ESO has organized an important online conference (22-26 June 2020) on the current significant tension in measuring the expansion rate of the Universe, i.e. the Hubble constant H0. The video of each day’s talks and the panel discussion is uploaded on YouTube by the organizers and is publicly available.
Here is a summary of day two of the meeting with links to specific parts of each speaker’s talk mainly for future personal reference, but hopefully useful for others as well. Check here for a summary of day 1.
After Antoine Mérand announced the start of the second day, the talks went on as follows:
Lennart Lindegren, Gaia astrometry and its contributions to the distance scale
- Intro:
An introduction to The Gaia Mission and its wide range of science objectives.
Collecting astrometric and spectroscopic data for > 1 billion sources, as well as radial velocities for > 100 million sources.
Detecting point sources with resolution ~0.1 arcsec.
Launched in 2013 and is currently in an orbit around L2.
Each part of the sky is visited by Gaia on average 14 times per year.
It has two telescopes set at a fixed angle (106.5 deg), and this enables it to accurately measure large angles in the sky which is important for measuring parallaxes. - Data Products:
astrometry (from 62 CCDs), low dispersion spectra photometry (from two red & blue photometry detectors), and radial velocities ( from a spectrometer). - Data Releases:
The early DR3 will come in a few months. - Gaia Astrometry:
Five astrometric parameters per source, 2 positions, 1 parallax, and 2 proper motions. Bright sources have radial velocity as well.
Positions are based on ~2800 quasars with accurate coordinates. - Random and systematic errors for parallaxes:
The systematic errors depend on the position, magnitude, and color of the sources.
The parallax zero-point is actually the systematic error, and “there is not just one parallax zero-point, it is a complicated function of a number of variables.” - A map of systematic error for different positions based on quasars
- Systematic vs. magnitude & color
- Estimates of Gaia DR2 zero-point
- The future of Gaia
Both systematic and random errors will improve in the next data releases. - Contributions of Gaia to distance scale
Grzegorz Pietrzynski, Absolute distances to standard candles
- Intro:
Introducing the Araucaria project: “improving the cosmic distance scale based on observations of several primary distance indicators in nearby galaxies”.
So far published ~150 papers in the past ~20 years. - The principal sources of error the in calibration of the cosmic distance ladder:
population effect, extinction, the zero-point, blending, crowding, and metallically. - Using eclipsing binaries for calibration of the distance ladder:
Obtaining the dimension of stars from the light and radial velocity curves with high precision (0.2%), measuring the angular diameter using surface brightness vs. color relation, and then measuring geometric distance. - Using 20 binary systems in LMC
A precise distance to LMC: 18.476 +/- 0.002 mag, Pietrzyński et al. (2019) published in Nature. - Comparing different independent distance measurements to LMC
- Using this geometric distance to calibrate the Cepheids in LMC
Need to be corrected for reddening. - Summary:
Soon <1% distance to LMC.
Caroline Huang, The Mira distance ladder
- Intro
The motivation for using Mira variables: Cepheids are young massive stars and can be used for measuring the distances to galaxies with recent star formation. However, Miras are older and less massive and can be used to measure distances to elliptical and S0 galaxies. - Intro to Mira variables
Tip of the asymptotic giant branch with periodic variability.
Similar to Cepheids, Miras’ periods are also correlated with their luminosities.
Long periods ~100 to ~ 1000 days. Currently P < 400 days are used for distance ladder.
While having similar luminosity to Cepheids, they are more common and can be detected in NIR due to larger pulsation amplitudes. - Miras in NGC 4258 (hosting a megamaser)
Three (inclusive) Mira samples resulting in consistent zero-point measurements. - Distance to NGC 1559 using Miras
Host to SN Ia 2005 with Cepheid distance. - Systematic and statistical uncertainties
Reddening and metallicity are the most challenging systematics. - H0 from Mira method
73.3 +/- 4.0 km/s.Mpc dominated by the random error from SNIa mag. - Future Work
Including long-period Miras (they follow a different P-L relation), but brighter hence covering a larger distance.
Four more galaxies to search for Mira variables. - Summary
Bruno Leibundgut, Supernovae as standardizable candles
- Intro:
“Supernovae as distance indicators”
Importance of SNe for H0.
“Best game in town right now to measure relative distances beyond 100 Mpc.” - Standardization of SNe Ia:
Most people use the SALT2 method. The distance modulus is corrected for light curve shape and color variations, also an additional correction for host galaxy mass.
There is a 3 sigma clipping involved which one should be careful about. - SNe Ia in NIR
Few to no corrections is needed for light curve shape, color, or host galaxy mass and reduced systematics (decreased influence of extinction).
Leading to a NIR Hubble diagram. - SN Ia absolute calibration:
They provide reliable “yardsticks”, and it is important where the yardstick is fixed.
“They are faithful translating whatever you calibrate into the absolute luminosity into the Hubble constant”. - Determination of H0
For z<0.2, the influence of cosmological model parameters is very small, Dhawan et al. (2020). - Type II SNe for cosmology
Can also be standardized hence can yield a Hubble diagram as well (after calibration). - adH0cc program
“accurate determination of H0 with core-collapse supernovae” - SN Summary
M. Rejkuba, M. Rigault, D. Scolnic, P. Kervella, and R. de Grijs
Reducing bias in SNe sample selections
Systematics due to SNe Ia environments
Systematics due to Cepheid environment:
LMC is not a spiral galaxy unlike SNe Ia host galaxies.
With Gaia parallaxes, Milky Way Cepheids can be used more reliably in near future.
Using the Milky Way as an anchor
Gaia parallaxes of Cepheids in the Milky Way
Can Gaia be used to help calibrate TRGB?
End of day two.
The images showing the first slide of each talk are screenshots obtained from the public YouTube video published by ESO. The featured image at the top of the post is obtained via the Wikimedia Commons, courtesy of NASA, ESA, A. Feild (STScI), and A. Riess (STScI/JHU).
