Missing Metals Problem Research Articles

The Cosmic Baryon and Metal Cycles

Characterizing the relationship between stars, gas, and metals in galaxies is a critical component of understanding the cosmic baryon cycle. We compile contemporary censuses of the baryons in collapsed structures and their chemical makeup and dust content. We show the following: ▪ The [Formula: see text] mass density of the Universe is well determined to redshifts [Formula: see text] and shows minor evolution with time. New observations of molecular hydrogen reveal its evolution mirrors that of the global star-formation rate density, implying a universal cosmic molecular gas depletion timescale. The low-redshift decline of the star-formation history is thus driven by the lack of molecular gas supply due to a drop in net accretion rate related to the decreased growth of dark matter halos. ▪ The metal mass density in cold gas ([Formula: see text] K) contains virtually all the metals produced by stars for [Formula: see text]. At lower redshifts, the contributors to the total amount of metals are more diverse; at [Formula: see text], most of the observed metals are bound in stars. Overall, there is little evidence for a “missing metals problem” in modern censuses. ▪ We characterize the dust content of neutral gas over cosmic time, finding the dust-to-gas and dust-to-metals ratios fall with decreasing metallicity. We calculate the cosmological dust mass density in the neutral gas up to [Formula: see text]. There is good agreement between multiple tracers of the dust content of the Universe.

Evidence for a Massive Warm–Hot Circumgalactic Medium around NGC 3221

Abstract We report a 3.4σ detection of the warm–hot, massive, extended circumgalactic medium (CGM) around an L ⋆ star-forming spiral galaxy NGC 3221, using deep Suzaku observations. The temperature of the gas is 106.1 K, comparable to that of the Milky Way CGM. The spatial extent of the gas is at least 150 kpc. For a β-model of density profile with solar abundance, the central emission measure is EM = (3 ± 1) × 10−5 cm−6 kpc and the central electron density is n eo = (4 ± 1) × 10−4 cm−3, with a slope of β = 0.56. We investigate a range of β values and find that the details of the density profile do not change our results significantly. The mass of the warm–hot gas, assuming a metallicity of Z ⊙, is (16 ± 3) × 1010 M ⊙, which is the most massive baryon component of NGC 3221. The baryon fraction is f b = 0.120 ± 0.036 (statistical) (systematic), consistent with the cosmological mean value, closing the baryon budget of this galaxy. We also investigated the missing metals problem in conjunction with the missing baryons problem and conclude that metals are likely to be preferentially expelled from the galaxy. Ours is the first detection of an extended warm–hot CGM around an external L ⋆ star-forming spiral galaxy, where the CGM likely accounts for the missing galactic baryons.

The missing metals problem - III. How many metals are expelled from galaxies?

We revisit the metal budget at z ≃ 2, and include the contribution of the intergalactic medium (IGM). Past estimates of the metal budget have indicated that, at redshift z ≃ 2.5, 90 per cent of the expected metals were missing. In the first two papers of this series, we have already shown that ∼30 per cent of the metals are observed in all z ∼ 2.5 galaxies detected in current surveys. This fraction could increase to ≤60 per cent if we extrapolate the faint end of the luminosity function (LF), leaving >40 per cent of the metals missing. Here, we extend our analysis to the metals outside galaxies (i.e. in the IGM), using (i) observational data and (ii) analytical calculations. Our results for both are strikingly similar. (i) Observationally, we find that, besides the small (5 per cent) contribution of damped Lya absorbers (DLAs), the forest and sub-DLAs contribute substantially to make ≤30-45 per cent of the metal budget; however, neither of these appear to be sufficient to close the metal budget. The forest accounts for 15-30 per cent depending on the ultraviolet background, and sub-DLAs for ≥2 to ≤17 per cent depending on the ionization fraction. Combining the metals in galaxies and in the IGM, it appears now that >65 per cent of the metals have been accounted for, and the 'missing metals problem' is substantially eased. (ii) We perform analytical calculations based on the effective yield-mass (y eff -V c ) relation, whose deficit for small galaxies is considered as evidence for supernova-driven outflows. As a test of the method, we show that, at z = 0, the calculation self-consistently predicts the total amount of metals expelled from galaxies. At z = 2, we find that the method predicts that 25-50 per cent of the metals have been ejected from galaxies into the IGM, consistent with the observations (≥35 per cent). The metal ejection is predominantly by L B < (1/3) L * B (z = 2) galaxies, which are responsible for 90 per cent of the metal enrichment, while the 50 percentile is at L ∼ (1/10) L * B (z = 2). As a consequence, if indeed 50 per cent of the metals have been ejected from galaxies, three to five bursts of star formation are required per galaxy prior to z = 2. The ratio between the mass of metals outside galaxies to those in stars has changed from z = 2 to z = 0; it was 1: 2 or 1: 1 and is now 1: 8 or 1: 9. This evolution implies that a significant fraction of the IGM metals will cool and fall back into galaxies.

The enrichment history of baryons in the Universe

We present predictions for the cosmic metal budget in various phases of baryons from redshift z = 6 → 0, taken from a cosmological hydrodynamic simulation that includes a well-constrained model for enriched galactic outflows. We find that substantial amounts of metals are found in every baryonic phase at all epochs, with diffuse intergalactic gas dominating the metal budget at early epochs and stars and halo gas dominating at recent epochs. We provide a full accounting of metals in the context of the missing metals problem at z ≈ 2.5, showing that ∼40 per cent of the metals are in galaxies, and the remainder are divided between diffuse intergalactic medium (IGM) gas and shocked gas in haloes and filamentary structures. Comparisons with available observations of metallicity and metal mass fraction evolution show broad agreement. We predict that stars have a mean metallicity of one-tenth solar already at z = 6, which increases slowly to one-half solar today, while stars just forming today typically have solar metallicity. Our H 1 column density weighted mean metallicity (comparable to damped Lyman a system metallicities) slowly increases from one-tenth to one-third solar from z = 6 → 1, and then falls to one-quarter solar at z = 0. The global mean metallicity of the universe tracks ∼50 per cent higher than that of the diffuse phase down to z ∼ 1, and by z = 0 it has a value around one-tenth solar. Metals move towards higher densities and temperatures with time, peaking around the mean cosmic density at z = 2 and an overdensity of 100 at z = 0. We study how carbon and oxygen ions trace the path of metals in phase space, and show that O III -O VII lines provide the most-practical option for constraining IGM metals at z ≤ 2.

The most metal-rich intervening quasar absorber known

The metallicity in portions of high-redshift galaxies has been successfully measured thanks to the gas observed in absorption in the spectra of quasars, in the Damped Lyman-alpha systems (DLAs). Surprisingly, the global mean metallicity derived from DLAs is about 1/10th solar at 0<z<4 leading to the so-called ``missing-metals problem''. In this paper, we present high-resolution observations of a sub-DLA system at z_abs=0.716 with super-solar metallicity toward SDSS J1323-0021. This is the highest metallicity intervening quasar absorber currently known, and is only the second super-solar absorber known to date. We provide a detailed study of this unique object from VLT/UVES spectroscopy. We derive [Zn/H]=+0.61, [Fe/H]=-0.51, [Cr/H]=<-0.53, [Mn/H] = -0.37, and [Ti/H] = -0.61. Observations and photoionisation models using the CLOUDY software confirm that the gas in this sub-DLA is predominantly neutral and that the abundance pattern is probably significantly different from a Solar pattern. Fe/Zn and Ti/Zn vary among the main velocity components by factors of \~ 3 and ~ 35, respectively, indicating non-uniform dust depletion. Mn/Fe is super-solar in almost all components, and varies by a factor of ~ 3 among the dominant components. It would be interesting to observe more sub-DLA systems and determine whether they might contribute significantly toward the cosmic budget of metals.

The missing metals problem – II. How many metals are in z ⋍ 2.2 galaxies?

Abstract In the context of the ‘missing metals problem’, the contributions of the ultraviolet-selected z ⋍ 2.2‘BX’ galaxies and z ⋍ 2.5‘distant red galaxies’ (DRGs) have not been discussed previously. Here we show that: (i) DRGs make only a marginal contribution to the metal budget (∼5 per cent); (ii) BX galaxies contribute as much as 18 per cent to the metal budget; and (iii) the K-bright subsample (K &amp;lt; 20) of the BX sample (roughly equivalent to the ‘BzK’ selected samples) contributes roughly half of this 18 per cent, owing to both their larger stellar masses and higher metallicities, implying that the rare K-bright galaxies at z &amp;gt; 2 are a major source of metals in the budget. We showed in the first paper of this series that submillimetre galaxies (SMGs) brighter than 3 mJy contribute ∼5 per cent (≲9 per cent as an upper limit) to the metal budget. Adding the contribution of SMGs and damped Lyα absorbers to the contribution of ultraviolet-selected galaxies implies that at least 30 per cent of the metals (in galaxies) have been accounted for at z ⋍ 2. The cosmic metal density thus accounted for is ρZ,galaxies ⋍ 1.3 × 106 M⊙ Mpc-3 or, in terms of the closure density, ΩZ= 9.6 × 10−6. This is a lower limit given that galaxies on the faint end of the luminosity function are not included. An estimate of the distribution of metals in local galaxies as a function of luminosity suggests that galaxies with luminosity &amp;lt;L⋆ contribute about half of the total mass of metals. If the metals in galaxies at z ∼ 2 are similarly distributed then faint galaxies alone cannot solve the ‘missing metals problem’. Galaxy populations at z ∼ 2 account for only about 50 per cent of the total metals predicted.

The Age-Metallicity Relation of the Universe in Neutral Gas: The First 100 Damped Ly Systems

We discuss accurate metallicity measurements for 125 damped Lyα (DLA) systems at 0.5 < z < 5, including ≈50 new measurements from our recently published Echellette Spectrograph and Imager surveys. This data set is analyzed to determine the age-metallicity relation of neutral gas in the universe. Contrary to previous analyses, this sample shows statistically significant evolution in the cosmic mean metallicity. The best linear fit for metallicity evolution is -0.26 ± 0.07 dex per unit redshift. The DLA systems continue to maintain a floor in metallicity of ≈1/700 solar, independent of observational effects. This metallicity threshold limits the prevalence of primordial gas in high-redshift galaxies and stresses the correspondence between damped systems and star formation. Finally, we comment on an apparent missing metals problem: the mean metallicity of the damped systems is ≈10 times lower than the value expected from their inferred star formation history. This problem is evident in current theoretical treatments of galaxy formation and may indicate a serious flaw in our understanding of the interplay between star formation and metal enrichment.

Cii* Absorption in Damped Lyα Systems. II. A New Window on the Star Formation History of the Universe

Starting from the star formation rate (SFR) per unit area, , determined for damped Lyα systems (DLAs) using the C II* method, we obtain the SFR per unit comoving volume, (z), at z ≈ 3. Pure warm neutral medium (WNM) solutions are ruled out since they generate more bolometric background radiation than observed, but the two-phase solutions dominated by the cold neutral medium (CNM) are consistent with the backgrounds. We find that (z) for DLAs agrees with the (z) for the Lyman break galaxies (LBGs). Although the mass of produced stars indicated by the SFRs is consistent with the current densities of known stellar populations, the mass of produced by z = 2.5 is 30 times larger than detected in absorption in DLAs. Of the three possible solutions to this missing metals problem, the most likely appears to be that star formation occurs in compact bulge regions. We search for evidence of feedback and find no correlations between and N(H I), but possible correlations between and low-ion velocity width and and metal abundance. We show that (1) the correlation between cooling rate and dust-to-gas ratio is positive evidence for grain photoelectric heating, (2) the cosmic microwave background (CMB) does not significantly populate the C II excited fine-structure states, and (3) the ratio of C II* to resonance-line optical depths is a sensitive probe of the multiphase structure of the DLA gas. We address recent arguments that DLAs are comprised only of WNM gas and show them to be inconclusive. Despite the rough agreement between (z) for DLAs and LBGs, current evidence indicates that these are distinct populations.