Direct detection spectroscopy at the cso with z spec and zeus
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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

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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

Matt Bradford

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 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

  • 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


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

  • 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 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

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-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

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




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.

Z-Spec Spectra from the CSO

Z-Spec Spectra from the CSO

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

Matt Bradford

Matt Bradford

APM 08279+5255 at z=3.91

Matt Bradford

Water Stacking of APM ?

Matt Bradford

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

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

[ 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

Nearby Seyfert NGC 1068

Z-Spec Commissioning At CSO

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