The evolution of the content of heavy elements in galaxies, the relative chemical abundances, their spatial distribution, and how these scale with various galactic properties, provide unique information on the galactic evolutionary processes across the cosmic epochs. In recent years major progress has been made in constraining the chemical evolution of galaxies and inferring key information relevant to our understanding of the main mechanisms involved in galaxy evolution. In this review we provide an overview of these various areas. After an overview of the methods used to constrain the chemical enrichment in galaxies and their environment, we discuss the observed scaling relations between metallicity and galaxy properties, the observed relative chemical abundances, how the chemical elements are distributed within galaxies, and how these properties evolve across the cosmic epochs. We discuss how the various observational findings compare with the predictions from theoretical models and numerical cosmological simulations. Finally, we briefly discuss the open problems the prospects for progress in this field in the nearby future.

/pdf/de-re-metallica-the-cosmic-chemical-evolution-of-galaxies-35saefoif6.pdf

De Re Metallica: The cosmic chemical evolution of galaxies

Pressure-regulated star formation is a simple variant on the usual supernova-regulated star formation efficiency that controls the global star formation rate as a function of cold gas content in star-forming galaxies, and accounts for the Schmidt-Kennicutt law in both nearby and distant galaxies. Inclusion of active galactic nucleus (AGN) induced pressure, by jets and/or winds that flow back onto a gas-rich disk, can lead, under some circumstances, to significantly enhanced star formation rates, especially at high redshift and most likely followed by the more widely accepted phase of star formation quenching. Simple expressions are derived that relate supermassive black hole growth, star formation, and outflow rates. The ratios of black hole to spheroid mass and of both black hole accretion and outflow rates to star formation rate are predicted as a function of time. I suggest various tests of the AGN-triggered star formation hypothesis.

https://iopscience.iop.org/article/10.1088/0004-637X/772/2/112/pdf

Unleashing Positive Feedback: Linking the Rates of Star Formation, Supermassive Black Hole Accretion, and Outflows in Distant Galaxies

We show how accretion rate governs the physical properties of a sample of unobscured broad-line, narrow-line, and lineless active galactic nuclei (AGNs). We avoid the systematic errors plaguing previous studies of AGN accretion rates by using accurate intrinsic accretion luminosities (L _(int)) from well-sampled multiwavelength spectral energy distributions from the Cosmic Evolution Survey, and accurate black hole masses derived from virial scaling relations (for broad-line AGNs) or host-AGN relations (for narrow-line and lineless AGNs). In general, broad emission lines are present only at the highest accretion rates (L _(int)/L _(Edd) > 10^(–2)), and these rapidly accreting AGNs are observed as broad-line AGNs or possibly as obscured narrow-line AGNs. Narrow-line and lineless AGNs at lower specific accretion rates (L _(int)/L _(Edd) 10^(–2) broad-line AGNs, since the unbound nature of the RIAF means it is easier to form a radio outflow. The IR torus signature also tends to become weaker or disappear from L _(int)/L _(Edd) < 10^(–2) AGNs, although there may be additional mid-IR synchrotron emission associated with the RIAF. Together, these results suggest that specific accretion rate is an important physical "axis" of AGN unification, as described by a simple model.

https://iopscience.iop.org/article/10.1088/0004-637X/733/1/60/pdf

Accretion rate and the physical nature of unobscured active galaxies

We present results from Keck I high resolution spectroscopy of the radio loud quasar PKS 0123+257 ($z_e$=2.364, V=17.5). In this object we detect Ly$\alpha$, N V 1238,1242, Si IV 1393,1402, and C IV 1548,1550 in an absorption system at a redshift of 2.369. The Ly$\alpha$ line has a square- bottomed profile suggesting a high column density of gas, yet the line does not reach zero intensity. The resolved C IV doublet ratio also clearly demonstrates that the absorbing clouds at this redshift do not fully occult the background light source along our line-of-sight. 
The absorption lines are positioned near the centers of the broad emission- lines and the coverage fraction of the strongest absorption lines varies inversely proportionally with the strength of the corresponding emission lines. This implies that although the absorption-line region may obscure the continuum source, it does not completely occult the broad emission-line region. This effect suggests that the lines are formed close to the QSO central region. A model is proposed in which the apparent coverage fraction derived for the weaker absorption lines may vary with the column density of the lines. 
Broad absorption-lines (which are known to be intrinsic) are found nearly exclusively in radio-quiet objects. Intrinsic narrow absorption lines have previously been found in radio quiet QSOs; it is therefore significant that an intrinsic absorption system has been verified in a radio loud quasar.

Keck High Resolution Spectroscopy of PKS 0123+257: Intrinsic Absorption in a Radio Loud Quasar

Pressure-regulated star formation is a simple variant on the usual supernova-regulated star formation efficiency that controls the global star formation rate as a function of cold gas content in star-forming galaxies, and accounts for the Schmidt-Kennicutt law in both nearby and distant galaxies. Inclusion of AGN-induced pressure, by jets and/or winds that flow back onto a gas-rich disk, can lead under some circumstances to significantly enhanced star formation rates, especially at high redshift and most likely followed by the more widely accepted phase of star formation quenching. Simple expressions are derived that relate supermassive black hole growth, star formation and outflow rates. The ratios of black hole to spheroid mass and of both black hole accretion and outflow rates to star formation rate are predicted as a function of time. I suggest various tests of the AGN-triggered star formation hypothesis.

Unleashing Positive Feedback: Linking the Rates of Star Formation, Supermassive Black Hole Accretion and Outflows in Distant Galaxies

During meiotic prophase I, spermatocytes must balance transcriptional activation with homologous recombination and chromosome synapsis, biological processes requiring extensive changes to chromatin state. We explored the interplay between chromatin accessibility and transcription through prophase I of mammalian meiosis by measuring genome-wide patterns of chromatin accessibility, nascent transcription, and processed mRNA. We find that Pol II is loaded on chromatin and maintained in a paused state early during prophase I. In later stages, paused Pol II is released in a coordinated transcriptional burst mediated by the transcription factors A-MYB and BRDT, resulting in ~3-fold increase in transcription. Transcriptional activity is temporally and spatially segregated from key steps of meiotic recombination: double strand breaks show evidence of chromatin accessibility earlier during prophase I and at distinct loci from those undergoing transcriptional activation, despite shared chromatin marks. Our findings reveal mechanisms underlying chromatin specialization in either transcription or recombination in meiotic cells. 

/pdf/a-myb-and-brdt-dependent-rna-polymerase-ii-pause-release-3i83us4c.pdf

A-MYB and BRDT-dependent RNA Polymerase II pause release orchestrates transcriptional regulation in mammalian meiosis

We present the results of an exploratory study of broad line region (BLR) metallicity in 34 2.2 < z < 4.6 quasars with far-infrared (FIR) luminosities (L_FIR) from 10^13.4 to 10^12.1 L_\odot . Quasar samples sorted by L_FIR might represent an evolutionary sequence if the star formation rates (SFRs) in quasar hosts generally diminish across quasar lifetimes. We use rest-frame ultraviolet spectra from the Sloan Digital Sky Survey to construct three composite spectra sorted by L_FIR, corresponding to average SFRs of 4980, 2130 and 340 M_\odot yr^-1 after correcting for a nominal quasar FIR contribution. The measured N V {\lambda} 1240/C IV {\lambda} 1550 and Si IV {\lambda} 1397+O IV] {\lambda} 1402/C IV {\lambda} 1550 emission line ratios indicate super-solar BLR metallicities in all three composites, with no evidence for a trend with the star formation rate. The formal derived metallicities, Z ~ 5-9 Z_\odot , are similar to those derived for the BLRs of other quasars at similar redshifts and luminosities. These results suggest that the ongoing star formation in the host is not responsible for the metal enrichment of the BLR gas. Instead, the BLR gas must have been enriched before the visible quasar phase. These results for high quasar metallicities, regardless of L_FIR, are consistent with evolution scenarios wherein visibly bright quasars appear after the main episode(s) of star formation and metal enrichment in the host galaxies. Finally, young quasars, those more closely associated with a recent merger or a blowout of gas and dust, may exhibit tracers of these events, such as redder continuum slopes and higher incidence of narrow absorption lines. With the caveat of small sample sizes, we find no relation between L_FIR and the reddening or the incidence of absorption lines.

Metallicity and Far-Infrared Luminosity of High Redshift Quasars

Rich complexes of associated absorption lines (AALs) in quasar spectra provide unique information about gaseous infall, outflows, and feedback processes in quasar environments. We study five quasars at redshifts 3.1 to 4.4 with AAL complexes containing from 7 to 18 CIV 1548, 1551 systems in high-resolution spectra. These complexes span velocity ranges $\lesssim$3600 km/s within $\lesssim$8200 km/s of the quasar redshifts. All are highly ionised with no measurable low-ionisation ions like SiII or CII, and all appear to form in the quasar/host galaxy environments based on evidence for line locking, partial covering of the background light source, strong NV absorption, and/or roughly solar metallicities, and on the implausibility of such complexes forming in unrelated intervening galaxies. Most of the lines in all five complexes identify high-speed quasar-driven outflows at velocity shifts $v\lesssim -1000$ km/s. Four of the complexes also have lines at smaller blueshifted velocities that might form in ambient interstellar clouds, low-speed outflows or at feedback interfaces in the host galaxies where high-speed winds impact and shred interstellar clouds. The partial covering we measure in some of the high-speed outflow lines require small absorbing clouds with characteristic sizes $\lesssim$1 pc or $\lesssim$0.01 pc. The short survival times of these clouds require locations very close to the quasars, or cloud creation in situ at larger distances perhaps via feedback/cloud-shredding processes. The AAL complex in one quasar, J1008+3623, includes unusually narrow CIV systems at redshifted velocities $350\lesssim v\lesssim640$ km/s that are excellent candidates for gaseous infall towards the quasar, e.g., ''cold-mode" accretion or a gravitationally-bound galactic fountain.

Nature and Origins of Rich Complexes of C iv Associated Absorption Lines

Narrow absorption line (NAL) outflows are an important yet poorly understood part of the quasar outflow phenomenon. We discuss one particular NAL outflow that has high speeds, time variability, and moderate ionizations like typical BAL flows, at an estimated location just ~5 pc from the quasar. It also has a total column density and line widths (internal velocity dispersions) ~100 times smaller than BALs, with no substantial X-ray absorption. We argue that radiative shielding (in the form of an X-ray/warm absorber) is not critical for the outflow acceleration and that the moderate ionizations occur in dense substructures that have an overall small volume filling factor in the flow. We also present new estimates of the overall incidence of quasar outflow lines; e.g., ~43% of bright quasars have a C IV NAL outflow while ~68% have a C IV outflow line of any variety (NAL, BAL, or mini-BAL).

Narrow UV Absorption Line Outflows from Quasars

/pdf/narrow-uv-absorption-line-outflows-from-quasars-u14ilx9axa.pdf

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A-MYB and BRDT-dependent RNA Polymerase II pause release orchestrates transcriptional regulation in mammalian meiosis

Metallicity and Far-Infrared Luminosity of High Redshift Quasars

Nature and Origins of Rich Complexes of C iv Associated Absorption Lines

Narrow UV Absorption Line Outflows from Quasars

Narrow UV Absorption Line Outflows from Quasars