skip to main content

Title: Uncertain times: the redshift–time relation from cosmology and stars
ABSTRACT Planck data provide precise constraints on cosmological parameters when assuming the base ΛCDM model, including a 0.17 per cent measurement of the age of the Universe, $t_0=13.797 \pm 0.023\, {\rm Gyr}$. However, the persistence of the ‘Hubble tension’ calls the base ΛCDM model’s completeness into question and has spurred interest in models such as early dark energy (EDE) that modify the assumed expansion history of the Universe. We investigate the effect of EDE on the redshift–time relation z↔t and find that it differs from the base ΛCDM model by at least ${\approx } 4{{\ \rm per\ cent}}$ at all t and z. As long as EDE remains observationally viable, any inferred t ← z or z ← t quoted to a higher level of precision do not reflect the current status of our understanding of cosmology. This uncertainty has important astrophysical implications: the reionization epoch – 10 > z > 6 – corresponds to disjoint lookback time periods in the base ΛCDM and EDE models, and the EDE value of t0 = 13.25 ± 0.17 Gyr is in tension with published ages of some stars, star clusters, and ultrafaint dwarf galaxies. However, most published stellar ages do not include an uncertainty in accuracy (due to, e.g. uncertain more » distances and stellar physics) that is estimated to be $\sim 7\!-\!10{{\ \rm per\ cent}}$, potentially reconciling stellar ages with $t_{0,\rm EDE}$. We discuss how the big data era for stars is providing extremely precise ages ($\lt 1{{\ \rm per\ cent}}$) and how improved distances and treatment of stellar physics such as convection could result in ages accurate to $4\!-\!5{{\ \rm per\ cent}}$, comparable to the current accuracy of t↔z. Such precise and accurate stellar ages can provide detailed insight into the high-redshift Universe independent of a cosmological model. « less
Award ID(s):
1910346 1752913
Publication Date:
Journal Name:
Monthly Notices of the Royal Astronomical Society
Page Range or eLocation-ID:
2764 to 2783
Sponsoring Org:
National Science Foundation
More Like this
  1. ABSTRACT The canonical Lambda cold dark matter (ΛCDM) cosmological model makes precise predictions for the clustering and lensing properties of galaxies. It has been shown that the lensing amplitude of galaxies in the Baryon Oscillation Spectroscopic Survey (BOSS) is lower than expected given their clustering properties. We present new measurements and modelling of galaxies in the BOSS LOWZ sample. We focus on the radial and stellar mass dependence of the lensing amplitude mismatch. We find an amplitude mismatch of around $35{{\ \rm per\ cent}}$ when assuming ΛCDM with Planck Cosmological Microwave Background (CMB) constraints. This offset is independent of halomore »mass and radial scale in the range Mhalo ∼ 1013.3−1013.9h−1 M⊙ and $r=0.1\!-\!60 \, h^{-1} \mathrm{Mpc}$ ($k \approx 0.05\!-\!20 \, h \, {\rm Mpc}^{-1}$). The observation that the offset is both mass and scale independent places important constraints on the degree to which astrophysical processes (baryonic effects, assembly bias) can fully explain the effect. This scale independence also suggests that the ‘lensing is low’ effect on small and large radial scales probably have the same physical origin. Resolutions based on new physics require a nearly uniform suppression, relative to ΛCDM predictions, of the amplitude of matter fluctuations on these scales. The possible causes of this are tightly constrained by measurements of the CMB and of the low-redshift expansion history.« less
  2. ABSTRACT Stellar and supernova nucleosynthesis in the first few billion years of the cosmic history have set the scene for early structure formation in the Universe, while little is known about their nature. Making use of stellar physical parameters measured by GALAH Data Release 3 with accurate astrometry from the Gaia EDR3, we have selected ∼100 old main-sequence turn-off stars (ages ≳12 Gyr) with kinematics compatible with the Milky Way stellar halo population in the Solar neighbourhood. Detailed homogeneous elemental abundance estimates by GALAH DR3 are compared with supernova yield models of Pop III (zero-metal) core-collapse supernovae (CCSNe), normal (non-zero-metal) CCSNe,more »and Type Ia supernovae (SN Ia) to examine which of the individual yields or their combinations best reproduce the observed elemental abundance patterns for each of the old halo stars (‘OHS’). We find that the observed abundances in the OHS with [Fe/H] > −1.5 are best explained by contributions from both CCSNe and SN Ia, where the fraction of SN Ia among all the metal-enriching SNe is up to 10–20 per cent for stars with high [Mg/Fe] ratios and up to 20–27 per cent for stars with low [Mg/Fe] ratios, depending on the assumption about the relative fraction of near-Chandrasekhar-mass SNe Ia progenitors. The results suggest that, in the progenitor systems of the OHS with [Fe/H] > −1.5, ∼ 50–60 per cent of Fe mass originated from normal CCSNe at the earliest phases of the Milky Way formation. These results provide an insight into the birth environments of the oldest stars in the Galactic halo.« less
  3. ABSTRACT We use FIRE simulations to study disc formation in z ∼ 0, Milky Way-mass galaxies, and conclude that a key ingredient for the formation of thin stellar discs is the ability for accreting gas to develop an aligned angular momentum distribution via internal cancellation prior to joining the galaxy. Among galaxies with a high fraction ($\gt 70{{\ \rm per\ cent}}$) of their young stars in a thin disc (h/R ∼ 0.1), we find that: (i) hot, virial-temperature gas dominates the inflowing gas mass on halo scales (≳20 kpc), with radiative losses offset by compression heating; (ii) this hot accretion proceedsmore »until angular momentum support slows inward motion, at which point the gas cools to $\lesssim 10^4\, {\rm K}$; (iii) prior to cooling, the accreting gas develops an angular momentum distribution that is aligned with the galaxy disc, and while cooling transitions from a quasi-spherical spatial configuration to a more-flattened, disc-like configuration. We show that the existence of this ‘rotating cooling flow’ accretion mode is strongly correlated with the fraction of stars forming in a thin disc, using a sample of 17 z ∼ 0 galaxies spanning a halo mass range of 1010.5 M⊙ ≲ Mh ≲ 1012 M⊙ and stellar mass range of 108 M⊙ ≲ M⋆ ≲ 1011 M⊙. Notably, galaxies with a thick disc or irregular morphology do not undergo significant angular momentum alignment of gas prior to accretion and show no correspondence between halo gas cooling and flattening. Our results suggest that rotating cooling flows (or, more generally, rotating subsonic flows) that become coherent and angular momentum-supported prior to accretion on to the galaxy are likely a necessary condition for the formation of thin, star-forming disc galaxies in a ΛCDM universe.« less
  4. ABSTRACT We study stellar-halo formation using six Milky-Way-mass galaxies in FIRE-2 cosmological zoom simulations. We find that $5{-}40{{\ \rm per\ cent}}$ of the outer (50–300 kpc) stellar halo in each system consists of in-situ stars that were born in outflows from the main galaxy. Outflow stars originate from gas accelerated by superbubble winds, which can be compressed, cool, and form co-moving stars. The majority of these stars remain bound to the halo and fall back with orbital properties similar to the rest of the stellar halo at z = 0. In the outer halo, outflow stars are more spatially homogeneous, metal-rich, and alpha-element-enhancedmore »than the accreted stellar halo. At the solar location, up to $\sim \!10 {{\ \rm per\ cent}}$ of our kinematically identified halo stars were born in outflows; the fraction rises to as high as $\sim \!40{{\ \rm per\ cent}}$ for the most metal-rich local halo stars ([Fe/H] >−0.5). Such stars can be retrograde and create features similar to the recently discovered Milky Way ‘Splash’ in phase space. We conclude that the Milky Way stellar halo could contain local counterparts to stars that are observed to form in molecular outflows in distant galaxies. Searches for such a population may provide a new, near-field approach to constraining feedback and outflow physics. A stellar halo contribution from outflows is a phase-reversal of the classic halo formation scenario of Eggen, Lynden-Bell & Sandange, who suggested that halo stars formed in rapidly infalling gas clouds. Stellar outflows may be observable in direct imaging of external galaxies and could provide a source for metal-rich, extreme-velocity stars in the Milky Way.« less
  5. ABSTRACT Recent searches for the hosts of z ∼ 4 damped Ly α absorbers (DLAs) have detected bright galaxies at distances of tens of kpc from the DLA. Using the FIRE-2 cosmological zoom simulations, we argue that these relatively large distances are due to a predominantly cool and neutral inner circumgalactic medium (CGM) surrounding high-redshift galaxies. The inner CGM is cool because of the short cooling time of hot gas in ${\lesssim}10^{12}\, {\rm M_{\odot }}$ haloes, which implies that accretion and feedback energy are radiated quickly, while it is neutral due to high volume densities and column densities at high redshiftmore »that shield cool gas from photoionization. Our analysis predicts large DLA covering factors (${\gtrsim}50{{\ \rm per\ cent}}$) out to impact parameters ∼0.3[(1 + z)/5]3/2Rvir from the central galaxies at z ≳ 1, equivalent to a proper distance of ${\sim}21\, M_{12}^{1/3} \left(\left(1+z\right)/5\right)^{1/2}\, {\rm kpc}$ (Rvir and M12 are the halo virial radius and mass in units of $10^{12}\, {\rm M_{\odot }}$, respectively). This implies that DLA covering factors at z ∼ 4 may be comparable to unity out to a distance ∼10 times larger than stellar half-mass radii. A predominantly neutral inner CGM in the early universe suggests that its mass and metallicity can be directly constrained by absorption surveys, without resorting to the large ionization corrections as required for ionized CGM.« less