Radio multiobject spectrograph C. Carilli, NRAO, GBT new instrumentation workshop, Sept 06

Multiobject Spectrographs: Revolution in Optical astronomy redshift surveys – 10,000’s redshifts SDSS,VIMOS, 2DF, DEIMOS…

10’s – 100’s galaxies per night

Slit positions pre-set based on optical/submm/radio/Xray imaging

Radio: typical cm/mm focal plane arrays: ‘Integral field units’ = continuous coverage of center of focal plane

MAMBO 250GHz bolometer camera/IRAM30

MMIC heterodyne array/FCARO

Needed for extragalactic radio astronomy: true multiobject spectrograph with adjustable ‘slit’ positions

Need for radio multiobject spectrograph I

Submm galaxies: formation of large spheroids in dusty starbursts at z=1 to 3 (SFR ~ 100’s to 1000 Mo/year?

MAMBO/30m

117 pixels

250 GHz

10.6”

0.9 mJy rms

~ 20 sources in typical 20’x20’ field

Bertoldi, Carilli, Schinnerer, Voss, Smolcic +

COSMOS/MAMBO 250GHz survey

Difficulty: Optical Ids and Redshifts

250GHz 1.4GHz Opt

Selection with 1.4 GHz imaging gets 50% to 75% at 10’s uJy sensitivity, with low z bias.

Missing most interesting sources = most distant?

PdBI dedicated study (10’s hours/source) of radio selected submm galaxies with optical redshifts (Greve et al. 2005)

Massive gas reservoirs (~1e10 Mo) = requisite fuel for star formation

Need multiobject spectrograph for unbiased search for CO, other molecules in complete submm galaxy sample.

z=2.4

Need for radio multiobject spectrograph II

Ly emitters into cosmic reionization: probing ‘first light’

~ 100 LAEs at z=5.8+/-0.1 in COSMOS Field (2square deg)

SFR (Ly) ~ 10 Mo/year

Represents ‘normal’ galaxy population during EoR?

Radio/MAMBO analysis: No bright, dust obscured starbursts

Need multiobject spectrograph to perform ‘stacking analysis’ to gain factor > 5 in effective sensitivity for <CO> properties, and to find rare, dusty starbursts

Radio stacking analysis:

<S1.4> < 3 uJy/beam

<SFR> < 120 Mo/year

Need for radio multiobject spectrograph III

Dense as tracers – HCN, HCO+, …

Trace > 1e3 cm^-3 => gas directly related to star forming clouds

Typically few to 10x fainter than CO => only seen in most pathologic and/or highly lensed sources

need multiobject spectrograph to perform stacking analysis on sample of submm galaxies to get mean dense gas properties

VLA obs of HCN in Cloverleaf 200 uJy!

Need for radio multiobject spectrograph IV

Nearby galaxies: Giant HII regions, GMCs, superstar clusters

K/Ka band lines: water, ammonia, methanol, C3H2, SO…

Need MOS to get spectra of many regions simultaneously

M101 Chen et al

10’

Feed ring Radius=30 cm

or 45cm

Use of Focal plane at GBT

(Norrod & Srikanth)

Offset = 50cm =>

Throw = 9arcmin

Efficiency = 60%

Specifications FoV = 10’ to 30’ (10’s submm gal, LAEs, egal SFRs…)

Number of receivers = 16 to 25 (space limitations?)

Reconfiguration (at most) once per day

Tracking = +/- few hours (=> rotate)

Spectrometer & IF (K/Ka/Q band)

LBGs and LAEs: z_spec ~ 100 km/s => ~20 MHz/source

Submm gals: z_phot ~ 0.2 => ~2 GHz/source

Potentially trade-off of ‘slits’ for bandwidth?

Sky removal: use full array?

Integral field spectroscopy: close-pack configuration?

Implementation

Individually adjustable receivers and feeds.

Challenge: Cryogenics?

Dense-packed receivers + adjustable feeds + flexible waveguide.

Challenge: Noise performance?

Dense-packed receivers and feeds + mirrors.

Challenge: Optics and tracking?

Very large array?

Overkill