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Register a new userThe MOSDEF Survey: Differences in SFR and Metallicity for Morphologically-Selected Mergers at z~2
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We study the properties of 55 morphologically-identified merging galaxy systems at z~2. These systems are flagged as mergers based on features such as tidal tails, double nuclei, and asymmetry. Our sample is drawn from the MOSFIRE Deep Evolution Field (MOSDEF) survey, along with a control sample of isolated galaxies at the same redshift. We consider the relationships between stellar mass, star formation rate (SFR), and gas-phase metallicity for both merging and non-merging systems. In the local universe, merging systems are characterized by an elevated SFR and depressed metallicity compared to isolated systems at a given mass. Our results indicate SFR enhancement and metallicity deficit for merging systems relative to non-merging systems for a fixed stellar mass at z~2, though larger samples are required to establish these preliminary results with higher statistical significance. In future work, it will be important to establish if the enhanced SFR and depressed metallicity in high-redshift mergers deviate from the fundamental metallicity relation, as is observed in mergers in the local universe, and therefore shed light on gas flows during galaxy interactions.
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We investigate the nature of the relation among stellar mass, star-formation rate, and gas-phase metallicity (the M$_*$-SFR-Z relation) at high redshifts using a sample of 260 star-forming galaxies at $zsim2.3$ from the MOSDEF survey. We present an analysis of the high-redshift M$_*$-SFR-Z relation based on several emission-line ratios for the first time. We show that a M$_*$-SFR-Z relation clearly exists at $zsim2.3$. The strength of this relation is similar to predictions from cosmological hydrodynamical simulations. By performing a direct comparison of stacks of $zsim0$ and $zsim2.3$ galaxies, we find that $zsim2.3$ galaxies have $sim0.1$ dex lower metallicity at fixed M$_*$ and SFR. In the context of chemical evolution models, this evolution of the M$_*$-SFR-Z relation suggests an increase with redshift of the mass-loading factor at fixed M$_*$, as well as a decrease in the metallicity of infalling gas that is likely due to a lower importance of gas recycling relative to accretion from the intergalactic medium at high redshifts. Performing this analysis simultaneously with multiple metallicity-sensitive line ratios allows us to rule out the evolution in physical conditions (e.g., N/O ratio, ionization parameter, and hardness of the ionizing spectrum) at fixed metallicity as the source of the observed trends with redshift and with SFR at fixed M$_*$ at $zsim2.3$. While this study highlights the promise of performing high-order tests of chemical evolution models at high redshifts, detailed quantitative comparisons ultimately await a full understanding of the evolution of metallicity calibrations with redshift.
We perform an aperture-matched analysis of dust-corrected H$alpha$ and UV SFRs using 303 star-forming galaxies with spectroscopic redshifts $1.36<z_text{spec}<2.66$ from the MOSFIRE Deep Evolution Field (MOSDEF) survey. By combining H$alpha$ and H$beta$ emission line measurements with multi-waveband resolved CANDELS/3D-HST imaging, we directly compare dust-corrected H$alpha$ and UV SFRs, inferred assuming a fixed attenuation curve shape and constant SFHs, within the spectroscopic aperture. Previous studies have found that H$alpha$ and UV SFRs inferred with these assumptions generally agree for typical star-forming galaxies, but become increasingly discrepant for galaxies with higher SFRs ($gtrsim$100 M$_odot$ yr$^{-1}$), with H$alpha$-to-UV SFR ratios being larger for these galaxies. Our analysis shows that this trend persists even after carefully accounting for the apertures over which H$alpha$ and UV-based SFRs (and the nebular and stellar continuum reddening) are derived. Furthermore, our results imply that H$alpha$ SFRs may be higher in the centers of large galaxies (i.e., where there is coverage by the spectroscopic aperture) compared to their outskirts, which could be indicative of inside-out galaxy growth. Overall, we suggest that the persistent difference between nebular and stellar continuum reddening and high H$alpha$-to-UV SFR ratios at the centers of large galaxies may be indicative of a patchier distribution of dust in galaxies with high SFRs.
We analyze the rest-optical emission-line ratios of z~1.5 galaxies drawn from the MOSFIRE Deep Evolution Field (MOSDEF) survey. Using composite spectra we investigate the mass-metallicity relation (MZR) at z~1.5 and measure its evolution to z=0. When using gas-phase metallicities based on the N2 line ratio, we find that the MZR evolution from z~1.5 to z=0 depends on stellar mass, evolving by $Deltarm log(rm O/H)sim0.25$ dex at $M_*<10^{9.75}M_{odot}$ down to $Deltarm log(rm O/H)sim0.05$ at $M_*>10^{10.5}M_{odot}$. In contrast, the O3N2-based MZR shows a constant offset of $Deltarm log(rm O/H)sim0.30$ across all masses, consistent with previous MOSDEF results based on independent metallicity indicators, and suggesting that O3N2 provides a more robust metallicity calibration for our z~1.5 sample. We investigated the secondary dependence of the MZR on SFR by measuring correlated scatter about the mean $M_*$-specific SFR and $M_*-log(rm O3N2)$ relations. We find an anti-correlation between $log(rm O/H)$ and sSFR offsets, indicating the presence of a $M_*$-SFR-Z relation, though with limited significance. Additionally, we find that our z~1.5 stacks lie along the z=0 metallicity sequence at fixed $mu=log(M_*/M_{odot})-0.6timeslog(rm SFR / M_{odot} yr^{-1})$ suggesting that the z~1.5 stacks can be described by the z=0 fundamental metallicity relation (FMR). However, using different calibrations can shift the calculated metallicities off of the local FMR, indicating that appropriate calibrations are essential for understanding metallicity evolution with redshift. Finally, understanding how [NII]/H$alpha$ scales with galaxy properties is crucial to accurately describe the effects of blended [NII] and H$alpha$ on redshift and H$alpha$ flux measurements in future large surveys utilizing low-resolution spectra such as with Euclid and the Roman Space Telescope.
We present new accurate measurements of the physical properties of a statistically significant sample of 103 galaxies at z~2 using near-infrared spectroscopy taken as part of the 3D-HST survey. We derive redshifts, metallicities and star formation rates (SFRs) from the [OII], [OIII] and Hbeta nebular emission lines and exploit the multi-wavelength photometry available in CANDELS to measure stellar masses. We find the mass-metallicity relation (MZR) derived from our data to have the same trend as previous determinations in the range 0<z<3, with lower mass galaxies having lower metallicities. However we find an offset in the relation compared to the previous determination of the z~2 MZR by Erb et al. 2006b, who measure metallicities using the [NII]/Halpha ratio, with metallicities lower at a given mass. Incorporating our SFR information we find that our galaxies are offset from the Fundamental Metallicity Relation (FMR) by ~0.3 dex. We investigate the photoionization conditions and find that our galaxies are consistent with the elevated ionization parameter previously reported in high-redshift galaxies. Using the BPT diagram we argue that, if this is the case, metallicity indicators based on [NII] and Halpha may not be consistent with the ones obtained via oxygen lines and Hbeta. Using a recent determination of the theoretical evolution of the star forming sequence in the BPT diagram we convert our measured [OIII]/Hbeta line ratios to [NII]/Halpha ratios. From the [NII]/Halpha ratio we infer systematically higher metallicities in better agreement with the FMR. Our results thus suggest the evolution of the FMR previously reported at z~2-3 may be an artifact of the differential evolution in metallicity indicators, and caution against using locally calibrated metallicity relations at high redshift which do not account for evolution in the physical conditions of star-forming regions.
Using the near-IR spectroscopy of the MOSFIRE Deep Evolution Field (MOSDEF) survey, we investigate the role of local environment in the gas-phase metallicity of galaxies. The local environment measurements are derived from accurate and uniformly calculated photometric redshifts with well-calibrated probability distributions. Based on rest-frame optical emission lines, [NII]$lambda6584$ and H$alpha$, we measure gas-phase oxygen abundance of 167 galaxies at $1.37leq zleq1.7$ and 303 galaxies at $2.09leq zleq2.61$, located in diverse environments. We find that at $zsim1.5$, the average metallicity of galaxies in overdensities with $M_*sim10^{9.8}M_odot, 10^{10.2}M_odot$ and $10^{10.8}M_odot$ is higher relative to their field counterparts by $0.094pm0.051$, $0.068pm0.028$ and $0.052pm0.043$ dex, respectively. However, this metallicity enhancement does not exist at higher redshift, $zsim2.3$, where, compared to the field galaxies, we find $0.056pm0.043$, $0.056pm0.028$ and $0.096pm 0.034$ dex lower metallicity for galaxies in overdense environments with $M_*sim10^{9.8}M_odot, 10^{10.2}M_odot$ and $10^{10.7}M_odot$, respectively. Our results suggest that, at $1.37leq zleq2.61$, the variation of mass-metallicity relation with local environment is small ($<0.1$dex), and reverses at $zsim2$. Our results support the hypothesis that, at the early stages of cluster formation, owing to efficient gas cooling, galaxies residing in overdensities host a higher fraction of pristine gas with prominent primordial gas accretion, which lowers their gas-phase metallicity compared to their coeval field galaxies. However, as the Universe evolves to lower redshifts ($zlesssim2$), shock-heated gas in overdensities cannot cool down efficiently, and galaxies become metal-rich rapidly due to the suppression of pristine gas inflow and re-accretion of metal-enriched outflows in overdensities.
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