Master thesis defense by Mark Beyer Stjerne

Tracing the dust and ISM properties of distant quiescent galaxies with ALMA and JWST

Abstract

Understanding the ceasing, or quenching, of star formation in massive galaxies is key to building a complete picture of galaxy evolution.

Recent observations with the James Webb Space Telescope (JWST) have extended the discovery frontier of massive quiescent galaxies (MQGs) to within 700 Myr after the Big Bang, offering a timely opportunity to investigate the mechanisms that govern quenching at high redshift.

Additionally, the current suite of semi-analytical and cosmological hydrodynamical models struggle to reproduce the observed MQG number counts, requiring improved constraints on the feedback mechanisms that suppress star formation in the early universe.

This work focuses on examining the dust and ionised interstellar medium (ISM) properties of high-redshift quiescent galaxies to constrain the role of feedback mechanisms that regulate star formation. We utilise a parent catalog of ~740 UVJ-selected MQGs spanning z ~ 2-7, identifying 119 galaxies with archival ALMA coverage that total ~300 observations across four ALMA bands.

By stacking this data, we place constraints on the dust-to-stellar mass ratio Mdust/Mstar = -2.89 (-0.30, +0.17) for a sample with median redshift z = 2.49, extending dust mass constraints into the z > 2.5 domain. Assuming a constant gas-to-dust ratio and current gas fraction and dust fraction relations for main-sequence galaxies, we construct a toy model to quantify the dust depletion timescales required to reproduce the observed dust-to-stellar mass ratio.

We find that star formation alone is insufficient, but that rapid dust depletion on a timescale of τ = 0.40 (-0.07, +0.06) Gyr must be invoked to reproduce the observed Mdust/Mstar. We complement these results by exploring the ionised ISM component of ~150 MQGs through the DeepDive compilation of medium-resolution (R~1000) JWST/NIRSpec grating spectra.

We apply, for the first time for MQGs, emission line diagnostics using [NeIII] λ3869, [OIII] λ5007, [OII] λλ3727, 3729, and Hβ λ4861, to characterise MQGs with an AGN-like ISM. We find that galaxies with a Dn4000 spectral index < 1.35 exhibit a higher AGN fraction than those with Dn4000 > 1.35, indicating stronger evidence of AGN activity shown in recently quenched galaxies.

We examine the presence of ionised AGN outflows, through identifying a broadened [NeIII] and [OIII] component in 3/38 (7.9%) galaxies with Dn4000 < 1.35, a 2.9x and 2.0x higher fraction than galaxies with 1.35 < Dn4000 < 1.5 and Dn4000 > 1.5 respectively. Together, these results support a scenario in which AGN-driven gas depletion plays a significant role in the evolution of MQGs at z > 2.

Zoom link

https://dtudk.zoom.us/j/62086920712?pwd=yLAMDf8nEG8DJrEUwS2W4Pu8aywvbX.1

Meeting ID: 620 8692 0712

Supervisors

Minju Lee,  William Michael Baker, Sune Toft 

Censor

Georgios Magdis (DTU Space)