Sub one per cent mass fractions of young stars in red massive galaxies

Abstract

Early-type galaxies are considered to be the endproducts of massive galaxy formation 1 . Optical spectroscopic studies reveal that massive early-type galaxies formed the bulk of their stars over short timescales (≤1 Gyr) and at high redshift (z≤2), followed by passive evolution to the present 2 . However, their optical spectra are insensitive to constrain small episodes of recent star formation, since they are dominated by old stars. Fortunately, this problem can be tackled in the ultraviolet range. While recent studies that make use of ultraviolet absorption lines have suggested the presence of young stars in a few early-type galaxies 3 , the age and mass fractions of young stars and their dependence on galaxy mass, is unknown. Here we report a detailed study of these young stellar populations, from high-quality stacked spectra of 28,663 galaxies from the BOSS survey 4 , analysing optical and ultraviolet absorption lines simultaneously. We find that residual star formation is ubiquitous in massive early-type galaxies, measuring average mass fractions of ~0.5% in young stars in the last 2 Gyr of their evolution. This fraction shows a decreasing trend with galaxy stellar mass, consistent with a down-sizing scenario 5 . We also find that synthetic galaxies from stateof-the-art cosmological numerical simulations 6 significantly overproduce both intermediate and young stellar populations. Therefore, our results pose stringent constraints on numerical simulations of galaxy formation 6,7 . Stephano Charlot for kindly providing us their new version of stellar population models extended to the UV spectral range in order to test how results change when using other SSP models.

Clues on the history of early-type galaxies from SDSS spectra and GALEX photometry

“…We detect signatures of stellar populations younger than 1 Gyr in a significant fraction of galaxies at all stellar masses. A similar result was found by Salvador-Rusiñol et al (2019) at 0.35 z 0.6. Despite differences in the methodology, the similarity between their results and ours shows that massive galaxies must be undergoing intermittent star-formation event from z ∼ 0.6 to z < 0.1.…”

“…Also using GALEX and SDSS photometry, Schawinski et al (2007) find that 30±3 per cent of massive ellipticals show signs of recent star-formation. Recently, Salvador-Rusiñol et al (2019) used spectra from the Baryon Oscillation Spectroscopic Survey Dawson et al (2013) covering the near-ultraviolet range to show that ∼ 0.5 per cent of the mass of their sample of ETGs was in stars younger than 2 Gyr, with the fraction decreasing with stellar mass according to a downsizing pattern. Several other studies also find that the UV colours of some ETGs cannot be explained by old populations alone (e.g.…”

Ubiquitous [OII] Emission in Quiescent Galaxies at z ~ 0.85

“…in 16% of quiescent galaxies at z ≈ 0.8; Chauke et al 2019, see also Akhshik et al 2021). Schawinski et al (2007) show that some star formation is occurring in 30% of bright (M r < −21.5) 0.05 < z < 0.10 early-type galaxies, and Treu et al (2005) show that the fractional contribution of secondary episodes of star formation increases with decreasing galaxy mass (from <1% for masses above 10 11.5 M to 20−40% below 10 11 M ; see also Kaviraj et al 2007;Thomas et al 2010;Salvador-Rusiñol et al 2020).…”

Reconstructing the mass accretion histories of nearby red nuggets with their globular cluster systems

“…A detailed exploration of the X-ray properties of NGC 1277 (c.f., Buote & Barth 2019) may bring important constraints on its dark matter halo, as will dynamical studies at large radii via kinematics of the GC system with James Webb Space Telescope. Upcoming analysis of its stellar populations in the UV will be used to search for possible star formation even at the sub-one percent level (e.g., Salvador-Rusiñol et al 2019), and infrared spectroscopy will bring further insights into the properties of its stellar populations.…”