The SLACS strong lens sample, debiased

Abstract

Strong gravitational lensing observations can provide extremely valuable information on the structure of galaxies, but their interpretation is made difficult by selection effects, which, if not accounted for, introduce a bias between the properties of strong lens galaxies and those of the general population. A rigorous treatment of the strong lensing bias requires, in principle, to fully forward model the lens selection process. However, doing so for existing lens surveys is prohibitively difficult. With this work we propose a practical solution to the problem: using an empirical model to capture the most complex aspects of the lens finding process, and constraining it directly from the data together with the properties of the lens population. We applied this method to real data from the SLACS sample of strong lenses. Assuming a power-law density profile, we recovered the mass distribution of the parent population of galaxies from which the SLACS lenses were drawn. We found that early-type galaxies with a stellar mass of $\log M_{\ast }/M_{\odot }=11.3$ and average size have a median projected mass enclosed within a $5$~kpc aperture of $\log M_5/M_{\odot }=11.332\pm 0.013$, and an average logarithmic density slope of $\gamma =1.99\pm 0.03$. These values are respectively $0.02$~dex and $0.1$ lower than inferred when ignoring selection effects. According to our model, most of the bias is due to the prioritisation of SLACS follow-up observations based on the measured velocity dispersion. As a result, the strong lensing bias in $\gamma$ reduces to $\sim 0.01$ when controlling for stellar velocity dispersion.

Submitted to Astronomy & Astrophysics. A 2-minute summary video is available at this link: https://youtu.be/wtrMRereBwE

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• The COSMOS-Web Lens Survey (COWLS) I: Discovery of >100 high redshift strong lenses in contiguous JWST imaging
• Velocity dispersion function evolution from strong lensing statistics
• The COSMOS-Web Lens Survey (COWLS) III: forecasts versus data
• The SLACS strong lens sample, debiased. II. Lensing-only constraints on the stellar IMF and dark matter contraction in early-type galaxies
• Galaxy-scale lens search in the PEARLS NEP TDF and CEERS JWST fields
• Optical+NIR analysis of a Newly Confirmed Einstein ring at z$\sim$1 from the Kilo-Degree Survey: Dark matter fraction, total and dark matter density slope and IMF
• Milky-Way-like stars in a galaxy core 8 billion years ago revealed by gravitational lensing