Models Of Cosmic Chemical Evolution Research Articles

Metallicity-constrained merger rates of binary black holes and the stochastic gravitational wave background

The recent detection of the binary black hole merger GW150914 demonstrates the existence of black holes more massive than previously observed in X-ray binaries in our Galaxy. This article explores different scenarios of black hole formation in the context of self-consistent cosmic chemical evolution models that simultaneously match observations of the cosmic star formation rate, optical depth to reionization and metallicity of the interstellar medium. This framework is used to calculate the mass distribution of merging black hole binaries and its evolution with redshift. We also study the implications of the black hole mass distribution for the stochastic gravitational wave background from mergers and from core collapse events.

A SEARCH FOR GALAXIES PRODUCING METAL-RICH QUASAR ABSORBERS

We have carried out optical and near-IR imaging of three fields with metal-rich damped or sub-damped Lyα quasar absorption systems, using the Goddard Fabry-Perot system and the Near-IR Camera and Fabry-Perot Spectrometer at the Apache Point Observatory 3.5 m telescope. The aim of these observations was to detect the underlying galaxies and search for their redshifted [O II] λ3727 emission and optical/near-IR continuum. Candidate absorber galaxies separated by 25-88 from the quasars are detected in two of three absorber fields in the H and KS bands at >3σ level. The potentially high success rate in finding galaxy counterparts of metal-rich absorbers suggests that the metal-rich absorbers may be easier to detect in imaging observations than the metal-poor absorbers targeted in many previous imaging studies of damped Lyα absorbers. Of course, spectroscopic confirmation of the candidate galaxies is necessary to establish whether they are at the absorber redshifts. Interestingly, no significant [O II] emission is detected in any of the fields at >3σ level, implying limits on the star formation rate (SFR) of 0.7-2.6 M ☉ yr–1 per 076 × 076 region, if no dust extinction is assumed. The 3σ sensitivity levels in the narrowband images (tuned to the expected positions of the redshifted [O II] λ3727) ranged between 1.3 × 10–17 and 3.3 × 10–17 erg s–1 cm–2 in observed frame. If the candidate galaxies are at the absorber redshifts, the lack of [O II] emission would suggest that the metal-rich absorbers may arise in early-type or S0 galaxies that do not have significant current star formation. We compare our results with those compiled from the literature and with predictions of global average SFR based on the models of cosmic chemical evolution. Together, these studies indicate SFRs much lower than the predictions for the global mean SFR.

The Role of Sub–Damped Lyα Absorbers in the Cosmic Evolution of Metals

Observations of low mean metallicity of damped Lyα (DLA) quasar absorbers at all redshifts studied appear to contradict the predictions for the global mean interstellar metallicity in galaxies from cosmic chemical evolution models. On the other hand, a number of metal-rich sub-DLA systems have been identified recently, and the fraction of metal-rich sub-DLAs appears to be considerably larger than that of metal-rich DLAs, especially at z < 1.5. In view of this, here we investigate the evolution of metallicity in sub-DLAs. We find that the mean Zn metallicity of the observed sub-DLAs may be higher than that of the observed DLAs, especially at low redshifts, reaching a near-solar level at z ≲ 1. This trend does not appear to be an artifact of sample selection, the use of Zn, the use of NH i weighting, or observational sensitivity. While a bias against very low metallicity could be present in the sub-DLA sample in some situations, this cannot explain the difference between the DLA and sub-DLA metallicities at low z. The primary reason for the difference between the DLAs and sub-DLAs appears to be the dearth of metal-rich DLAs. We estimate the sub-DLA contribution to the total metal budget using measures of their metallicity and comoving gas density. These calculations suggest that at z ≲ 1, the contribution of sub-DLAs to the total metal budget may be several times that of DLAs. At higher redshifts also, there are indications that the sub-DLAs may contribute significantly to the cosmic metal budget.

A Fabry‐Perot Imaging Search for Lyα Emission in Quasar Absorbers atz∼ 2.4

We have carried out a deep narrow-band imaging survey of six fields with heavy-element quasar absorption lines, using the Goddard Fabry-Perot (FP) system at the Apache Point Observatory (APO) 3.5-meter telescope. The aim of these observations was to search for redshifted Ly-$\alpha$ emission from the galaxies underlying the absorbers at $z = 2.3-2.5$ and their companion galaxies. The 3 $\sigma$ sensitivity levels ranged between $1.9 \times 10^{-17}$ and $5.4 \times 10^{-17}$ erg s$^{-1}$ cm$^{-2}$ in observed-frame Ly-$\alpha$ flux. No significant Ly-$\alpha$ emitters were detected at $> 3 \sigma$ level. The absence of significant Ly-$\alpha$ emission implies limits on the star formation rate (SFR) of 0.9-2.7 $M_{\odot}$ yr$^{-1}$ per 2-pixel x 2-pixel region, if no dust attenuation is assumed. We compare our results with those from other emission-line studies of absorber fields and with predictions for global average SFR based on the models of cosmic chemical evolution. Our limits are among the tightest existing constraints on Ly-$\alpha$ emission from galaxies in absorber fields, but are consistent with many other studies. In the absence of dust attenuation, these studies suggest that SFRs in a large fraction of objects in the absorber fields may lie below the global mean SFR. However, it is possible that dust attenuation is responsible for the low emission line fluxes in some objects. It is also possible that the star-forming regions are compact and at smaller angular separations from the quasar than the width of our point spread function and, get lost in the quasar emission. We outline future observations that could help to distinguish between the various possibilities.

The evolution of damped Ly-$\alpha$ absorbers: metallicities and star formation rates

Damped Lyman- $\alpha$ (DLA) and sub-DLA quasar absorption lines provide powerful probes of the evolution of metals, gas, and stars in galaxies. One major obstacle in trying to understand the evolution of DLAs and sub-DLAs has been the small number of metallicity measurements at $z , an epoch spanning $\sim 70$ % of the cosmic history. In recent surveys with the Hubble Space Telescope and Multiple Mirror Telescope, we have doubled the DLA Zn sample at $z . Combining our results with those at higher redshifts from the literature, we find that the global mean metallicity of DLAs does not rise to the Solar value at low redshifts. These surprising results appear to contradict the near-Solar mean metallicity observed for nearby ( $z \approx 0$ ) galaxies and the predictions of cosmic chemical evolution models based on the global star formation history. Finally, we discuss direct constraints on the star formation rates (SFRs) in the absorber galaxies from our deep Fabry-Perot Ly- $\alpha$ imaging study and other emission-line studies in the literature. A large fraction of the observed heavy-element quasar absorbers at $0 appear to have SFRs substantially below the global mean SFR, consistent with the low metallicities observed in the spectroscopic studies.

Hubble Space TelescopeObservations of Element Abundances in Low‐Redshift Damped Lyα Galaxies and Implications for the Global Metallicity‐Redshift Relation

Most models of cosmic chemical evolution predict that the mass-weighted mean interstellar metallicity of galaxies should rise with time from a low value $\sim 1/30$ solar at $z \sim 3$ to a nearly solar value at $z = 0$. In the absence of any selection effects, the damped Lyman-alpha absorbers (DLAs) in quasar spectra are expected to show such a rise in global metallicity. However, it has been difficult to determine whether or not DLAs show this effect, primarily because of the very small number of DLA metallicity measurements at low redshifts. In an attempt to put tighter constraints on the low-redshift end of the DLA metallicity-redshift relation, we have observed Zn II and Cr II lines in four DLAs at $0.09 < z < 0.52$, using the Space Telescope Imaging Spectrograph (STIS) onboard the Hubble Space Telescope (HST). These observations have provided the first constraints on Zn abundances in DLAs with $z < 0.4$. In all the three DLAs for which our observations offer meaningful constraints on the metallicity, the data suggest that the metallicities are much lower than the solar value. These results are consistent with recent imaging studies indicating that these DLAs may be associated with dwarf or low surface brightness galaxies. We combine our results with higher redshift data from the literature to estimate the global mean metallicity-redshift relation for DLAs. We find that the global mean metallicity shows at most a slow increase with decreasing redshift. ...(Please see the paper for the complete abstract).

Evolution of Metals and Stars in Damped Lyman-Alpha Galaxies

AbstractDamped Lyman-alpha absorbers in quasar spectra provide a unique tool to directly measure the abundances of elements in galaxies at red-shifts 0 &lt; z &lt; 5, and hence probe the chemical evolution of galaxies over &gt; 90% of the age of the Universe. Since cosmic chemical evolution models predict the global metallicity of galaxies to increase with time, it is of great interest to determine whether DLAs actually show such a trend. We discuss statistical analysis of existing DLA Zn data to examine the metallicity-redshift relation, and a comparison of the observed data with models of cosmic chemical evolution. We also describe efforts to expand the DLA abundance sample at z &lt; 1.5, where the current data are particularly sparse. Finally, we discuss emission-line imaging studies of the absorber galaxies and compare constraints on their star formation rates with models based on the global star formation history.

Metallicity Evolution of Damped Lyα Galaxies

We have reanalyzed the existing data on zinc abundances in damped Lyα (DLA) absorbers to investigate whether their mean metallicity evolves with time. Most models of cosmic chemical evolution predict that the mass-weighted mean interstellar metallicity of galaxies should rise with time from a low value of ~1/30 solar at z ~ 3 to a nearly solar value at z ~ 0. However, several previous analyses have suggested that there is little or no evolution in the global metallicity of DLAs. The main problem is that the effective number of systems that dominate the N(H I)-weighted mean metallicity is very small. We have used a variety of statistical techniques to quantify the global metallicity-redshift relation and its uncertainties, taking into account both measurement and sampling errors. Three new features of our analysis are (1) an unbinned N(H I)-weighted nonlinear χ2 fit to an exponential relation, (2) survival analysis to treat the large number of limits in the existing data, and (3) a comparison of the data with several models of cosmic chemical evolution based on an unbinned N(H I)-weighted χ2. We find that a wider range of evolutionary rates is allowed by the present data than claimed in previous studies. The slope of the exponential fit to the N(H I)-weighted mean Zn metallicity versus redshift relation is -0.20 ± 0.11 counting limits as detections and -0.27 ± 0.12 counting limits as zeros. Similar results are also obtained if the data are binned in redshift and if survival analysis is used. These slopes are marginally consistent with no evolution but are also consistent with the rates predicted by several models of cosmic chemical evolution (e.g., slopes of -0.61 to -0.25 for the models of Pei & Fall, Malaney & Chaboyer, and Pei et al.). The χ2 values obtained for most of these models are somewhat worse than that for the exponential model because the models lie above the observed data points, but still suggest that the present DLA data could indicate some evolution of the metallicity with redshift. Finally, we outline some future measurements necessary to improve the statistics of the global metallicity-redshift relation.

The cosmic supernova neutrino background

Abstract Models of cosmic chemical evolution, observations of damped Lyα systems, and faint galaxy surveys provide reliable estimates of the global star formation history of the universe to redshifts beyond unity. The associated evolution of the supernova rate can thus be determined with reasonable accuracy. Core collapse supernovae are copious sources of neutrinos, and produce an integrated neutrino flux at Earth of ∼ 10 neutrinos cm−2 s−1 for each flavor. While terrestrial and extraterrestrial backgrounds exceed this signal below ∼ 15 MeV and above ∼ 40 MeV, the intermediate energy band may allow detection of the neutrino background from supernovae, thus providing a unique tracer of the cosmic star formation history. We employ recent cosmic chemical evolution studies to derive the global supernova rate and provide a simple scheme to clearly indicate the dependencies on cosmological, galactic, and stellar evolution parameters. We estimate a typical flux ∼ 0.2 ± 0.1 cm−2 s−1 in the relevant 20–30 MeV window. This flux is significantly lower than current limits, but is potentially detectable.

Cosmic Emissivity and Background Intensity from Damped Lyman-Alpha Galaxies

We present a new method to compute the cosmic emissivity ν and background intensity Jν. Our method is based entirely on data from quasar absorption-line studies, namely, the comoving density of H I and the mean metallicity and dust-to-gas ratio in damped Lyα galaxies. These observations, when combined with models of cosmic chemical evolution, are sufficient to determine the comoving rate of star formation as a function of redshift. From this, we compute ν and Jν using stellar population synthesis models. Our method includes a self-consistent treatment of the absorption and reradiation of starlight by dust. In all of our calculations, the near-UV emissivity declines rapidly between z ≈ 1 and z = 0, in agreement with estimates from the Canada-France Redshift Survey. The background intensity is consistent with a wide variety of observational limits and with a tentative detection at far-IR wavelengths.