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 and the prospects for major progress in this field in the nearby future.
ABSTRACT
Forthcoming observational facilities will make the exploration of the early universe routine, likely probing large populations of galaxies at very low metallicities. It will therefore be ...important to have diagnostics that can solidly identify and distinguish different classes of objects in such low metallicity regimes. We use new photoionization models to develop diagnostic diagrams involving various nebular lines. We show that combinations of these diagrams allow the identification and discrimination of the following classes of objects in the early universe: PopIII and direct collapse black holes (DCBH) in pristine environments, PopIII and DCBH embedded in slightly enriched interstellar medium (ISM; $\rm Z\sim 10^{-5}-10^{-4}$), and (metal poor) PopII and active galactic nucleus in enriched ISM. Diagnostics involving rest-frame optical lines (that will be accessible by James Webb Space Telescope) have a better discriminatory power but also rest-frame ultraviolet diagnostics can provide very useful information. Interestingly, we find that metal lines such as O iiiλ5007 and C ivλ1549 can remain relatively strong (about a factor of 0.1–1 relative H β and He iiλ1640, respectively), even in extremely metal poor environments ($\rm Z\sim 10^{-5}-10^{-4}$), which could be embedding PopIII galaxies and DCBH.
Local galaxies are broadly divided into two main classes, star-forming (gas-rich) and quiescent (passive and gas-poor). The primary mechanism responsible for quenching star formation in galaxies and ...transforming them into quiescent and passive systems is still unclear. Sudden removal of gas through outflows or stripping is one of the mechanisms often proposed. An alternative mechanism is so-called "strangulation", in which the supply of cold gas to the galaxy is halted. Here we report an analysis of the stellar metallicity (the fraction of elements heavier than helium in stellar atmospheres) in local galaxies, from 26,000 spectra, that clearly reveals that strangulation is the primary mechanism responsible for quenching star formation, with a typical timescale of four billion years, at least for local galaxies with a stellar mass less than 10(11) solar masses. This result is further supported independently by the stellar age difference between quiescent and star-forming galaxies, which indicates that quiescent galaxies of less than 10(11) solar masses are on average observed four billion years after quenching due to strangulation.
Abstract
Several models have predicted that stars could form inside galactic outflows and that this would be a new major mode of galaxy evolution. Observations of galactic outflows have revealed that ...they host large amounts of dense and clumpy molecular gas, which provide conditions suitable for star formation. We have investigated the properties of the outflows in a large sample of galaxies by exploiting the integral field spectroscopic data of the large MaNGA-SDSS4 galaxy survey. We find evidence for prominent star formation occurring inside at least 30 per cent of the galactic outflows in our sample, whilst signs of star formation are seen in up to half of the outflows. We also show that even if star formation is prominent inside many other galactic outflows, this may have not been revealed as the diagnostics are easily dominated by the presence of even faint active galactic nucleus and shocks. If very massive outflows typical of distant galaxies and quasars follow the same scaling relations observed locally, then the star formation inside high-z outflows can be up to several 100 $\rm M_{\odot }~yr^{-1}$ and could contribute substantially to the early formation of the spheroidal component of galaxies. Star formation in outflows can also potentially contribute to establishing the scaling relations between black holes and their host spheroids. Moreover, supernovae exploding on large orbits can chemically enrich in situ and heat the circumgalactic and intergalactic medium. Finally, young stars ejected on large orbits may also contribute to the reionization of the Universe.
We derive new empirical calibrations for strong-line diagnostics of gas-phase metallicity in local star-forming galaxies by uniformly applying the T sub( e) method over the full metallicity range ...probed by the Sloan Digital Sky Survey (SDSS). To measure electron temperatures at high metallicity, where the auroral lines needed are not detected in single galaxies, we stacked spectra of more than 110 000 galaxies from the SDSS in bins of logOii/H... and logOiii/H... This stacking scheme does not assume any dependence of metallicity on mass or star formation rate, but only that galaxies with the same line ratios have the same oxygen abundance. We provide calibrations which span more than 1 dex in metallicity and are entirely defined on a consistent absolute T sub( e) metallicity scale for galaxies. We apply our calibrations to the SDSS sample and find that they provide consistent metallicity estimates to within 0.05 dex. (ProQuest: ... denotes formulae/symbols omitted.)
We investigate the properties of star-formation-driven outflows by using a large spectroscopic sample of ~160 000 local “normal” star-forming galaxies drawn from the Sloan digital sky survey (SDSS), ...spanning a wide range of star formation rates (SFRs) and stellar masses (M∗). The galaxy sample is divided into a fine grid of bins in the M∗−SFR parameter space, for each of which we produced a composite spectrum by stacking the SDSS spectra of the galaxies contained in that bin together. We exploited the high signal-to-noise of the stacked spectra to study the emergence of faint features of optical emission lines that may trace galactic outflows and are otherwise too faint to detect in individual galaxy spectra. We have adopted a novel approach that relies on the comparison between the line-of-sight velocity distribution (LoSVD) of the ionised gas (as traced by the OIIIλ5007 and Hα+NIIλλ6548, 6583 emission lines) and the LoSVD of the stars, which are used as a reference for tracing virial motions. Significant deviations in the gas kinematics from the stellar kinematics in the high-velocity tail of the LoSVDs are interpreted as a signature of outflows. Our results suggest that the incidence of ionised outflows increases with SFR and specific SFR. The outflow velocity (vout) is found to correlate tightly with the SFR for SFR> 1 M⊙ yr-1, whereas the dependence of vout on SFR is nearly flat at lower SFRs. The outflow velocity appears to also increase with the stellar velocity dispersion (σ∗), although this relation has a much larger scatter than the one with SFR, and we infer velocities as high as vout ~ (6−8)σ∗. Strikingly, we detect the signature of ionised outflows only in galaxies located above the main sequence (MS) of star-forming galaxies in the M∗−SFR diagram, and the incidence of such outflows increases sharply with the offset from the MS. This result suggests that star-formation-driven outflows may be responsible for shaping the upper envelope of the MS by providing a self-regulating mechanism for star formation. Finally, our complementary analysis of the stellar kinematics reveals the presence of blue asymmetries of a few 10 km s-1 in the stellar LoSVDs. The origin of such asymmetries is not clear, but a possibility is that they trace the presence of a large number of high velocity runaway stars and hypervelocity stars in radial trajectories in local galaxies.
We present the stacking analysis of a sample of 48 quasi-stellar objects (QSOs) at 4.5 < z < 7.1 detected by the Atacama Large Millimetre Array (ALMA) in the CII λ158 μm emission line to ...investigate the presence and the properties of massive, cold outflows associated with broad wings in the CII profile. The high sensitivity reached through this analysis allows us to reveal very broad CII wings tracing the presence of outflows with velocities in excess of 1000 km s−1. We find that the luminosity of the broad CII emission increases with LAGN, while it does not significantly depend on the star formation rate of the host galaxy, indicating that the central active galactic nucleus (AGN) is the main driving mechanism of the CII outflows in these powerful, distant QSOs. From the stack of the ALMA cubes, we derive an average outflow spatial extent of ∼3.5 kpc. The average atomic neutral mass outflow rate inferred from the stack of the whole sample is Ṁout ∼ 100 M⊙ yr−1, while for the most luminous systems it increases to ∼200 M⊙ yr−1. The associated outflow kinetic power is about 0.1% of LAGN, while the outflow momentum rate is ∼LAGN/c or lower, suggesting that these outflows are either driven by radiation pressure onto dusty clouds or, alternatively, are driven by the nuclear wind and energy conserving but with low coupling with the interstellar medium. We discuss the implications of the resulting feedback effect on galaxy evolution in the early Universe.
We present zoom-in, adaptive mesh refinement, high-resolution (...30 pc) simulations of high-redshift (z ... 6) galaxies with the aim of characterizing their internal properties and interstellar ...medium. Among other features, we adopt a star formation model based on a physically sound molecular hydrogen prescription, and introduce a novel scheme for supernova feedback, stellar winds and dust-mediated radiation pressure. In the zoom-in simulation, the target halo hosts 'Dahlia', a galaxy with a stellar mass M... = 1.6 x 10 super( 10) M..., representative of a typical z ~ 6 Lyman-break galaxy. Dahlia has a total H2 mass of 108.5 M... that is mainly concentrated in a disc-like structure of effective radius ...0.6 kpc and scale height ...200 pc. Frequent mergers drive fresh gas towards the centre of the disc, sustaining a star formation rate per unit area of ...15 M...yr super( -1) kpc super( -2). The disc is composed of dense (n ... 25 cm super( -3)), metal-rich (Z ... 0.5 Z...) gas that is pressure supported by radiation. We compute the 158 ...m Cii emission arising from Dahlia, and find that ...95 per cent of the total Cii luminosity ($$L_...\rm C\,\small ...II......\simeq 10 greater than or equal to ..7.5...\,...\rm L..._...\odot ...$$) arises from the H2 disc. Although 30 per cent of the Cii mass is transported out of the disc by outflows, such gas negligibly contributes to Cii emission, due to its low density (n ... 10 cm super( -3)) and metallicity (Z ... 10 super( -1) Z...). Dahlia is underluminous with respect to the local Cii-SFR relation; however, its luminosity is consistent with upper limits derived for most z ~ 6 galaxies. (ProQuest: ... denotes formulae/symbols omitted.)
Abstract
The increasing observational evidence of galactic outflows is considered as a sign of active galactic nucleus (AGN) feedback in action. However, the physical mechanism responsible for ...driving the observed outflows remains unclear, and whether it is due to momentum, energy, or radiation is still a matter of debate. The observed outflow energetics, in particular the large measured values of the momentum ratio ($\dot{p}/(L/c) \sim 10$) and energy ratio ($\dot{E}_{\rm k}/L \sim 0.05$), seems to favour the energy-driving mechanism; and most observational works have focused their comparison with wind energy-driven models. Here, we show that AGN radiation pressure on dust can adequately reproduce the observed outflow energetics (mass outflow rate, momentum flux, and kinetic power), as well as the scalings with luminosity, provided that the effects of radiation trapping are properly taken into account. In particular, we predict a sublinear scaling for the mass outflow rate ($\dot{M} \propto L^{1/2}$) and a superlinear scaling for the kinetic power ($\dot{E}_{\rm k} \propto L^{3/2}$), in agreement with the observational scaling relations reported in the most recent compilation of AGN outflow data. We conclude that AGN radiative feedback can account for the global outflow energetics, at least equally well as the wind energy-driving mechanism, and therefore both physical models should be considered in the interpretation of future AGN outflow observations.
We show that the mass–metallicity relation observed in the local universe is due to a more general relation between stellar mass M★, gas-phase metallicity and star formation rate (SFR). Local ...galaxies define a tight surface in this 3D space, the fundamental metallicity relation (FMR), with a small residual dispersion of ∼0.05 dex in metallicity, i.e. ∼12 per cent. At low stellar mass, metallicity decreases sharply with increasing SFR, while at high stellar mass, metallicity does not depend on SFR. High-redshift galaxies up to z∼ 2.5 are found to follow the same FMR defined by local Sloan Digital Sky Survey (SDSS) galaxies, with no indication of evolution. In this respect, the FMR defines the properties of metal enrichment of galaxies in the last 80 per cent of cosmic time. The evolution of the mass–metallicity relation observed up to z= 2.5 is due to the fact that galaxies with progressively higher SFRs, and therefore lower metallicities, are selected at increasing redshifts, sampling different parts of the same FMR. By introducing the new quantity μα= log (M★) −α log (SFR), with α= 0.32, we define a projection of the FMR that minimizes the metallicity scatter of local galaxies. The same quantity also cancels out any redshift evolution up to z∼ 2.5, i.e. all galaxies follow the same relation between μ0.32 and metallicity and have the same range of values of μ0.32. At z > 2.5, evolution of about 0.6 dex off the FMR is observed, with high-redshift galaxies showing lower metallicities. The existence of the FMR can be explained by the interplay of infall of pristine gas and outflow of enriched material. The former effect is responsible for the dependence of metallicity with SFR and is the dominant effect at high redshift, while the latter introduces the dependence on stellar mass and dominates at low redshift. The combination of these two effects, together with the Schmidt–Kennicutt law, explains the shape of the FMR and the role of μ0.32. The small-metallicity scatter around the FMR supports the smooth infall scenario of gas accretion in the local universe.
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