Selected Past Projects

Chronologically, by end-date.

MAKO (2012 – 2015)

Figure 3. left: MAKO focal-plane array (Chris McKenney). right: Sgr B2 imaged at 350 μm with MAKO, and reduced by CRUSH.

MAKO was a detector & multiplexing technology demonstration camera built for the CSO, operating at 350 μm and 850 μm wavelengths, a project led by C. D. Dowell at Caltech. It's main purpose was to demonstrate the technologies needed for the large, 100-kilopixel or megapixel, cameras for the future. Specifically, it is to demonstrate low-cost production, with high-multiplexing and low readout cost, in order to meet the ultimate goal of $1/pixel combined expense.

My role in the project was twofold: providing a readout solution ( see above) for its detectors, and its data reduction facility, CRUSH (see above). MAKO had its first successful run in April 2013, with newer generation detectors tested in May 2015, and produced some beautiful images with the help of CRUSH.

S-Z Clusters (2013 – 2015)

Figure 9. An S-Z Cluster observed with GISMO, reduced by CRUSH.

I became interested in Sunyaev-Zel'dovich (SZ) clusters, partly because of GISMO. The SZ effect is the upscattering of cold Cosmic Microwave Background (CMB) photons as the radiation passes through the hot ionized material of intergalactic gas. It results in a partial depletion of photons at millimeter wavelengths, which is most pronounced around 2 mm (GISMO's band), producing the unique negative signature in maps.

The IRAM 30-m telescope has a niche for clusters. Most other SZ studies are performed either on small (10-m class or smaller) telescopes, or else dedicated interferometers that have short (~10-m baselines) to retain sensitivity on the arcminute scales of most clusters. The 30-m telescope offers more collecting area than the smaller dishes or the SZ interferometers, and a resolution in-between those. It is a sweet spot, offering both excellent sensitivity and sufficient resolution for cluster sub-structure, and/or to resolve dusty galaxies (e.g. bright cluster-core galaxies) embedded within.

GISMO (2009 – 2013)

Figure 1. The 8×16 TES array of GISMO (left). The IRAM telescope beam at 2 mm imaged by GISMO and reduced by CRUSH – 8'x8' size, 50+ dB dynamic range, logarithmic scale (right).

GISMO is a 2-mm camera built by J. Staguhn and his team at NASA Goddard Space Flight Center (GSFC) for the IRAM 30-m telescope near Granada, Spain. Its 128 pixels fill about half the telescope's ~4' field of view. GISMO has been operational since 2008.

For me, GISMO marked the beginning of CRUSH-2, the new data reduction platform capable of supporting virtually any submillimeter camera. I have also been active in improving the GISMO's performance, from ~70 mJy s^0.5 in 2010 to around 8 mJy s^0.5 by 2012 (grounding, bias optimization, and optical troubleshooting – such as spillover, stray-light rejection). Now, we are using all we have learned in the process to build the larger, better successor instrument: GISMO-2...

APEX Cameras (2006 – 2012)

Figure 2. APEX (left), and LABOCA's 295-pixel focal plane (right).

My efforts at the Max Planck Institute (MPIfR) were much focused on the APEX bolometer cameras, especially LABOCA (850 μm) and SABOCA (350 μm). Partly due to my contributions, LABOCA saw an order-of-magnitude improvement in sensitivity during each of four test runs, until it became the fastest (though, not background-limited) 850 μm camera in the world.

I have contributed to troubleshooting and optimizing its electronics (AC bias level and frequency), its signal chain (bias locked sampling, anti-alias filtering and downsampling, and microphonic avoidance), and optics (spillover and bandpass). Part of this effort saw the creation of the APEX Bridge software, a configurable anti-alias filtering and downsampling unit, which can be inserted between the control system (APECS/FitsWriter) and any telescope backend, in a transparent manner. My contributions to SABOCA were also similar in nature and in scope.

For the optimal reduction of data, I have forked my SHARC-2-specific CRUSH code, and created miniCRUSH for the APEX cameras. (By now, both the original CRUSH-1.xx and miniCRUSH have been retired in favor of the new CRUSH-2 platform, which does it all.)

Most recently, I have worked on supporting a chopped photometry mode for the APEX cameras, with a novel reduction approach that goes far beyond the traditional chop-nod analysis. Exploiting more of the redundancies present in the data stream, CRUSH is able to rid the chopped data from many of its traditional artifacts, yielding improved photometric sensitivity.

PolKa (2009 – 2011)

Figure 3. Unpolarized (left) and polarized (right) 850 μm emission in OMC-1, imaged with PolKa/LABOCA, reduced by CRUSH.

PolKa (Siringo et al. 2012) is a polarimetry frontend for imaging cameras, conceived and built by G. Siringo. Its unique feature is the room-temperature reflective half-wave plate (a wire grid at λ/4 spacing in front of a flat mirror), which makes it a seamless addition to any instrument.

Giorgio and I have beaten the odds to demonstrate its operation, by successfully separating the polarized emission of OMC-1 from the unpolarized part, despite bad weather and many technical problems. I have innovated a new polarimetry data reduction approach for fast scanning with a rotating waveplate (i.e. the telescope moves by more than a beam during a polarization cycle!). I am convinced that this mapping mode is superior to the more traditional approaches, resulting in cleaner polarization maps. Most of the data from a second, more extended, PolKa run in 2011 is awaiting analysis...

Properties of SMGs (2003 – 2010)

Figure 4. An SMG triplet. Its three components are unresolved by most submillimeter surveys.

Submillimeter-selected galaxies (SMGs) yield an unbiased characterization and history of star-formation in the Universe – provided we can learn enough about them. Whereas detecting them (typically near 1 mm wavelength) is relatively easy, seeing them in other bands is not, and measuring their distances is extremely challenging.

Based on the first limited redshift survey of SMGs (Chapman et al. 2005), I conducted a 350 μm follow-up, with SHARC-2 (Kovács et al. 2006). The addition of a shorter wavelength datum, near the SED peak, enabled the first true far-infrared (FIR) characterization (dust temperatures, masses and luminosities) of the SMG population. I concluded that SMGs were dusty galaxies typically around 35 K (although possibly getting hotter at higher redshifts), and that they strongly abide by the radio-FIR correlation (Helou et al. 1988) of local star-forming galaxies. As such, the SMG luminosities are predominantly fueled by star-formation, with no significant heating by an AGN, even when these are co-present.

In collaboration with A. Omont, we showed (Kovács et al. 2010) that the z~2 starburst galaxies selected by Spitzer (bumpies with a strong 24 μm excess due to the 7.7 μm PAH feature redshifting into that band) were essentially also SMGs, with nearly identical properties to the original Chapman et al. 2005 sample. Our study also provided one of the first hints that some of the mm-bright sources were in fact multiplets (we caught a triplet!), and that SMGs may be strongly clustered on short angular scales.

The LABOCA Deep Field (LESS) (2007 – 2010)

Figure 5. The E-CDFS by LABOCA with sources circled (CRUSH reduction).

Axel Weiss and I co-headed the deep 850 μm mapping of a half-degree by half-degree patch of the Extended CHANDRA Deep Field South (ECDFS) with LABOCA, a joint project between the Max Planck Institutes and ESO consuming over 300 hours of net observing time. The result was the deepest (around 1.2 mJy/beam rms over the full 1/4 sqdeg field), cleanest (purely Gaussian noise!), and most complete census of submillimeter galaxies selected at 850 μm, yielding some 120 SMGs.

Apart from contributing to designing and conducting the survey, I was most active in pushing the data reduction to the limit, and was responsible for source extraction, the P(D) number counts analysis, and the deboosting of fluxes – all areas in which I have pioneered new approaches.

One of the major conclusions of the survey was that we saw no evidence for a break in the powerlaw, describing the number of SMGs vs their brightness, despite the larger range of sensitivity we had vs prior surveys. We suspected that such breaks were a by-product of the circular, catalog-based number counts approach used by some of the preceding surveys (which we confirmed explicitly via simulations). A second major conclusion was that our number count models accounted for most (80–100%) of the FIR background at 850 μm as measured by COBE. Thus, we have indirectly proved that the mm-wave background is composed (almost) entirely of SMGs.

The LABOCA Survey of Nearby Galaxies (2007 – 2008)

Figure 6. Nearby galaxies with LABOCA: NGC 253 (left) and a composite image of Centaurus A (right), with the LABOCA image shown in orange hue.

Axel Weiss and I jointly led the program to map some nearby galaxies at 870 μm with LABOCA. Local galaxies are faint at 850 μm and are also quite extended, which is why sensitive 850 μm measurements (or at longer wavelengths) were not available before, even though they were much needed to constrain the massive amount of cold dust in these galaxies.

Deep observations of a handful of galaxies confirmed the value of the mm-wave measurement in determining the amount of cold star-forming material in these objects (Weiss et al. 2008). It also proved that single-temperature models, quite common until then, were inadequate for characterizing galaxies as a whole. This, in turn inspired me to develop my empirical multi-temperature SED model for describing the thermal emission of galaxies, one of my research areas today.

SIS Mixer Design (2002 – 2004)

Figure 7. Layout masks of the new CSO mixer chips, with twin 12 kA/cm² AlN junctions.

J. Kooi and I set out to replace the aging CSO facility receivers in all four bands (180–280, 280–420, 380–540, and 580–730 GHz) with new state-of-the-art technology. The new receivers were designed to be ultra wide band (up to 50% fractional RF bandwidth), tunerless, with 4 GHz IF (the mixers I designed support 12 GHz flat IF bandwidth!), and provide unprecedented stability. We opted for a balanced configuration, instead of side-band separation, to suppress LO noise – a choice, which was vindicated by the exceptional noise performance (i.e. sensitivity) of the new receivers.

The wide-band operation (up to 160 GHz) posed challenges for the mixer design with the medium current-density (12 kA/cm²) AlN barrier junctions supplied by JPL Microdevices Lab at the time. I used SuperMix simulations to determine the most robust design capable supporting that bandwidth, resulting in a minimalistic matching network in combination with twin SIS junctions. The mixers operate near or slightly above unity gain in all bands (180 – 730 GHz). I suggested a similar design for HIFI Band 1 as a consultant to LERMA. Even though the lack of manpower delayed delivery until now, they will be some of the best receivers in the world today...

SHARC-2 (1997 – 2003)

Figure 8. The 32×12 focal plane array of SHARC-2 (Dowell et al. 2003).

Much of my time in grad-school was focused on the development of SHARC-2, the 384-pixel 350 μm camera built by C. D. Dowell for the CSO. SHARC-2 was not only the largest bolometer camera of its time (and for a long time to come), it also was a game changer in the way we operate submillimeter arrays. I insisted on total-power observing (instead of position-switched differencing), and we were one of the first to rely on this mode of operation. Later, total-power observing became the standard mode of operation for most submillimeter cameras (owing in part to the success of SHARC-2). To enable total-power operation, I drafted its readout scheme (delayed demodulation of the AC-biased detectors, filtering, DC-subtraction), developed new observing modes (such as Lissajous scans), and pioneered a new data-reduction approach (CRUSH) to handle total-power bolometer timestreams.

I also wrote SHARC-2's first control and data acquisition software (JSharc), before we received IRC from NASA/Goddard. After commissioning in November 2002, I developed a direct calibration-scheme for the non-linear loading-dependent bolometer response, line-of-sight measurement (direct tau), a server for the polynomial tau fits (MaiTau), and a characterization of the 350um atmosphere (sky noise spectrum and effective NEFD statistics). 10 years after it began operation, SHARC-2 was still the best 350 μm camera on the ground(!). But, now we hope to de-throne it with MAKO ;-)...

Millimeter-wave Spectroscopy of Carbon-chain Radicals (1996 – 1997)

Figure 9. The cumulene carbenes H₂C₅ and H₂C₆ detected with the FTS.

I was a very lucky undergrad to have worked in the millimeter-wave spectroscopy lab of Pat Thaddeus, lead by C. Gottlieb. At the time, the lab used rotational spectroscopy exclusively to identify the exotic species of molecules that populate the interstellar medium (ISM). My job was to run some of the experiements. Using the 'old' glow-discharge absorption spectrometer, I detected C₈H, C₇H, and HC₁₃N (in that order), leading to several papers (McCarthy et al. 1996, Travers et al. 1996a, b; McCarthy et al. 1997a). The detection of C₇H marked a turning point in laboratory spectroscopy: it was the first time that lab spectroscopy preceded astronomical detection (Guelin et al. 1997), which then became the new norm.

Later, I was charged with getting the new Fourier Transform Spectrometer (FTS) to work. I wrote a large part of its control and acquisition software, and proceeded with optimizing the operational parameters for detection. My efforts were rewarded by getting its first detections: the cumulene carbenes H₂C₅ and H₂C₆ (McCarthy et al. 1997b), the latter of which was subsequently detected in space also (Langer et al. 1997).