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