Reliable Tests of Faint-end UV Luminosity Functions in Strong Lensing Fields

Abstract

Dark matter comprises ~85% of the entire mass of the Universe, but the fundamental nature of its constituent particles remains elusive. In this thesis, I test for two competitive dark matter models: the conventional heavy particle paradigm, and dark matter being ultralight bosons of mass $\sim 10^{-22}$eV ($ψ$DM). More specifically, I test for the faint-end turnover induced by $ψ$DM models, exploiting the strong lensing power by massive galaxy clusters to probe intrinsically fainter magnitudes. A key challenge for such an analysis would be contamination by low-z galaxies sharing similar observed SEDs as high-z galaxies. As I will demonstrate, such a contamination issue is generally severe and may wash out the faint-end turnover signatures. I also show that $\sim 50\%$ of the purported $3.5\leq z\leq 5.5$ galaxies within existing photometric redshift catalogs constructed for Hubble Frontier Fields (HFF) are in fact low-z interlopers. Luckily, individual mitigation of interlopers can be achieved with the combination of deep HST and JWST observations. For fields without supplementary data, machine learning methods will be shown useful in preserving the mitigating power. Cleaner $3.5\leq z\leq 5.5$ and $6\leq z\leq 10$ samples are derived for a more reliable test in strong lensing field of MACS J0416, with which I found no evidence for faint-end turnovers, leading to a constraint on the $ψ$DM mass of $>2.97\times10^{-22}$eV at 95\% confidence. This constraint will also be interpreted in an scheme where dark matter is composed of multiple particle copies, where I argue the derived mass bound is likely on an effective de Broglie scale governing the collective behavior of the entire $ψ$DM budget under gravitational equilibrium established.

Figures (71)

• Figure 1: Left: Galaxy UV Luminosity Functions as measured by Bouwens2021 at different redshifts, exhibiting an exponential suppression at bright magnitudes and steadily increasing to fainter plotted magnitudes following a power-law. Solid lines are the best fit Schechter functional form to the measurements. Right: Overall amplitude $\phi^*$ of UV LFs at different redshifts based on the fitting shown in the left panel. It is seen that $\phi^*$ increases faster at higher redshifts than at lower redshifts, suggesting an "accelerated" evolution.
• Figure 2: Left: UV LFs with baryon physics induced faint end turnover at $M_{UV}\sim -12$ as obtained by cosmological simulation of Gnedin2014. Dashed and dotted curves correspond to two different mass resolutions, and data points are a compilation of observations available at the time. Right: UV LFs with $\psi$DM induced faint end turnover in comparison with UV LFs observed by Leung2018 (black data point) and others available at the time. The black dotted curve corresponds to the heavy particle CDM paradigm with UV LF steadily increasing to faint magnitudes, and colored curves correspond to UV LFs anticipated in the $\psi$DM paradigm ($m_{22}$ is $\psi$DM mass in unit of $10^{-22}$eV) as parametrized by Schive2016.
• Figure 3: Demonstration of effects brought by lensing magnification with simulated images. On the left, we present an unlensed view hosting five bright galaxies. The lensed view toward the same observing direction is shown on the right, in which many fainter galaxies are newly magnified to be observable. But the sky area probed by the lensed view is also seen reduced, as indicated by the red contour in the left panel.
• Figure 4: Comparison of UV LF at $z\sim5$(left) and $z\sim6$ (right) constructed in HFF lensing fields by various studies and also that from deep blank fields by Bouwens2021 (black solid line indicating faint end extrapolation). Measurements by Leung2018Atek2018 and Bouwens2017 all hint at a faint end turnover, despite at different magnitudes, whereas measurements by Bouwens202205 suggest the opposite.
• Figure 5: Comparison of SED in Hubble Frontier Fields filters of a $z_{spec}$=0.29 galaxy (blue curve) with a $z_{spec}$=3.95 galaxy (orange curve). The values are taken from the S18 catalog, and the SED for the low spec-z source is scaled up by a factor of 2.75 for a better comparison of shapes. The scaled SEDs are seen to be very similar in shape, which leads to a misidentification problem of the low-z sources, as is demonstrated here by their photo-z estimates. In the inserted figures, we also show a 1.8 arcsec$^2$ F160W band cutout image (using similar colors as their SEDs) for both galaxies. Both galaxies are of similar apparent size, but the high-z galaxies are seen to be slightly more elongated. This slight visual difference, however, is not enough to refute the wrongly estimated photo-z for the low spec-z galaxy: the blue cutout could also be interpreted as the lensed image of a compact high-z galaxy affected less by lensing shear.