Gamma ray burst studies with THESEUS

Gamma-ray Bursts (GRBs) are the most powerful transients in the Universe, over–shining for a few seconds all other γ-ray sky sources. Their emission is produced within narrowly collimated relativistic jets launched after the core–collapse of massive stars or the merger of compact binaries. THESEUS will open a new window for the use of GRBs as cosmological tools by securing a statistically significant sample of high-z GRBs, as well as by providing a large number of GRBs at low–intermediate redshifts extending the current samples to low luminosities. The wide energy band and unprecedented sensitivity of the Soft X-ray Imager (SXI) and X-Gamma rays Imaging Spectrometer (XGIS) instruments provide us a new route to unveil the nature of the prompt emission. For the first time, a full characterisation of the prompt emission spectrum from 0.3 keV to 10 MeV with unprecedented large count statistics will be possible revealing the signatures of synchrotron emission. SXI spectra, extending down to 0.3 keV, will constrain the local metal absorption and, for the brightest events, the progenitors’ ejecta composition. Investigation of the nature of the internal energy dissipation mechanisms will be obtained through the systematic study with XGIS of the sub-second variability unexplored so far over such a wide energy range. THESEUS will follow the spectral evolution of the prompt emission down to the soft X–ray band during the early steep decay and through the plateau phase with the unique ability of extending above 10 keV the spectral study of these early afterglow emission phases.

Minimally modified gravity fitting Planck data better than $$\Lambda $$ CDM

We study the phenomenology of a class of minimally modified gravity theories called $$f(\mathcal {H})$$ f ( H ) theories, in which the usual general relativistic Hamiltonian constraint is replaced by a free function of it. After reviewing the construction of the theory and a consistent matter coupling, we analyze the dynamics of cosmology at the levels of both background and perturbations, and present a concrete example of the theory with a 3-parameter family of the function f. Finally, we compare this example model to Planck data as well as some later-time probes, showing that such a realization of $$f(\mathcal {H})$$ f ( H ) theories fits the data significantly better than the standard $$\Lambda $$ Λ CDM model, in particular by modifying gravity at intermediate redshifts, $$z\simeq 743$$ z ≃ 743 .

K-CLASH: spatially resolving star-forming galaxies in field and cluster environments at z ≈ 0.2–0.6

ABSTRACT We present the KMOS-CLASH (K-CLASH) survey, a K-band Multi-Object Spectrograph (KMOS) survey, of the spatially resolved gas properties and kinematics of 191 (pre-dominantly blue) H α-detected galaxies at 0.2 ≲ z ≲ 0.6 in field and cluster environments. K-CLASH targets galaxies in four Cluster Lensing And Supernova survey with Hubble (CLASH) fields in the KMOS IZ-band, over 7 arcmin radius (≈2–3 Mpc) fields of view. K-CLASH aims to study the transition of star-forming galaxies from turbulent, highly star-forming disc-like and peculiar systems at z ≈ 1–3, to the comparatively quiescent, ordered late-type galaxies at z ≈ 0, and to examine the role of clusters in the build-up of the red sequence since z ≈ 1. In this paper, we describe the K-CLASH survey, present the sample, and provide an overview of the K-CLASH galaxy properties. We demonstrate that our sample comprises star-forming galaxies typical of their stellar masses and epochs, residing both in field and cluster environments. We conclude K-CLASH provides an ideal sample to bridge the gap between existing large integral-field spectroscopy surveys at higher and lower redshifts. We find that star-forming K-CLASH cluster galaxies at intermediate redshifts have systematically lower stellar masses than their star-forming counterparts in the field, hinting at possible ‘downsizing’ scenarios of galaxy growth in clusters at these epochs. We measure no difference between the star formation rates of H α-detected, star-forming galaxies in either environment after accounting for stellar mass, suggesting that cluster quenching occurs very rapidly during the epochs probed by K-CLASH, or that star-forming K-CLASH galaxies in clusters have only recently arrived there, with insufficient time elapsed for quenching to have occurred.

The stellar population of metal-poor galaxies at z ≈ 0.8 and the evolution of the mass–metallicity relation

ABSTRACT We present results from deep Spitzer/Infrared Array Camera (IRAC) observations of 28 metal-poor, strongly star-forming galaxies selected from the DEEP2 Galaxy Survey. By modelling infrared and optical photometry, we derive stellar masses and other stellar properties. We determine that these metal-poor galaxies have low stellar masses, M⋆ ≈ 108.1–109.5 M⊙. Combined with the Balmer-derived star formation rates (SFRs), these galaxies have average inverse SFR/M⋆ of ≈100 Myr. The evolution of stellar mass–gas metallicity relation to z ≈ 0.8 is measured by combining the modelled masses with previously obtained spectroscopic measurements of metallicity from [O iii] λ4363 detections. Here, we include measurements for 79 galaxies from the Metal Abundances across Cosmic Time Survey. Our mass–metallicity relation is lower at a given stellar mass than at z = 0.1 by 0.27 dex. This demonstrates a strong evolution in the mass–metallicity relation, $(1+z)^{-1.45^{+0.61}_{-0.76}}$. We find that the shape of the z ≈ 0.8 mass-metallicity relation, a steep rise in metallicity at low stellar masses, transitioning to a plateau at higher masses, is consistent with z ∼ 0.1 studies. We also compare the evolution in metallicity between z ≈ 0.8 and z ∼ 0.1 against recent strong-line diagnostic studies at intermediate redshifts and find good agreement. Specifically, we find that lower mass galaxies (4 × 108 M⊙) built up their metal content 1.6 times more rapidly than higher mass galaxies (1010 M⊙). Finally, we examine whether the mass–metallicity relation has a secondary dependence on SFR, and statistically concluded that there is no strong secondary dependence for z ≈ 0.8 low-mass galaxies.

Chlorine-bearing molecules in molecular absorbers at intermediate redshifts

We use observations of chlorine-bearing species in molecular absorbers at intermediate redshifts to investigate chemical properties and 35Cl/37Cl isotopic ratios in the absorbing sightlines. Chloronium (H2Cl+) is detected along three independent lines of sight in the z = 0.89 and z = 0.68 molecular absorbers located in front of the lensed quasars PKS 1830−211 and B 0218+357, respectively. Hydrogen chloride (HCl) was observed only toward PKS 1830−211, and is found to behave differently from H2Cl+. It is detected in one line of sight with an abundance ratio [H2Cl+] / [HCl] ∼1, but remains undetected in the other, more diffuse, line of sight, with a ratio [H2Cl+] / [HCl] > 17. The absorption profiles of these two chlorine-bearing species are compared to other species and discussed in terms of the physical properties of the absorbing gas. Our findings are consistent with the picture emerging from chemical models where different species trace gas with different molecular hydrogen fraction. The 35Cl/37Cl isotopic ratios are measured in the different lines of sight and are discussed in terms of stellar nucleosynthesis.

Emission-line properties of the most luminous AGNs in massive galaxies at intermediate redshifts

ABSTRACT We have analysed the emission-line properties of 6019 Type II active galactic nuclei (AGNs) at redshifts in the range 0.4–0.8 with [O iii] luminosities greater than $3 \times 10^8 \, \mathrm{L}_{\odot }$, characteristic of the Type II quasars first identified in population studies by Zakamska et al. The AGNs are drawn from the CMASS sample of galaxies with stellar masses greater than $10^{11} \, \mathrm{M}_{\odot }$ that were studied as part of the Baryon Oscillation Spectroscopic Survey (BOSS) and comprise 0.5 per cent of the total population of these galaxies. Individual spectra have low S/N, so the analysis is carried out on stacked spectra in bins of [O iii] luminosity and estimated stellar age. The emission line ratios of the stacks are well fit with simple uniform-density photoionization models with metallicities between solar and twice solar. In the stacks, a number of emission lines are found to have distinct broad components requiring a double Gaussian rather than a single Gaussian fit, indicative of outflowing ionized gas. These are: [O iii] λ4959, [O iii] λ5007, [O ii] λ3727,3729, and H αλ6563. Higher ionization lines such as [Ne iii] λ3869 and [Ne v] λ3345 are detected in the stacks, but are well fit by single Gaussians. The broad components typically contain a third of the total line flux and have widths of 600 km s−1 for the oxygen lines and 900 km s−1 for H α. The fraction of the flux in the broad component and its width are independent of [O iii] luminosity, stellar age, radio, and mid-IR luminosity. The stellar mass of the galaxy is the only parameter we could identify that influences the width of the broad line component.