Attendance: Al, Shaul, Julian, Lloyd, Raphael
Notes by: Karl
Spectrometer conversation with Al
Hi Al,
Can you pose this question to the group: what spectrometer would it take to get to the silk damping signal at >5 sigma? What aperture or set of apertures? Is the measurement feasible given the foregrounds? What are the other challenges to the measurement? Would you gain anything by sacrificing the polarization sensitivity?
Shaul
Hi Shaul,
Max and Colin have run some analyses to see what is required for a
spectrometer to detect the spectral distortion (mu distortion) from
Silk damping at better than 5 sigma. For specificity, the sensitivities
are calculated relative to PIXIE 12 month observation. Sensitivity
scales as sqrt(time) and sqrt(etendu).
If there were no foregrounds, reaching 5 sigma requires 13x PIXIE sensitivity. This could be PIXIE with 153 months of observations, or a larger Super-PIXIE spectrometer with (say) 4 sets of mirrors and 54 months observation.
Foregrounds seriously degrade the prospects of detection. A rough scaling is that the synchrotron amplitude and spectral index need to be measured to 0.1%. Achieving this with PIXIE's baseline 15
GHz spectral bin width requires 390x PIXIE sensitivity for a worst-case scenario (no help from spatial information) and ~80x PIXIE sensitivity for a more realistic scenario with foreground cleaning such as SMICA that includes spatial information. This could be achieved using some combination of longer integration time (10 year mission like WMAP), fatter beams (7 deg tophat like FIRAS) and multiple cloned FTS assemblies (12 copies).
Foreground subtraction is substantially improved by adding channels below 30
GHz. Fourier transform spectroscopy is not ideal for such low-frequency observations, so this would better be addressed by adding an absolute photometer of some sort, sharing the same calibrator as the FTS. Adding a low-frequency photometer could reduce the sensitivity requirement of the FTS from the 390x PIXIE sensitivity above to perhaps 30x PIXIE. One such scenario would have 4 years integration, 5 deg beams, and 3 copies of the FTS assembly.
Much of this analysis is captured in https://arxiv.org/pdf/1705.01534.pdf. It seems that a Super-PIXIE that includes a low-frequency photometer could indeed get a reliable detection of the Silk damping signal.
Cheers, Al
General notes
Foreground Workshop – skipped everyone online knows already.
Spectrometer status
time for final tally of opions. Shaul will poll those not on call.
decision of inclusion of spectrometer at possible expense of imager aperture and/or sensitivity.
measurement of silk dampening at 5 sigma? needs Super-PIXIE or larger. details above.
Al: mini-PIXIE valuable insurance. garuntees high S/N dust foregrounds. Also different systematics especially on large scales. Gives 2 methods to measure reionization bump which is more compelling. Reionization bump is major science goal.
Raphael: Agree we need Reionization bump. But worry about loss of resolution in small case –> can't delense with PICO alone. What about just probe spectrometer?
Al: should get reionization bump cleanly. But can't delense. So need ground synergy to get recombination bump down to r ~ 10^-4. Risk because don't know what ground will be able to do.
Shaul: How well does probe-PIXIE do low frequency foregrounds? Al: depends on your bandwidth choices to get lowest frequencies. Then FTS does poorly in lowest bands. Need a low frequency photometer.
Shaul: Early on look at Super-PIXIE concluded that benefit was only recombination lines. Not a large gain over $250M PIXIE. We can reconsider this.
Raphael: Goal seems like B-modes + X. What is X in spectrometer case? What are differences in r measurement for various instrument cases?
Al: comes down to what scales matter most for inflation science. Large imager overlaps in l with S4, l > 50 will be down from ground. Prime reason for space is lowest l's. Spectrometer focuses almost exclusively on lowest l's, and get spectral distortions which can't be done from ground. Goal is see reionization bump and overlap only somewhat with ground l range. Review panel of PIXIE thought there was no strong driver for space observation as r would be seen from ground.
Raphael: Current STM largely mirrors S4. Reionization lines would be interesting.
Lloyd: High frequency accessible from space is valuable. Resolution loss limits galaxy formation science, which is largest benefit at high frequency.
Raphael: Combination seems weakest. They take away from each other.
Lloyd: Agree, hybrid loses too much from both.
Julian: Limits range of both. See concerns that you're doubling risk.
Raphael: 1 benefit is hybrid could garauntee reionizaiton bump. Don't know if imager alone can get this yet.
Lloyd: looks like spectrometer doesn't add a lot. Imager has sufficient frequency bands to control dust.
Shaul: hybrid is concerning. We don't know if spectrometer is critical so its insurance may not be needed. Science loss is large. For now, keep large imager as this gives most science deliverables. Hybrid does worse in science deliverables than either single case.
Shaul: For now, move to TEAM-X with large imager. If engineering resources allow can broach idea of combination. See how the costs look. Will contact others, can still entertain other options in future.
Efforts in near future