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Register a new userThe stellar mass versus stellar metallicity relation of star-forming galaxies at $1.6le zle3.0$ and implications for the evolution of the $alpha$-enhancement
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We measure the relationship between stellar mass and stellar metallicity, the stellar mass--metallicity relation (MZR), for 1336 star-forming galaxies at $1.6le zle3.0$ (<z>=2.2) using rest-frame far-ultraviolet spectra from the zCOSMOS-deep survey. High signal-to-noise composite spectra containing stellar absorption features are fit with population synthesis model spectra of a range of metallicity. We find stellar metallicities, which mostly reflect iron abundances, scaling as $(Z_{Fe,ast}/Z_{Fe,odot})=-(0.81pm0.01)+(0.32+0.03)log(M_ast/10^{10}M_odot)$ across the mass range of $10^9lesssim M_ast/M_odotlesssim10^{11}$, being $approx6times$ lower than seen locally at the same masses. The instantaneous oxygen-to-iron ratio ($alpha$-enhancement) inferred using the gas-phase oxygen MZRs, is on average found to be [O/Fe]$approx0.47$, being higher than the local [O/Fe]$approx0$. The observed changes in [O/Fe] and [Fe/H] are reproduced in simple flow-through gas-regulator models with steady star-formation histories (SFHs) that follow the evolving main sequence. Our models show that the [O/Fe] is determined almost entirely by the instantaneous specific star formation rate alone while being independent of the SFHs, mass, and the gas-regulation characteristics of the systems. We find that the locations of $sim10^{10}M_odot$ galaxies at z~2 in the [O/Fe]--metallicity planes are in remarkable agreement with the sequence of low-metallicity thick-disk stars in our Galaxy. This manifests a beautiful concordance between the results of Galactic archaeology and observations of high-redshift Milky Way progenitors. However, there remains a question of how and when the old metal-rich, low-$alpha$/Fe stars seen in the bulge had formed by z~2 because such a stellar population is not seen in our data and difficult to explain in the context of our models.
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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, $M_{star}$ $approx10^{8.1}$-$10^{9.5}$ $M_{odot}$. Combined with the Balmer-derived star formation rates (SFRs), these galaxies have average inverse SFR/$M_{star}$ of $approx$100 Myr. The evolution of stellar mass-gas metallicity relation to $zapprox0.8$ is measured by combining the modelled masses with previously obtained spectroscopic measurements of metallicity from [O III] $lambda$4363 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 $zapprox0.8$ mass-metallicity relation, a steep rise in metallicity at low stellar masses, transitioning to a plateau at higher masses, is consistent with $zsim0.1$ studies. We also compare the evolution in metallicity between $zapprox0.8$ and $zsim0.1$ against recent strong-line diagnostic studies at intermediate redshifts and find good agreement. Specifically, we find that lower mass galaxies ($4times10^8$ $M_{odot}$) built up their metal content 1.6 times more rapidly than higher mass galaxies ($10^{10}$ $M_{odot}$). 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 $zapprox0.8$ low-mass galaxies.
The stellar mass-stellar metallicity relation (MZR) is an essential approach to probe the chemical evolution of galaxies. It reflects the balance between galactic feedback and gravitational potential as a function of stellar mass. However, the current MZR of local dwarf satellite galaxies (M* <~ 10^8 Msun, measured from resolved stellar spectroscopy) may not be reconcilable with that of more massive galaxies (M* >~ 10^9.5 Msun, measured from integrated-light spectroscopy). Such a discrepancy may result from a systematic difference between the two methods, or it may indicate a break in the MZR around 10^9 Msun. To address this question, we measured the stellar metallicity of NGC 147 from integrated light using the Palomar Cosmic Web Imager (PCWI). We compared the stellar metallicity estimates from integrated light with the measurements from resolved stellar spectroscopy and found them to be consistent within 0.1 dex. On the other hand, the high-mass MZR overpredicts the metallicity by 0.6 dex at the mass of NGC 147. Therefore, our results tentatively suggest that the discrepancy between the low-mass MZR and high-mass MZR should not be attributed to a systematic difference in techniques. Instead, real physical processes cause the transition in the MZR. In addition, we discovered a positive age gradient in the innermost region and a negative metallicity gradient from the resolved stars at larger radii, suggesting a possible outside-in formation of NGC 147.
We quantify evolution in the cluster scale stellar mass - halo mass (SMHM) relations parameters using 2323 clusters and brightest central galaxies (BCGs) over the redshift range $0.03 le z le 0.60$. The precision on inferred SMHM parameters is improved by including the magnitude gap ($rm m_{gap}$) between the BCG and fourth brightest cluster member (M14) as a third parameter in the SMHM relation. At fixed halo mass, accounting for $rm m_{gap}$, through a stretch parameter, reduces the SMHM relations intrinsic scatter. To explore this redshift range, we use clusters, BCGs, and cluster members identified using the Sloan Digital Sky Survey C4 and redMaPPer cluster catalogs and the Dark Energy Survey redMaPPer catalog. Through this joint analysis, we detect no systematic differences in BCG stellar mass, $rm m_{gap}$, and cluster mass (inferred from richness) between the datsets. We utilize the Pareto function to quantify each parameters evolution. We confirm prior findings of negative evolution in the SMHM relations slope (3.5$sigma$) and detect negative evolution in the stretch parameter (4.0$sigma$) and positive evolution in the offset parameter (5.8$sigma$). This observed evolution, combined with the absence of BCG growth, when stellar mass is measured within 50kpc, suggests that this evolution results from changes in the clusters $rm m_{gap}$. For this to occur, late-term growth must be in the intra-cluster light surrounding the BCG. We also compare the observed results to Illustris TNG 300-1 cosmological hydrodynamic simulations and find modest qualitative agreement. However, the simulations lack the evolutionary features detected in the real data.
We present an analysis of the predictions made by the Galform semi-analytic galaxy formation model for the evolution of the relationship between stellar mass and halo mass. We show that for the standard implementations of supernova feedback and gas reincorporation used in semi-analytic models, this relationship is predicted to evolve weakly over the redshift range 0<z<4. Modest evolution in the median stellar mass versus halo mass (SHM) relationship implicitly requires that, at fixed halo mass, the efficiency of stellar mass assembly must be almost constant with cosmic time. We show that in our model, this behaviour can be understood in simple terms as a result of a constant efficiency of gas reincorporation, and an efficiency of SNe feedback that is, on average, constant at fixed halo mass. We present a simple explanation of how feedback from active galactic nuclei (AGN) acts in our model to introduce a break in the SHM relation whose location is predicted to evolve only modestly. Finally, we show that if modifications are introduced into the model such that, for example, the gas reincorporation efficiency is no longer constant, the median SHM relation is predicted to evolve significantly over 0<z<4. Specifically, we consider modifications that allow the model to better reproduce either the evolution of the stellar mass function or the evolution of average star formation rates inferred from observations.
We measure the gas-phase oxygen abundances of ~3000 star-forming galaxies at z=0.05-0.75 using optical spectrophotometry from the AGN and Galaxy Evolution Survey (AGES), a spectroscopic survey of I_AB<20.45 galaxies over 7.9 deg^2 in the NOAO Deep Wide Field Survey (NDWFS) Bootes field. We use state-of-the-art techniques to measure the nebular emission lines and stellar masses, and explore and quantify several potential sources of systematic error, including the choice of metallicity diagnostic, aperture bias, and contamination from unidentified active galactic nuclei (AGN). Combining volume-limited AGES samples in six independent redshift bins and ~75,000 star-forming galaxies with r_AB<17.6 at z=0.05-0.2 selected from the Sloan Digital Sky Survey (SDSS) that we analyze in the identical manner, we measure the evolution of the stellar mass-metallicity (M-Z) between z=0.05 and z=0.75. We find that at fixed stellar mass galaxies at z~0.7 have just 30%-60% the metal content of galaxies at the present epoch, where the uncertainty is dominated by the strong-line method used to measure the metallicity. Moreover, we find no statistically significant evidence that the M-Z relation evolves in a mass-dependent way for M=10^9.8-10^11 Msun star-forming galaxies. Thus, for this range of redshifts and stellar masses the M-Z relation simply shifts toward lower metallicity with increasing redshift without changing its shape.
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