21 cm Cosmology Workshop

Aug 30, 2022, 7:00 AM → Sep 2, 2022, 2:00 PM America/Chicago

B343 Sterling Hall (UW Madison)

To advance progress in the practice of 21 cm intensity mapping we are organizing a community workshop on 21 cm cosmology instrumentation in Madison Wisconsin.  The workshop focuses on sharing recent progress and challenges from all the major 21 cm intensity mapping efforts, both EoR and post-EoR, to help this field reach its full potential. 

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Workshop topics:

Beam and gain calibration

• the practice of calibration from the sky, redundant baselines and their combination
• use of drones, holography and EM simulation
• pass band structures

Sources of correlated signals (other than sky)

• cross talk, feed cross coupling, ground pickup

Advances in mitigation of foregrounds

• foreground wedge and its leakage in practice
• identifying the 21 cm signal in the presence of foregrounds
• cross-correlations as a tool:
  • for identifying systematics
  • for validation

Systematics associated with digital signal processing

Validation- building confidence in the reality of an extracted 21 cm   signal;  use of independent simulations and analysis pipelines; blind analysis

 Software tools

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The Scientific Organizing Committee

Reza Ansari
Xuelei Chen
Danny Jacobs
Miguel Morales
Laura Newburgh
Albert Stebbins
Peter Timbie

 

WorkshopVisuals.pdf

Tuesday, August 30

6:00 PM → 8:00 PM Welcoming Reception - Memorial Union - Third Floor - Old Madison Room

Wednesday, August 31

6:30 AM → 7:00 AM Conference Room Open

7:00 AM → 8:30 AM Wednesday Morning - Calibration I ; Session Chair - John Marriner

7:00 AM Welcome to Madison!

Speaker: Peter Timbie

7:15 AM X-coupling, calibration in HERA

Instrumental systematics pose a significant threat to 21 cm telescopes aiming to detect a weak cosmological signal behind bright foreground emission. I will discuss the kinds of instrumental coupling systematics seen in HERA data and the tools we are developing to mitigate these systematics in 21 cm power spectrum analyses, including extended calibration techniques and complex visibility Fourier filters.

Speaker: Nick Kern (MIT)

Kern_HERA_Systematics.pdf

7:40 AM Achieving Precision Calibration for 21 cm Cosmology with DWCal

One of the primary challenges of 21 cm cosmology analyses is overcoming calibration error. Established calibration approaches in the field require an exquisitely accurate sky model. Even low-level sky model errors introduce calibration errors that corrupt the cosmological signal and prevent a detection. We present a novel approach called Delay-Weighted Calibration, or DWCal. DWCal enables precise calibration even in the presence of sky model error. In simulation, DWCal substantially outperforms traditional sky-based calibration when the calibration sky model has realistic levels of error.
21 cm cosmology efforts are predicated on the principle that the faint cosmological signal can be separated from bright foreground emission based on its unique spectral properties. Foreground emission is spectrally smooth and therefore compact in power spectrum space, occupying a limited number of power spectrum modes known as the “foreground wedge.” However, typical calibration approaches extend this error beyond the foreground wedge into the “EoR window,” contaminating the cosmological measurement. We show that, in simulation, DWCal mitigates the impact of sky model error in calibration and reduces contamination of 21 cm measurements.
DWCal exploits the fact that sky model error is compact in power spectrum space to avoid coupling error in the foreground wedge into the EoR window. It quantifies the expected sky model error in each power spectrum mode and incorporates this information into the calibration operation. DWCal imposes no prior assumptions about the instrument’s bandpass response and can accurately calibrate even when the instrumental bandpass has substantial frequency structure. It fits the same number of free calibration parameters as traditional sky-based calibration, avoiding the computational challenges, over-fitting concerns, and degeneracies associated with many-parameter calibration approaches. The DWCal approach can be combined with other advanced calibration techniques such as redundant calibration, for which it improves the accuracy of the absolute calibration step.

Speaker: Ruby Byrne (CalText)

Byrne_DWCal.pdf

8:05 AM Validation of Software Pipelines: the HERA Experience

The extraction of the desired signal from 21 cm cosmology experiments faces steep challenges: the combined issues of data size, weak signal, strong foregrounds, and complex instruments couple with sophisticated, constantly-evolving, multi-step software pipelines developed by large groups to produce a situation which undermines confidence in the final analysis. The challenges of producing robust scientific results from complicated instrumentation and analysis are of course not unique to 21 cm cosmology, and many strategies have been developed for these purposes, the most important for our purposes being end-to-end simulations of the instrument, its systematics, and all expected signals, processed with a high degree of realism through the entire analysis chain, for which I will term “validation” as shorthand. The possibility of confirmation bias argues for having the validation effort be independent of the data analysis team. I will take as an example the HERA validation effort, which was developed rather late in the instrument and analysis process, and describe how we interface with the analysis pipeline group, including adequately understanding and simulating systematic effects, and the instrument builders and their measurements and simulations (antenna response, instrument noise, bandpass) to produce a validation of the HERA pipeline. Specific and unexpected concerns which arose as part of this process, and potential lessons for other groups will be discussed.

Speaker: James Aguirre (UPenn / HERA)

ValidationOfSoftwarePipelines.pdf

8:30 AM → 9:00 AM Refreshment Break

9:00 AM → 11:05 AM Wednesday Morning - Foregrounds and Sky Model; Session Chair - Albert Stebbins

9:00 AM Practical Foreground Removal with CHIME

Speaker: Richard Shaw (UBC)

9:25 AM H I constraints from the cross-correlation of eBOSS galaxies and Green Bank Telescope intensity maps

I will present the joint analysis of HI Intensity Mapping observations with three galaxy samples: the Luminous Red Galaxy (LRG) and Emission Line Galaxy (ELG) samples from the eBOSS survey, and the WiggleZ Dark Energy Survey sample. The H I intensity maps are Green Bank Telescope observations of the redshifted 21cm emission on 100deg2 covering the redshift range 0.6 < z < 1.0. I will show the results of the foreground removal by fastICA, corrected for H I signal loss. In our study, we cross-correlate the cleaned H I data with the galaxy samples and study the overall amplitude as well as the scale dependence of the power spectrum. The cross-correlations constrain the quantity Ω_HI b_HI r_HI,opt at an effective scale keff, where ΩHI is the H I density fraction, b_HI is the H I bias, and r_HI,opt the galaxy-hydrogen correlation coefficient, which is dependent on the H I content of the optical galaxy sample.

Speaker: Laura Wolz (Manchester)

21cmcosmology_madison_laurawolz.pdf

9:50 AM 21cm Intensity Mapping: opportunities and challenges on the road to the SKA Observatory

A key point for 21cm Intensity Mapping studies in the post-reionization Universe is the subtraction of the bright foregrounds,orders of magnitude stronger than the pristine cosmological signal. In this talk, I will briefly describe the status of MeerKLASS, an Intensity Mapping survey with the MeerKAT telescope. Moreover, I will report the results of an effort, led by the SKA Intensity Mapping Focus Group, to construct a realistic mock data cube with improved sky model and instrument characterization,and to assess through simulations the performance of foreground cleaning methods. I will discuss current limitations and the roadmap to the SKAO era.

Speaker: Marta Spinelli (ETH)

Spinelli_Workshop21_Aug22.pdf

10:15 AM MeerKAT HI intensity mapping cross-correlations with overlapping galaxy surveys at z~0.4

I will present details of the first detection of cosmological signal using HI intensity mapping with a multi-dish array. For this we used the 64 dish MeerKAT telescope, a pathfinder for the Square Kilometre Array Observatory (SKAO), operating in single-dish mode, whereby the array is used as a collection of scanning auto-correlation dishes rather than as an interferometer. The detection is achieved in cross-correlation with overlapping galaxies from the WiggleZ Dark Energy Survey. The intensity mapping data was collected from 10.5 hours of pilot observations using MeerKAT's L-band over six nights, covering ~200deg^2 in the 11hr field of WiggleZ. We used data in the frequency range 971-1023 MHZ (0.39 < z < 0.46 in redshift), where impact from RFI is most minimal. This detection is the first evidence of its kind to suggest a multi-dish array operating in single-dish mode can probe cosmological signals. This marks an important milestone in the roadmap for the large scale structure science case with the full SKAO.

Speaker: Steve Cunnington (Manchester/ MeerKAT)

10:40 AM Improved upper limits on the 21-cm signal power spectrum of neutral hydrogen from LOFAR

Direct detection of the Epoch of Reionization via the redshifted 21-cm line will have unprecedented implications on the study of structure formation in the early Universe. This exciting goal is challenged by the difficulty of extracting the feeble 21-cm signal buried under bright astrophysical foregrounds and contaminated by numerous systematics. The LOFAR-EoR project has recently made considerable progress in this direction. We have now published the deepest upper limit on the signal power spectrum at redshift 9, which has made it possible to set a few constraints on the physics of the IGM during the EoR. We are also making progress towards a deep upper limit at multiple redshifts. In this talk, I will present our new upper limits with a focus on the latest developments in instrument calibration and foreground suppression that have enabled these progresses.

Speaker: Florent Mertens (LOFAR)

Mertens_LOFAR-EoR.pdf

11:05 AM → 12:45 PM Lunch Break - Box Lunches

12:45 PM → 2:00 PM Wednesday Afternoon - Foregrounds, Continued; Session Chair - Albert Stebbins

12:45 PM Deep learning approach for identification of HII regions and 21-cm signal recovery from SKA reionisation observations

(SKA-Low) will map the distribution of neutral hydrogen during reionisation and produce a tremendous amount of 3D tomographic data. These image cubes will be subject to instrumental limitations, such as systematic noise, foreground contamination, radio frequency interference (RFI) and limited resolution. The challenge of this astronomy image is the undesired astronomical and instrumental noise that outshines the 21-cm signal. Therefore, when studying the properties of the 21-cm signal for EoR, considering foregrounds and instrumental imprint on the data is of great importance, as they are orders of magnitude larger than the actual signal and further increase the data cleaning complexity.
Here we present SegU-Net, a stable and reliable method for identifying neutral and ionised regions in these images. SegU-Net is a U-Net architecture-based convolutional neural network (CNN) for image segmentation. It can segment our image data into meaningful features (ionised and neutral regions) with greater accuracy than previous methods. We can estimate the true ionisation history from our mock observation of SKA with an observation time of 1000 h with more than 87 per cent accuracy. Our network can be used to recover various topological summary statistics that characterise the non-Gaussian nature of the reionisation process.
Moreover, we also show an extended version of SegU-Net that can recover the 21-cm signal from the foreground contaminated tomographic dataset. The updated version of our network employs the segmented maps as position and shape prior to a guided recovery of the simulated 21-cm. We finally derive summary statistics from evaluating the applicability of the expected data from SKA.

Speaker: Michele Bianco (EPFL LASTRO)

MBianco_21cmWorkshop_220831.pdf

1:10 PM RFI removal with Neural Networks

Speaker: Ben Saliwanchik (BNL)

Saliwanchik_RFI_NN.pdf

1:35 PM Low frequency global sky model

Accurate modelling of the Galactic radio foreground is essential for detecting the cosmological 21cm signal. Such a model can aid foreground mitigation in both ongoing searches for the global EoR signal and upcoming efforts by instruments such as HERA to detect the 21cm brightness temperature power spectrum. In this talk, we outline a new low frequency global sky model, the Bayesian Global Sky Model (B-GSM). We extend on traditional PCA based models e.g. GSM by introducing a Bayesian framework to determine components and spectra.

In B-GSM, nested sampling is used to sample the posterior distributions of component maps and their spectra, and to determine Bayesian evidence values for different variations of model. Model comparisons, using this Bayesian evidence, allow the choice of number of components and spectral parametrisation to be guided by the data. This results in a data driven reconstruction of the Galactic foreground, with the components and spectra inferred from data across the whole sky, and with inherent quantification of uncertainty in the models predictions.

We present preliminary results from this model and outline future work that we will conduct in this area

Speaker: George Carter (Cambridge)

2:00 PM → 3:00 PM Wednesday Afternoon - Calibration I Discussion; Moderator - Danny Jacobs; Report Editor - Chris DiLullo

3:00 PM → 3:30 PM Refreshment Break

3:30 PM → 4:30 PM Wednesday Afternoon - Foregrounds Discussion; Moderator- Miguel Morales; Report Editor - John Podczerwinski

5:00 PM → 6:00 PM Canoeing on Lake Mendota. Bring suitable clothes! Dinner on your own (see `Accommodations' for suggestions)

Thursday, September 1

6:30 AM → 7:00 AM Conference Room Open

7:00 AM → 9:00 AM Thursday Morning - From Observations to Signals; Session Chair - Richard Shaw

7:00 AM FAST

Speaker: Yougang Wang (NAOC)

HI_survey_fast.pdf

7:25 AM Tianlai Cylinder Array

Speaker: Shifan Zuo (NAOC)

ValidationOfSoftwarePipelines.pdf

7:50 AM Progress Toward Constraint of the Cosmic Dawn from 21 cm Measurements with the OVRO-LWA Stage III

The highly redshifted 21 cm emission line from neutral hydrogen has the potential to reveal the temperature, density, and ionization fraction of the IGM during the Cosmic Dawn, when the first stars and galaxies illuminated the universe. The Long Wavelength Array at the Owens Valley Radio Observatory (OVRO-LWA) is currently undergoing an upgrade that substantially improves its sensitivity to this cosmological signal at redshifts z ≈ 16-100. We describe progress upgrading the array and developing a precision analysis pipeline to deep limits on the 21 cm power spectrum from the Cosmic Dawn.
The current upgrade to the OVRO-LWA is the instrument’s second major upgrade, bringing it into “Stage III” of operation. Eastwood et al. 2019 used data from the array’s previous Stage II to produce an upper limit on the 21 cm at z ≈ 18.4. However, due to systematic limitations, that result does not meaningfully constrain cosmological models. We discuss three aspects of the instrument upgrade that will improve 21 cm power spectrum results:
1. The upgrade adds an additional 64 fiber-fed outrigger antennas for a total of 352 antennas. The new antennas extend the array’s longest baseline to 2.4 km and will improve imaging resolution and precision calibration.
2. The overhauled analog signal processing and digital backend improves channel isolation, significantly reducing instrumental cross-talk. Additionally, the upgraded signal path reduces signal reflections and a produces a smoother bandpass response.
3. The adoption of state-of-the-art calibration and power spectrum estimation techniques improves foreground isolation and 21 cm sensitivity.
Commissioning of the OVRO-LWA Stage III is currently underway. The expanded array, improved signal processing, and refined analysis techniques will enable 21 cm power spectrum analyses to produce improved constraints on the Cosmic Dawn.

Speaker: Ruby Byrne (CalTech)

Byrne OVRO-LWA.pdf

8:15 AM Digital Systematics

Speakers: Miguel Morales (University of Washington), Miguel Morales

9:00 AM → 9:30 AM Refreshment Break

9:40 AM → 11:25 AM Thursday Morning - Calibration II Beams and EM Simulations; Session Chair - George Carter

9:45 AM Characterization of the LWA Antenna and Station Beam Pattern

I will present ongoing work to better characterize the response of the Long Wavelength Array (LWA) antenna. Recent work has focused on directly measuring the impedance mismatch between the antenna and front end electronics (FEEs). Custom calibration and testing fixtures were created which allows for measurement of the antenna impedance at the feed points on the FEE boards. These measurements should yield better characterization of the LWA antenna response across frequency than simulations which were previously used to model the impedance mismatch. The impedance mismatch directly affects both 21 cm experiments using the LWA antenna and LWA sky survey results. I will also present ongoing work which aims to better understand the effects of sidelobes on efforts to detect the global 21 cm signal using beamforming techniques with LWA-SV. This work will help highlight potential avenues to improve systematics which currently obscure detection of the signal.

Speaker: Chris DiLullo (NASA Goddard)

DiLullo_21cmWorkshop.pdf

10:10 AM Beam calibration using holography for CHIME

Speaker: Alex Reda (Yale)

Reda_CHIME_holography.pdf

10:35 AM HIRAX Electromagnetic Design

Speaker: Ben Saliwanchik (Brookhaven National Lab)

Saliwanchik_HIRAX_EM_Sims.pdf

11:00 AM Capturing the shape and variations of beams with holography

With unprecedented sensitivity and field of view, new generation radiotelescopes at low frequencies have to face new calibration challenges. One of the main drivers to cope with direction-dependent distortion (due to strong off-axis sources) is to develop a knowledge of beamshapes and how they vary in frequency and direction. I am going to present recent modelling work carried on VLA and MeerKAT beam modelling, required for high dynamic imaging with these large interferometers. At low frequencies with the NenuFAR radiotelescope, I will also discuss the impact of this modelling on current Cosmic Dawn observations.

Speaker: Julien Girard (Observatoire de Paris)

JGIRARD talk Beam holography 21cm HQ.mp4

JNGIRARD-Beam holography-21cmCosmologyWorkshop-31-2sep.pdf

11:25 AM → 1:15 PM Lunch Break - Box Lunches

1:15 PM → 2:05 PM Thursday Afternoon - From Observations to Signals (cont'd); Session Chair - George Carter

1:15 PM Tianlai Dish Array: Beam Calibration, Correlated Noise, Ground Pickup

Speaker: Albert Stebbins (Fermilab)

Stebbins_20220911_GroundPickup_Madison.pdf

1:40 PM PAON4 X-correlated noise, satellites for beam, status of electronics (IDROGEN)

Speaker: Olivier Perdereau (IJCLab)

perdereau.pdf

2:05 PM → 3:00 PM Thursday Afternoon - From Observations to Signals - Discussion; Moderator - Adam Beardsley ; Report Editor - Reza Ansari

3:00 PM → 3:30 PM Refreshment Break

3:30 PM → 4:45 PM Thursday Afternoon - Calibration II Beams & EM Simulations - Discussion; Moderator - Laura Newburgh; Report Editor - Will Tyndall

7:00 PM → 9:00 PM Conference Dinner - Memorial Union, Third Floor, Old Madison Room

Friday, September 2

7:30 AM → 8:00 AM Conference Room Open

8:00 AM → 8:30 AM Friday Morning - Summary of Calibration Discussion; Danny Jacobs & Chris DiLullo

8:30 AM → 9:00 AM Friday Morning - Summary of Foregrounds Discussion; Miguel Morales & John Podczerwinski

9:00 AM → 9:30 AM Friday Morning - Summary of Beams & EM Simulations; Laura Newburgh & Will Tyndall

9:30 AM → 10:00 AM Refreshment Break

10:00 AM → 10:30 AM Friday Morning - Summary of Observations to Signals; Adam Beardsley & Reza Ansari

10:30 AM → 11:00 AM Friday Morning - Conclusions and Next Steps

11:00 AM → 1:00 PM Lunch Break - Box Lunches