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Direct-Detection Spectroscopy at the CSO with Z-Spec and ZEUS. Probing galaxies near and far with two new bolometers-based grating spectrometers Matt Bradford with input from Gordon Stacey August 4, 2008. Dominant gas coolants are in the far-IR / submm Redshifted to the submm / mm.

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direct detection spectroscopy at the cso with z spec and zeus
Direct-Detection Spectroscopy at the CSO with Z-Spec and ZEUS

Probing galaxies near and far with

two new bolometers-based grating spectrometers

Matt Bradford with input from Gordon Stacey

August 4, 2008

Matt Bradford

dominant gas coolants are in the far ir submm redshifted to the submm mm
Dominant gas coolants are in the far-IR / submmRedshifted to the submm / mm

CSO @ z=0

CSO @ z=1.2

CSO @ z=2.6

CSO @ z=4.4







SED courtesy A. Blain

Matt Bradford


Direct-Detection SpectroscopyA survey capability which complements the high spatial and spectral resolution of interferometers (CARMA / ALMA)

  • Submillimeter is the region of overlap between coherent (heterodyne) and incoherent (direct-detection) techniques for astrophysics.
  • Coherent approaches have yielded much of the spectroscopic work to date.
    • High spectral resolution required for Galactic cores.
    • Large bandwidths not essential for Galactic sources or nearby galaxies.
    • SIS mixers near quantum limit, (also near background limit at 1 mm).
    • Until recently, direct detectors neither sufficiently sensitive, nor sufficiently arrayed to be compelling.
    • Direct-detection spectrometers (gratings and Fabry-Perots) for long wavelengths are large and expensive.
  • Direct-detection submillimeter spectrometers are now compelling
    • Submillimeter spectroscopy coming of age as an extragalactic probe.
    • Spectral resolution greater than few x 1000 not required.
    • Direct detectors are now readily background-limited, and are undergoing a revolution in format (driven by cameras).
    • Large fractional bandwidth presents a new discovery space for measuring redshifts and multiple lines.
    • Multi-object capability a natural progression with a direct-detection system.

Matt Bradford

the world s only submillimeter grating spectrometers
The World’s Only Submillimeter Grating Spectrometers


The Redshift (z) and Early Universe Spectrometer

Stacey et al. (Cornell) w/ GSFC, NIST

  • Short submm windows: 350mm, 450 mm
  • Slit-fed echelle grating, 4th and 5th order
  • Resolving power ~1200 (300 km/s)
  • 20 GHz (~2-4%) instantaneous bandwidth
  • 1x32 bolometer array, but TES upgrade underway


Glenn (U. Colorado); Bradford, Bock, Zmuidzinas, (Caltech), Aguirre (CU-> Penn), Matsuhara (ISAS)

  • 1 mm window: 195-310 GHz
  • Single beam w/ new waveguide grating architecture
  • Resolving power ~300 (1000 km/s)
  • 100 GHz (40%) instantaneous bandwidth
  • 160 individually-mounted Ge-sensed bolometers

Both with sensitivity very close to fundamental limits at the CSO

Matt Bradford

primary scientific objectives
Primary Scientific Objectives
  • ZEUS
  • J=6->5 and 7->6 in both 12CO and 13CO and [CI] J=2->1 constrain the mass and energy budget of the warm molecular gas.
  • Z-Spec
  • Complete 1-mm spectrum includes multiple high-critical-density species: CN, CS, HCN, HNC, HCO+. A rapid census of the dense molecular gas.
  • Embedded energy sources and conditions of star forming gas in local-universe infrared-bright galaxies (LIRGS and ULIRGS).
  • Interstellar medium conditions and spatial extent of star formation extent in the era of peak star-formation history (z=0.5 to 2) and prior.
  • Evolutionary history of energy release via unbiased redshift surveys.
  • ZEUS
  • C+ at z=1--1.2 and 1.8--2. C+ to dust continuum ratio measures the UV field intensity, constrains the extent of the starburst.
  • Access [OI], [NII], [OIII] at the highest redshifts.
  • Z-Spec
  • Full band provides at least 2 CO transitions to measure redshift as well as temperature, density, and mass of the molecular gas.
  • Unexplored rest-frame short-submm.
  • C+, other fine-structure transitions accessible beyond z=6.

Matt Bradford

zeus optical path
ZEUS: Optical Path


BP Filter Wheel


LP Filter 2


Detector Array




4He Cold Finger

LP Filter 1

Entrance Beam


Quartz & LP Filter 1

M5: Primary


Scatter Filter

Dual Stage 3He Refrigerator


4He cryostat

  • 35 cm long R2 echelle grating blazed for 5th order @ 359 m
  • There is a series of a scatter, quartz, 2 long  pass, and a bandpass filter in series to achieve dark performance (Cardiff U.)
  • Total optical efficiency: ~ 30%, or 15% including bolometer DQE

Matt Bradford

zeus on the cso
ZEUS on the CSO
  • Mounted on the left Nasmyth focus
  • Has been co-mounted and co-scheduled with both Z-Spec and Bolocam
  • First light in 2006
  • Thesis project of Steve Hailey-Dunsheath (Cornell PhD 2008)

Matt Bradford

zeus observations of ngc 253 first extragalactic detection of 13 co 6 5


ZEUS observations of NGC 253: First Extragalactic Detection of 13CO(6-5)
  • Line is bright ~ 10% that of the 12CO(6-5) line indicating optically thick emission in the main line.
  • We also re-observed (and mapped) the CO(7-6) line to constrain LVG models
    • 35 to 55% of the molecular ISM is warm and dense: T~ 120 K, n~104 cm-3
  • Heating this much gas is difficult:

likely due to that X-rays from the starburst or the decay of micro-turbulence within clouds must dominate the heating.

  • These processes are powered by the starburst -> the starburst is self-regulating.

Hailey-Dunsheath et al. in prep.

  • [CI] (2-1) line only 1000 km/s to the blue and always within ZEUS band.

Matt Bradford

zeus observations of lirgs ulirgs
ZEUS Observations of LIRGs & ULIRGs

IRAS 18293

Arp 220 CO(6-5)


IRAS 17208 CO(6-5)

NGC 6240 CO(6-5)


NGC 6240 CO(8-7)

NGC 6240 [CI] (2-1) & CO (7-6)

VLSR(km/s) VLSR(km/s)

  • Pre-ZEUS: 1 ULIRG in CO 6-5 (Mrk 231, Papadopoulos et al. 07)
  • ZEUS has observed ~19 LIRGs and ULIRGs to date
    • Most in CO (6-5)
    • Some also in CO (7-6) & [CI] (2-1) or CO (8-7)
  • Fractional mid-J CO luminosity decreases in the most powerful sources (as with C+).
  • -> More concentrated systems than the less-luminous starbursts.

Matt Bradford

Nikola et al. in prep.

zeus high r edshift e xample cii from mips j142824 0 352619
ZEUS High-Redshift Example: [CII] from MIPS J142824.0 +352619
  • Identified as red object in MIPS Bootes field (Borys et al. 2006)
  • Integrated far-IR SED indicates Lfar-IR ~ 3.21013 L
  • Likely a mildly lensed super-starburst galaxy
  • ZEUS/CSO detection in April 2008 -- 1.5 hours of good (but not great) weather (225 GHz ~ 0.05 to 0.06)
    • I[CII] ~ 6 K-km/sec
    • Fline ~ 9.0  10-18 W m-2
    • L[CII] ~ 2.5  1010 L
  • CII / far-IR ratio much greater than in local ULIRGs. Conclude that starburst is 2-3 kpc in extent – “galaxy wide starburst”

Hailey-Dunsheath et al. 2008

Matt Bradford

z spec a new ultracompact waveguide grating

curved grating in parallel plate waveguide

Z-Spec: A New Ultracompact Waveguide Grating
  • Propagation confined in parallel-plate waveguide
    • 2-D Geometry
    • Stray light eliminated
  • Curved grating diffracts and focuses
    • Efficient use of space
    • No additional optical elements
  • Custom “stigmatic” grating design possible at long wavelengths

H.A. Rowland, 1883, Phil. Mag 16

K.A. McGreer, 1996, IEEE Phot. Tech. 8

Matt Bradford

z spec layout
Z-Spec Layout




Individually mounted

SiN bolometers

Focal ARC

CSO, Mauna Kea




Matt Bradford


Z-Spec graduate students @ 13,400 ft

Lieko Earle (Colorado),

Bret Naylor (Caltech)

Matt Bradford


Z-Spec 1 mm survey of NGC 253

Lieko Earle, U. Colorado Ph.D. ‘08

3.5 hours telescope time

19 ID’d transitions > 3s

+4 unID’d as of yet.


Molecular Gas in Local-Universe Galaxies, ex. M82

B. Naylor et al., ApJ in prep.

Compile all transitions, use RADEX to model excitation & transfer in the lines

-> Generate Bayesian likelihoods





Also include:



SO2Combine in a

single model:

-> evidence of cold, dense gas component

-> the material actually forming the stars?

Matt Bradford

plans for the next cycle zeus z spec instrument programs funded
Plans for the next cycleZEUS & Z-Spec instrument programs funded

Z-Spec Survey Program

Funded by NSF AAG

(Aguirre et al. U. Penn)

  • Dense gas in Local-Universe dense molecular gas surveys.
    • 50 galaxies, 8 hours per
  • Mid-J CO + spectral discovery in high-z objects with and without prior redshifts.
    • 20 galaxies, 24 hours per
  • Excellent use of low-frequency time at CSO.
    • Baseline 300 hours per year, could be more.
    • Helium recycler under study to reduce cryogen costs.

ZEUS upgrade to ZEUS-2

Funded by NSF MRI

  • Incorporating (3) NIST 2-d TES bolometer arrays which share the focal plane and can operate simultaneously:
    • 10 x 24 at 200 mm
    • 9 x 40 at 350-450 mm
    • 5 x 12 at 650 mm
  • Up to 5 lines simultaneously (in extended sources)
  • Some imaging capability (9-10 beams)
  • Closed cycle refrigerators

Matt Bradford


Backup material

Matt Bradford

cii far ir constrains starburst extent

Starlight that contributes to  but not G

[CII]/far-IR continuum luminosity ratio vs. density for various G (from Kaufman 1999).

[CII]/far-IR Constrains Starburst Extent

L[CIII] ~ 2.5  1010 L

Lfar-IR ~ 3.2  1013 L

30% of [CII] from ionized medium

R =5.5  10-4

 G ~ 2000

far-IR = L/(4D2) = 14

DL~ 9.2 Gpc

 = IR/(G 2) = 3.5 x 10-3

 = beam = 0.083(”)2

d ~ 0.32”  2.75 kpc


- 700

Galaxy-wide starburst supports the contention that hyper-luminous systems may be giant elliptical galaxies in formation (unlike local ULIRGs)

Matt Bradford


Z-Spec channel spectral response

Measured with long-path FTS

(~100 MHz resolution)

  • Range: 185--305 GHz
  • Resolving power: 250--400
  • (Not over sampled)
  • (750 < v < 1200 km/s)
  • Complete coverage from channel to channel -> no gaps

Matt Bradford


Z-Spec Sensitivity

Observed noise white with atmospheric 1/f

Relative to an imaging system, fundamental noise levels are lower, but some systematic aspects are easier.

Chopping -> response to a single frequency

Narrow spectrometer bandwidth helps

NEFsky ~ 

NEFGaussian noise ~ sqrt()

Scaling consistent with e.g. Bolocam observations


Z-Spec Sensitivity

Clear scaling with , very close to photon background limit

Blue -> achieved at =0, 0.1, 0.2

Black -> simple model for Z-Spec at CSO

Det, amplifier, & internal load NEP: 6.4e-18 W/sqrt(Hz)

(not tracking detector parameters in detail)

measured instrument trans (~0.25)

Aperture efficiency per taper + Ruze (60-70%)

measured chop duty cycle (65%)

photon noise from sky + telescope the most important term

-> additional factor of 1.2







Z-Spec labor force

James Aguirre -> U. Penn

Jansky Fellow

Colorado, NRAO

Bret Naylor

Recent Caltech Ph.D.

Lieko Earle

Colorado Ph.D. student

(finishing Spring 08)

ulirg survey preliminary results
ULIRG Survey Preliminary Results

[ Line fluxes in Jy km/s, HCN / CO ratio corrected for to TMB ]

Line fluxes SNR 4 - 20

Not finding overluminous HNC / HCN 3-2 ratio as per Aalto, Cernicharo.

will follow-up further at CSO.

Matt Bradford