X-Spec: a Multi-Object Wideband Spectrograph for CCAT:

Concept and Technology Development

© 2013. CCAT. All Rights Reserved.CCAT Engineering Design Phase is partially supported by funding by the National Science Foundation’s Division of Astronomical Sciences

Motivation On-Chip SpectrometerWe are developing an on-chip millimeter-wave spectrometer technology for CCAT/X-Spec and other imaging spectroscopy applications such as intensity mapping. The approach is to create a filterbank in superconducting transmission line: each channel of the filterbank is a half-wave resonator coupled to a KID.

Canadian Consortium

First light X-Spec will cover both Band 1 and Band 2 for each of 25 independently-steered feeds. The upper black curve shows the worst case sensitivity (single pol, 2×photon-noise, line extracted from a total bandwidth of R=250), while the lower curve is the goal (dual pol, sqrt(2) × photon noise, effective noise bandwidth of R=700). For both, the instrument efficiency is 25%, forward efficiency is 92% (8% warm spillover), taper efficiency is 85%, and the Ruze efficiency factor assumes a total surface RMS of 12 microns. The sky is assumed to have 700 μm of water and be viewed at a 40 deg zenith angle. Model galaxy spectra are overplotted in color.

The bulk of our information about this early epoch originates in rest-frame optical/ultraviolet (observed-frame optical/near-IR). In many galaxies, the UV is a tiny fraction of the total bolometric energy, so substantial (and uncertain) dust corrections are required. CCAT cameras will identify broadly high-redshift dusty galaxies using flux ratios, but flux ratios do not provide an accurate redshift (unlike in the UV / optical in which there is a break). Charting a detailed history requires spectroscopic redshifts, and ALMA is not optimal for survey large numbers of objects. CCAT X-Spec is designed for spectral surveys. As redshifts are measured, atomic line fluxes (e.g. C+) also provide constraints on the temperature, density and UV field strength in the interstellar gas, revealing the conditions of star formation.

The top scientific priority identified for CCAT is to chart the cosmic history of the star formation and the buildup of heavy elements. Of particular interest is the early period, from approximately 500 million to 3 billion years after the Big Bang (corresponding to redshift ~10 to redshift ~2), in which the first generation of stars began enriching the medium, the Universe was reionized,, and the star formation rate increased to a what appears to have been a broad peak or plateau. This period of galaxy infancy witnessed the greatest logarithmic growth in the density of stars, black holes, and heavy elements, and set the stage for the relatively modest evolution for the remaining 10 billion years of the Universe’s history.

Star formation rate history compiled by Hopkins and Beacom, 2006. The epoch prior to z~3 relies on rest-frame UV measurements, some of which have substantial dust opacity corrections.

• The first multi-object spectrometer for the mm / submm.• Covers the full 195-520 GHz range at R~700.• Based on an on-chip lithographically-patterned spectrometer.• Uses RF-multiplexed kinetic inductance detectors (KIDs) based on titanium nitride (TiN), like the CCAT SWCam and LWCam. Shares readouts. 250 mK operating temperature.• ~2000 detectors per dual-polarization module. • First-light X-Spec will have at least 25 spectrometer elements.• Source density is 1/100 to 1/10 per beam, so each spectrometer is fed with an articulated quasioptical feed. Will be faster than ALMA for full-band surveys of galaxies with unknown z. • Will eventually be a large integral-field imaging spectrometer with ~1000 spectrometer ‘pixels’, ~30x faster than ALMA for spectral surveys.

C.M. Bradford for the X-Spec study teamWe are developing X-Spec, a multi-object wide-band direct-detection spectrometer for CCAT. X-Spec is designed for rapid full-band (195-510 GHz), moderate resolution (R~700) spectral surveys of galaxies, measuring the bright atomic fine-structure and molecular rotational transitions which cool the interstellar gas. Detected lines will provide redshifts for and interstellar gas conditions in tens of thousands of galaxies ranging from the early universe (z > 6) to the present day, a survey scale which is impossible with ALMA. X-Spec is based on an on-chip spectrometer in which the spectrometer and kinetic inductance detectors are defined lithographically in a single few-cm 2-sized package which offers the potential for a massive integral-field spectrometer limited only by available detector format. For the initial X-Spec to be deployed at or near CCAT first light, we are designing toward 50,000 total detectors; they will be arrayed in 25 spectrometer units, each individually steered to a target galaxy with a (warm) articulated quasioptical feed.

Hopkins and Beacom, 2006

Joaquin Vieira

350/870 μm flux ratio

350/870 micron ‘color’ as a function of redshift for submm / mm selected galaxies. The large range in dust properties translates into a large redshift uncertainty.

X-Spec at a Glance

Sensitivity

The system uses noibium microstrip on silicon nitride. Each KID consists of and inductor that absorbs radiation, and a capacitor, both patterned in TiN. We have built and begun testing a Band 1 80-channel prototype, shown here. (E. Shirokoff et al., chip design.)

KID coupling capacitors

mm-wave feedline (niobium, traveling horizontally)

KID resonator capacitors (titanium nitride, interdigitated)

mm-wave half-wave resonator (U-shape, niobium)

mm-wave absorber = meandered KID inductor (titanium nitride)

7 mm

Initial optical tests show basic functionality and that materials have sufficiently low loss. Next step is smaller KID inductor for increased response.

www.CCATObservatory.org

Steering System ApproachX-Spec will couple each spectrometer to a galaxy position with a 2-axis quasioptical steering system located just exterior to the cryostat vacuum jacket. The concept shown here is based on a pair of commercially-available Aerotech rotary stages.

Goldsmith & Seiffert, 2009

Full 1-deg (3-m, curved) CCAT focal plane

accommodates ~96 units

Lupe Banales et al.

S. Hailey-Dunsheath, T. Reck

5 cm

~30 cm

J. Zmuidzinas

Chary+Pope

SPT

12700

4900

2300

1000350 μm dropouts