We present Chandra observations of 23 galaxy groups and low-mass galaxy clusters at 0.03 < z < 0.15 with a median temperature of ∼ 2 keV. The sample is a statistically complete flux-limited subset of the 400 deg 2 survey. We investigated the scaling relation between X-ray luminosity (L) and temperature (T ), taking selection biases fully into account. The logarithmic slope of the bolometric L − T relation was found to be 3.29 ± 0.33, consistent with values typically found for samples of more massive clusters. In combination with other recent studies of the L − T relation we show that there is no evidence for the slope, normalisation, or scatter of the L − T relation of galaxy groups being different than that of massive clusters. The exception to this is that in the special case of the most relaxed systems, the slope of the core-excised L − T relation appears to steepen from the self-similar value found for massive clusters to a steeper slope for the lower mass sample studied here. Thanks to our rigorous treatment of selection biases, these measurements provide a robust reference against which to compare predictions of models of the impact of feedback on the X-ray properties of galaxy groups.Galaxy clusters are the largest gravitationally bound systems in the Universe, ranging in size from 2 -10 Mpc, with X-ray luminosities of ∼ 10 43 − 10 45 erg s −1 . The mass content of clusters consists of ∼ 85% dark matter, ∼ 12% X-ray bright, low-density intra-cluster medium (ICM), and ∼ 3% stars. Studying galaxy clusters is motivated by two complementary goals, investigating the formation and evolution of clusters and their galaxies, and using clusters as cosmological probes.If the ICM is only heated by the conversion, via shocks, of its gravitational potential energy to internal energy during its infall into the cluster, then its properties will exhibit selfsimilar behaviour. This will lead to simple power-law correlations between the X-ray observables, such as the temper-ature (T ) and luminosity (L) of the gas (Kaiser 1986). Importantly, any deviations of observed clusters from this selfsimilar behaviour points to the action of non-gravitational energy input to the ICM, such as mechanical and radiative energy from supernova-driven galaxy winds, or outflows powered by active galactic nuclei (AGN).The correlation between X-ray luminosity and temperature (the L − T relation) has been extensively studied, due to the relative ease with which those properties can be measured (e.g. Edge & Stewart 1991;Markevitch 1998;Pratt et al. 2009;Maughan et al. 2012;Lovisari et al. 2014;Bharadwaj et al. 2014). A consensus has emerged that the L − T relation is steeper than the self-similar prediction, in the sense that lower mass clusters are hotter and/or less luminous than expected (although Maughan et al. (2012)

While molecular quasar absorption systems provide unique probes of the physical and chemical properties of the gas as well as original constraints on fundamental physics and cosmology, their detection remains challenging. Here we present the results from a complete survey for molecular gas in thirty-nine absorption systems selected solely upon the detection of neutral carbon lines in Sloan Digital Sky Survey (SDSS) spectra, without any prior knowledge of the atomic or molecular gas content. H 2 is found in all twelve systems (including seven new detections) where the corresponding lines are covered by the instrument setups and measured to have log N(H 2 ) 18, indicating a self-shielded regime. We also report seven CO detections (7/39) down to log N(CO) ∼ 13.5, including a new one, and put stringent constraints on N(CO) for the remaining 32 systems. N(CO) and N(C I) are found to be strongly correlated with N(CO)/N(C I) ∼ 1/10. This suggests that the C I-selected absorber population is probing gas deeper than the H I-H 2 transition in which a substantial fraction of the total hydrogen in the cloud is in the form of H 2 . We conclude that targeting C I-bearing absorbers is a very efficient way to find high-metallicity molecular absorbers. However, probing the molecular content in lower-metallicity regimes as well as high-column-density neutral gas remains to be undertaken to unravel the processes of gas conversion in normal high-z galaxies.

The Dark Energy Spectroscopic Instrument (DESI) embarked on an ambitious 5 yr survey in 2021 May to explore the nature of dark energy with spectroscopic measurements of 40 million galaxies and quasars. DESI will determine precise redshifts and employ the baryon acoustic oscillation method to measure distances from the nearby universe to beyond redshift z > 3.5, and employ redshift space distortions to measure the growth of structure and probe potential modifications to general relativity. We describe the significant instrumentation we developed to conduct the DESI survey. This includes: a wide-field, 3.°2 diameter prime-focus corrector; a focal plane system with 5020 fiber positioners on the 0.812 m diameter, aspheric focal surface; 10 continuous, high-efficiency fiber cable bundles that connect the focal plane to the spectrographs; and 10 identical spectrographs. Each spectrograph employs a pair of dichroics to split the light into three channels that together record the light from 360–980 nm with a spectral resolution that ranges from 2000–5000. We describe the science requirements, their connection to the technical requirements, the management of the project, and interfaces between subsystems. DESI was installed at the 4 m Mayall Telescope at Kitt Peak National Observatory and has achieved all of its performance goals. Some performance highlights include an rms positioner accuracy of better than 0.″1 and a median signal-to-noise ratio of 7 of the [O ii] doublet at 8 × 10−17 erg s−1 cm−2 in 1000 s for galaxies at z = 1.4–1.6. We conclude with additional highlights from the on-sky validation and commissioning, key successes, and lessons learned.