Beecroft Institute for Particle Astrophysics and Cosmology

Abstract:

Our comprehension of the history of star formation at z > 3 strongly relies on rest-frame ultraviolet observations. However, this selection systematically misses the dustiest and most massive sources, resulting in an incomplete census at earlier times. Infrared facilities such as Spitzer and the James Webb Space Telescope have shed light on a hidden population lying at z = 3 − 6 characterised by extreme red colours named HIEROs (HST-to-IRAC extremely red objects), identified by the colour criterion H E − ch2 > 2.25. Recently, Euclid Early Release Observations (EROs) have opened the possibility to further study such objects, exploiting the comparison between Euclid and ancillary Spitzer /IRAC observations. The aim of this study was to investigate the effectiveness of this synergy in characterising the population of a small test area of 232 arcmin 2 . We utilised catalogues in the Perseus field across the VIS and NISP bands, supplemented by data from the four Spitzer channels and several ground-based MegaCam bands ( u , g , r , H α , i , and z ) already included in the ERO catalogue. We selected 121 HIEROs by applying the H E − ch2 > 2.25 colour cut, cleaned this sample of globular clusters and brown dwarfs, and then inspected by eye the multi-band cutouts of each source, ending with 42 reliable HIEROs. Photometric redshifts and other physical properties of the final sample were estimated using the spectral-energy-distribution-fitting software Bagpipes . From the z phot and M * values, we computed the galaxy stellar mass function at 3.5 < z < 5.5. When we exclude all galaxies that could host an active galactic nucleus, or whose stellar masses might be overestimated, we still find that the high-mass end of the galaxy stellar mass function is similar to previous estimates, indicating that the true value could be even higher. This investigation highlights the importance of a deeper study of this still mysterious population, in particular to assess its contribution to the cosmic star-formation rate density and its agreement with current galaxy evolution and formation models. These early results demonstrate Euclid ’s capabilities to push the boundaries of our understanding of obscured star formation across a wide range of epochs.

Abstract:

Current cosmological data seem to show that dark energy is evolving in time and that it possibly crossed the phantom divide in the past. So far the only theories that lead to such a behavior involve a nontrivial coupling between dark energy, in the form of a scalar field, and the gravitational or matter sector. We show that there is another possibility involving both a nontrivial kinetic sector in a cubic Galileon theory and a scalar field potential that breaks the Galileon shift symmetry, which can lead to a similar phenomenology on large scales. We perform a full Bayesian analysis using the latest cosmological data, including DESI DR2 baryonic acoustic oscillation measurements, type Ia SNe measurements from DESY5, Union3, and Pantheon+, and cosmic microwave background data from Planck and ACT. We find that it is statistically strongly favored over a universe dominated by a cosmological constant (with a Bayes factor of <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"> <mi>log</mi> <mi>B</mi> <mo>≃</mo> <mn>6.5</mn> </math> ). Yet, as with other nonminimally coupled theories, it has severe ancillary gravitational effects. These can be mitigated to some extent, but as with other viable theories, the penalty is ever more elaborate scalar field models of dark energy.

Abstract:

Abstract We explore how the fraction of red (quenched) galaxies varies in the dwarf galaxy regime (107 M⊙ < M⋆ < 109.5 M⊙), using a mass-complete sample of ∼5900 dwarfs at z < 0.15, constructed using deep multi-wavelength data in the COSMOS field. The red fraction decreases steadily until M⋆ ∼ 108.5 M⊙ and then increases again towards lower stellar masses. This ‘U’ shape demonstrates that the traditional notion of ‘downsizing’ (i.e. that progressively lower mass galaxies maintain star formation until later epochs) is incorrect – downsizing does not continue uninterrupted into the dwarf regime. The U shape persists regardless of environment, indicating that it is driven by internal processes rather than external environment-driven mechanisms. Our results suggest that, at M⋆ ≲ 108 M⊙, the quenching of star formation is dominated by supernova (SN) feedback and becomes more effective with decreasing stellar mass, as the potential well becomes shallower. At M⋆ ≳ 109 M⊙, the quenching is driven by a mix of SN feedback and AGN feedback (which becomes more effective with increasing stellar mass, as central black holes become more massive). The processes that quench star formation are least effective in the range 108 M⊙ < M⋆ < 109 M⊙, likely because the potential well is deep enough to weaken the impact of SN feedback, while the effect of AGN feedback is still insignificant. The cosmological simulations tested here do not match the details of how the red fraction varies as a function of stellar mass – we propose that the red fraction vs stellar mass relation (particularly in the dwarf regime) is a powerful calibrator for the processes that regulate star formation in galaxy formation models.