.5?– Do we include other target distributions (QSOs from 1<z<2)?– Can we coordinate with DES mocks?

• Resources– Requirements?– NERSC time available?– European or Chinese contributions?

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• DEEP2 luminosity function for constant volume number density

• Flux limit scales with DL-2, volume density, AND wavelength of [OII] at zmax

• Exposure times are currently scaled to 1800s at z=2 for S/N=8 and [OII]=5E-17 flux limit

Scaling FluxScaling Flux