Andrea Bolamperti

Galaxy-scale strong lensing systems

Galaxy-scale strong gravitational lenses represent a powerful tool in several extragalactic astrophysics research fields. When two or more sources are multiply imaged by the same lens galaxy, it is possible to measure the values of some cosmological parameters describing the geometry of the Universe. Although more massive lenses, like galaxy clusters, can generate dozens of multiple images from sources in a large redshift range, they have a complex total mass distribution which includes several mass components, that might introduce degeneracies between the model parameters.
As a result, the discovery of ultra massive galaxies that act as lenses for two or more background sources plays a key role in measuring the values of cosmological parameters through strong lensing modeling. In fact, by considering the ratio of the two Einstein radii observable in a double-source system, one can constrain the value of $\Omega_m$ independently of that of $H_0$. The presence of more than two multiply-imaged sources can further reduce the uncertainties on the total mass profile of the lens and on the cosmological parameters, but is extremely rare, with only a few known cases to date.

The next system: SDSS J0100+1818

I studied a unique deflector, SDSS J0100+1818, analyzing its total and baryonic mass distributions. I started working on this system during my Bachelor's and, after working on a different project (still lensing-related) in my Master's, I further analyzed it in the last year, and it worths the attention. In fact, this system is composed by an ultra-massive early-type galaxy at $z=0.581$, with uncommon stellar velocity dispersion of ($450 \pm 40$) km s$^{-1}$, surrounded by ten multiple images of three background sources, two of which (A and B) spectroscopically confirmed at $z=1.880$. Exploiting high-resolution HST photometry and VLT/X-shooter spectroscopy I conduct a strong lensing analysis with the software GLEE to reconstruct the observed position of the images first, and the complex surface brightness distributions of the background sources over approximately 7200 HST pixels then (see image below).

We found that:

1. We have combined the available multiband photometry from PanSTARRS, NOT, and HST to model the spectral energy distribution of the main lens galaxy. The best-fit model results in a stellar mass value of $(1.5 \pm 0.3) \times 10^{12}$ M$_{\odot}$.
2. We have used the public software pPXF to estimate the value of the stellar velocity dispersion $\sigma$ of the main lens galaxy from its X-Shooter spectrum
3. With the point-like source modeling, we have found a total mass value projected within the Einstein radius (of approximately 42 kpc) of $(9.1 \pm 0.1) \times 10^{12}$ M$_\odot$. The source C is predicted at $z_C=1.72$. The best-fit value of the logarithmic slope is shallower than for a singular isothermal profile.
4. We confirmed the results, with much more accuracy, by adopting the extended modeling.
5. We have reconstructed the surface brightness distributions of the background sources and measured their half-light radii from the luminosity profiles. We have successfully recovered the two-peaked structure of the AB source, with a small physical separation of about 2 kpc at $z=1.880$. We have measured values of the effective radius between $0.5$ and $1$ kpc at $z=1.880$ for the A and B components, depending on the adopted model. Our measured sizes are in agreement with those of strongly lensed clumps in star-forming galaxies at $z\sim 1-3$, with similar moderately-magnified sources.

New MUSE data: an accepted and observed 5-hours proposal as PI

SDSS J0100+1818 has been targeted with VLT/MUSE (program 0110.A-0248, PI: A. Bolamperti) to obtain integral field spectroscopy of the system to identify other galaxies on the lens plane, spectroscopically confirm the redshift of system C, enhance our study on the total and dark-matter mass profiles, and measure the stellar velocity dispersion profile of the main deflector. In the 1'$\times$1' MUSE field of view we find other 18 galaxies at $z \approx 0.58$, confirming that SDSS J0100+1818 is not an isolated galaxy, but it is more likely the brightest galaxy of a group residing in a dark matter halo. Furthermore, we confirm the stellar velocity dispersion measured with X-shooter, but we are now able to derive its radial profile. Most importantly, we spectroscopically confirm that C1 and C2 are two multiple images of the same background source and we identify, through their Lyman-$\alpha$ emission, two additional sources at higher redshift, each with four multiple images.

Cosmography with SDSS J0100+1818 (Bolamperti et al. 2024)

I have exploited the new VLT/MUSE data to expand the previous study. I have confirmed the redshift of the background sources A and B, at $z = 1.880$, and measured the redshift of source C, at $z = 1.698$ (very close to the best-fit values of Bolamperti et al. 2023a!). Furthermore, I discovered two additional background Ly$\alpha$ emitters, E and F, at $z = 4.95$.
I have securely measured the redshifts of 65 objects in the field of view, identifying 19 group members (squares in the figure on the left, including the Brightest Group Galaxy; BGG), and measured the stellar velocity dispersion profile of the BGG, peaking at $(386 ± 6) \mathrm{km/s}$.
Through an enhanced strong lensing model, developed with GLEE, I reconstructed the total and baryonic mass profiles of the deflector. They are consistent with the previous results but, thanks to the redshift of C and the inclusion of E and F, has lower uncertainties.
Most importantly, I developed new lensing models with the cosmological parameters $\Omega_m$ and $w$ left as free parameters, in $\Lambda$CDM and $w$CDM Universes, respectively. We found that:

1. The average 1$\sigma$ uncertainty is on the order of 0.1 for $\Omega_{\rm m}$, and of 0.4 for $w$. We have measured $\Omega_{\rm m} = 0.14^{+0.16}_{-0.09}$ (with a 95.4\% upper limit of 0.47) in the flat $\Lambda$CDM case, and $\Omega_{\rm m} = 0.19^{+0.17}_{-0.10}$ (with a 95.4\% upper limit of 0.59) and %$w$ with a $\approx 30\%$ uncertainty, $w = -1.27_{-0.48}^{+0.43}$ (with a 95.4\% upper limit of $-0.53$) in the flat $w$CDM case.

2. We have considered models adopting a multi-plane strong lensing modeling approach. These models have included an additional PIEMD mass distribution at $z=1.880$, representing the total mass term associated to sources A and B, and contributing to the total deflection angle. We have found consistent median values for the cosmological parameters $\Omega_{\rm m}$ and $w$, with a similar uncertainty on $w$ and an increased uncertainty on $\Omega_{\rm m}$ by a factor of approximately 1.5 (red lines in the figure on the right, versus the single-plane deflector curves in black and blue).
3. The accuracy on the cosmological measurements obtained with SDSS~J0100+1818 is similar to that obtained with the galaxy-scale multi-source-plane strong lensing system known as the ``Jackpot'' and with cluster-scale probes.

Upcoming new data: 15 ks of observations with XMM-Newton

Fossil groups are dynamically evolved systems dominated by a central elliptical galaxy, and immersed in hot gas halos emitting in the X-ray band. The growth of fossil groups occurs over very long timescales, due to the gradual angular momentum loss by dynamical friction, and thus the discovery of distant fossil groups is crucial in testing any galaxy evolution model. SDSS J0100 fulfills all the fossil-group criteria, but lacks the last one, the X-ray detection of the hot gas halo. With XMM data we will measure its X-ray luminosity and gas temperature to finally confirm its fossil status and put it in the broader context of the evolution of structures in the Universe.
It was accepted for observations in AO-24 of XMM-Newton

Want to read more?

You can find more details, and useful figures, in our published papers, Bolamperti et al. 2023a and Bolamperti et al. 2024.




References

Collett et al. 2014, 2017, 2020; Smith & Collett 2020; Tu et al. 2009; Tanaka et al. 2016; Schuldt et al. 2019; Wang et al. 2022; Suyu et al. 2010, 2012; Jullo et al. 2010; Caminha et al. 2016, 2022; Grillo et al. 2015, 2020; Bergamini et al. 2020, 2022; Schneider2014