94 research outputs found
Raising the bar: new constraints on the Hubble parameter with cosmic chronometers at z2
One of the most compelling tasks of modern cosmology is to constrain the
expansion history of the Universe, since this measurement can give insights on
the nature of dark energy and help to estimate cosmological parameters. In this
letter are presented two new measurements of the Hubble parameter H(z) obtained
with the cosmic chronometer method up to . Taking advantage of
near-infrared spectroscopy of the few very massive and passive galaxies
observed at available in literature, the differential evolution of this
population is estimated and calibrated with different stellar population
synthesis models to constrain H(z), including in the final error budget all
possible sources of systematic uncertainties (star formation history, stellar
metallicity, model dependencies). This analysis is able to extend significantly
the redshift range coverage with respect to present-day constraints, crossing
for the first time the limit at . The new H(z) data are used to
estimate the gain in accuracy on cosmological parameters with respect to
previous measurements in two cosmological models, finding a small but
detectable improvement (5 %) in particular on and .
Finally, a simulation of a Euclid-like survey has been performed to forecast
the expected improvement with future data. The provided constraints have been
obtained just with the cosmic chronometers approach, without any additional
data, and the results show the high potentiality of this method to constrain
the expansion history of the Universe at these redshifts.Comment: 5 pages, 4 figures. Accepted for publication as MNRAS letter. The
H(z) data can be downloaded at
http://www.physics-astronomy.unibo.it/en/research/areas/astrophysics/cosmology-with-cosmic-chronometer
Cosmological constraints from a joint analysis of cosmic growth and expansion
Combining measurements on the expansion history of the Universe and on the
growth rate of cosmic structures is key to discriminate between alternative
cosmological frameworks and to test gravity. Recently, Linder (2017) proposed a
new diagram to investigate the joint evolutionary track of these two
quantities. In this letter, we collect the most recent cosmic growth and
expansion rate datasets to provide the state-of-the-art observational
constraints on this diagram. By performing a joint statistical analysis of both
probes, we test the standard CDM model, confirming a mild tension
between cosmic microwave background predictions from Planck mission and cosmic
growth measurements at low redshift (). Then we test alternative models
allowing the variation of one single cosmological parameter at a time. In
particular, we find a larger growth index than the one predicted by general
relativity ). However, also a standard model with
total neutrino mass of eV provides a similarly accurate
description of the current data. By simulating an additional dataset consistent
with next-generation dark-energy mission forecasts, we show that growth rate
constraints at will be crucial to discriminate between alternative
models.Comment: 5 pages, 3 figures. Accepted for publication in MNRAS lette
Addressing the Hubble tension with cosmic chronometers
Twenty years after the discovery that the expansion of the Universe is
accelerating, a new finding is now challenging our understanding of the cosmos.
Recent studies have shown that the Hubble constant, the speed of expansion
measured today, provides values in significant tension when measured from the
Cosmic Microwave Background in the primordial Universe or from Cepheids and
Supernovae Type Ia in the local Universe. Whether this tension is hinting
towards new physics or some issue in the measurements, is still under debate;
but it is clearly calling for new independent cosmological probes to provide
additional pieces of evidence to solve this puzzle. This chapter introduces the
method of cosmic chronometers, a new emerging cosmological probe that can
provide cosmology-independent estimates of the Universe's expansion history.
This method is based on the fact that the expansion rate of the Universe can be
directly derived from measuring how much the Universe has changed in age
between two different redshifts, i.e. by estimating the slope of the
age--redshift relation. First, the main ingredients of the method will be
discussed, presenting the main equations involved and how to estimate from the
observables the needed quantities. After, it will be presented how to reliably
select a sample of tracers to map the age evolution of the Universe coherently.
Next, different methods to robustly measure the differential age of a
population, the fundamental quantity involved in the method, will be reviewed.
Finally, the main measurements obtained will be presented, providing forecasts
for future surveys and discussing how these data can provide useful feedback to
address the Hubble tension.Comment: Invited chapter for the edited book Hubble Constant Tension (Eds. E.
Di Valentino and D. Brout, Springer Singapore, expected in 2024
Revisiting oldest stars as cosmological probes: new constraints on the Hubble constant
Despite the tremendous advance of observational cosmology, the value of the
Hubble constant () is still controversial (the so called ``Hubble
tension'') because of the inconsistency between local/late-time measurements
and those derived from the cosmic microwave background. As the age of the
Universe is very sensitive to , we explored whether the present-day oldest
stars could place independent constraints on the Hubble constant. To this
purpose, we selected from the literature the oldest objects (globular clusters,
stars, white dwarfs, ultra-faint and dwarf spheroidal galaxies) with accurate
age estimates. Adopting a conservative prior on their formation redshifts () and assuming , we
developed a method based on Bayesian statistics to estimate the Hubble
constant. We selected the oldest objects ( Gyr) and estimated both
for each of them individually and for the average ages of homogeneous
subsamples. Statistical and systematic uncertainties were properly taken into
account. The constraints based on individual ages indicate that
km/s/Mpc when selecting the most accurate estimates. If the ages are averaged
and analyzed independently for each subsample, the most stringent constraints
imply with a probability of 90.3% and errors around 2.5 km/s/Mpc. We
also constructed an ``accuracy matrix'' to assess how the constraints on
become more stringent with further improvements in the accuracy of stellar ages
and . The results show the high potential of the oldest stars
as independent and competitive cosmological probes.Comment: 11 pages, 5 figures, 3 tables (including appendix). Submitted to Ap
Catching galaxies in the act of quenching star formation
Detecting galaxies when their star-formation is being quenched is crucial to
understand the mechanisms driving their evolution. We identify for the first
time a sample of quenching galaxies selected just after the interruption of
their star formation by exploiting the [O III]5007/Halpha ratio and searching
for galaxies with undetected [O III]. Using a sample of ~174000 star-forming
galaxies extracted from the SDSS-DR8 at 0.04 < z < 0.21,we identify the ~300
quenching galaxy best candidates with low [O III]/Halpha, out of ~26000
galaxies without [O III] emission. They have masses between 10^9.7 and 10^10.8
Mo, consistently with the corresponding growth of the quiescent population at
these redshifts. Their main properties (i.e. star-formation rate, colours and
metallicities) are comparable to those of the star-forming population,
coherently with the hypothesis of recent quenching, but preferably reside in
higher-density environments.Most candidates have morphologies similar to
star-forming galaxies, suggesting that no morphological transformation has
occurred yet. From a survival analysis we find a low fraction of candidates
(~0.58% of the star-forming population), leading to a short quenching timescale
of tQ~50Myr and an e-folding time for the quenching history of tauQ~90Myr, and
their upper limits of tQ<0.76 Gyr and tauQ<1.5Gyr, assuming as quenching
galaxies 50% of objects without [O III] (~7.5%).Our results are compatible with
a 'rapid' quenching scenario of satellites galaxies due to the final phase of
strangulation or ram-pressure stripping. This approach represents a robust
alternative to methods used so far to select quenched galaxies (e.g. colours,
specific star-formation rate, or post-starburst spectra).Comment: 22 pages, 23 figures, accepted for publication in MNRA
A methodology to select galaxies just after the quenching of star formation
We propose a new methodology aimed at finding star-forming galaxies in the
phase which immediately follows the star-formation (SF) quenching, based on the
use of high- to low-ionization emission line ratios. These ratios rapidly
disappear after the SF halt, due to the softening of the UV ionizing radiation.
We focus on [O III] 5007/H and [Ne III] 3869/[O II]
3727, studying them with simulations obtained with the CLOUDY
photoionization code. If a sharp quenching is assumed, we find that the two
ratios are very sensitive tracers as they drop by a factor 10 within
10 Myr from the interruption of the SF; instead, if a smoother and
slower SF decline is assumed (i.e. an exponentially declining star-formation
history with -folding time 200 Myr), they decrease by a factor
2 within 80 Myr. We mitigate the ionization -- metallicity
degeneracy affecting our methodology using pairs of emission line ratios
separately related to metallicity and ionization, adopting the [N II]
6584/[O II] 3727 ratio as metallicity diagnostic. Using a
Sloan Digital Sky Survey galaxy sample, we identify 10 examples among the most
extreme quenching candidates within the [O III] 5007/H vs. [N
II] 6584/[O II] 3727 plane, characterized by low [O III]
5007/H, faint [Ne III] 3869, and by blue
dust-corrected spectra and colours, as expected if the SF quenching has
occurred in the very recent past. Our results also suggest that the observed
fractions of quenching candidates can be used to constrain the quenching
mechanism at work and its time-scales.Comment: Accepted for publication in MNRAS; 19 pages, 21 figures, 1 tabl
Toward a Better Understanding of Cosmic Chronometers: A new measurement of H(z) at z~0.7
We analyze the stellar ages obtained from a combination of Lick indices in Borghi et al. for 140 massive and passive galaxies selected in the LEGA-C survey at 0.6 < z < 0.9. From their median age-redshift relation, we derive a new direct measurement of H(z) without any cosmological model assumption using the cosmic chronometer approach. We thoroughly study the main systematics involved in this analysis: the choice of the Lick indices combination, the binning method, the assumed stellar population model, and the adopted star formation history; these effects are included in the total error budget. We obtain H(z = 0.75) = 98.8 +/- 33.6 km s(-1) Mpc(-1). In parallel, we also propose a simple framework based on a cosmological model to describe the age-redshift relations in the context of galaxy downsizing. This allows us to derive constraints on the Hubble constant H (0) and the typical galaxy formation time. This new H(z) measurement, whose accuracy is currently limited by the scarcity of the sample analyzed, paves the road for the joint study of the stellar populations of individual passive galaxies and the expansion history of the universe in light of future spectroscopic surveys
Constraining the time evolution of dark energy, curvature and neutrino properties with cosmic chronometers
We use the latest compilation of observational H(z) measurements obtained
with cosmic chronometers in the redshift range to place constraints on
cosmological parameters. We consider the sample alone and in combination with
other state-of-the art cosmological probes: CMB data from the latest Planck
2015 release, the most recent estimate of the Hubble constant , a
compilation of recent BAO data, and the latest SNe sample. Since cosmic
chronometers are independent of the assumed cosmological model, we are able to
provide constraints on the parameters that govern the expansion history of the
Universe in a way that can be used to test cosmological models. We show that
the H(z) measurements obtained with cosmic chronometer from the BOSS survey
provide enough constraining power in combination with CMB data to constrain the
time evolution of dark energy, yielding constraints competitive with those
obtained using SNe and/or BAO. From late-Universe probes alone we find that
and , and when combining also CMB data we
obtain and . These new constraints imply
that nearly all quintessence models are disfavoured, only phantom models or a
pure cosmological constant being allowed. For the curvature we find
, including CMB data. Cosmic chronometers data are
important also to constrain neutrino properties by breaking or reducing
degeneracies with other parameters. We find that , thus
excluding the possibility of an extra (sterile) neutrino at more than
, and put competitive limits on the sum of neutrino masses, eV at 95% confidence level. Finally, we constrain the redshift
evolution of dark energy, and find w(z) consistent with the CDM model
at the 40% level over the entire redshift range . [abridged]Comment: 19 pages, 9 figures, 6 tables, submitted to JCAP. The cosmic
chronometers data used in this analysis can be downloaded at
http://www.physics-astronomy.unibo.it/en/research/areas/astrophysics/cosmology-with-cosmic-chronometer
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