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Register a new userThe Role of Environment in the Mass-Metallicity Relation
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Michael C. Cooper
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Using a sample of 57,377 star-forming galaxies drawn from the Sloan Digital Sky Survey, we study the relationship between gas-phase oxygen abundance and environment in the local Universe. We find that there is a strong relationship between metallicity and environment such that more metal-rich galaxies favor regions of higher overdensity. Furthermore, this metallicity-density relation is comparable in strength to the color-density relation along the blue cloud. After removing the mean dependence of environment on color and luminosity, we find a significant residual trend between metallicity and environment that is largely driven by galaxies in high-density regions, such as groups and clusters. We discuss the potential source of this relationship between metallicity and local galaxy density in the context of feedback models, with special attention paid to quantifying the impact of environment on the scatter in the mass-metallicity relation. We find that environment is a non-negligible source of scatter in this fundamental relation, with > 15% of the measured scatter correlated with environment.
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We examine the relation between gas-phase oxygen abundance and stellar mass---the MZ relation---as a function of the large scale galaxy environment parameterized by the local density. The dependence of the MZ relation on the environment is small. The metallicity where the MZ relation saturates and the slope of the MZ relation are both independent of the local density. The impact of the large scale environment is completely parameterized by the anti-correlation between local density and the turnover stellar mass where the MZ relation begins to saturate. Analytical modeling suggests that the anti-correlation between the local density and turnover stellar mass is a consequence of a variation in the gas content of star-forming galaxies. Across $sim1$ order of magnitude in local density, the gas content at a fixed stellar mass varies by $sim5%$. Variation of the specific star formation rate with environment is consistent with this interpretation. At a fixed stellar mass, galaxies in low density environments have lower metallicities because they are slightly more gas-rich than galaxies in high density environments. Modeling the shape of the mass-metallicity relation thus provides an indirect means to probe subtle variations in the gas content of star-forming galaxies.
Active galactic nuclei (AGNs) are characterized by a clear correlation between luminosity and metallicity (L_AGN-Z_AGN relation). The origin of this correlation is not clear. It may result from a relation between the black hole mass (M_BH) and metallicity, or from a relation between the accretion rate (L/L_Edd) and metallicity. To investigate the origin of the L_AGN-Z_AGN relation, we use optical spectra of 2383 quasars at 2.3 < z < 3.0 from the Sloan Digital Sky Survey. By using this data set, we have constructed composite spectra of 33 subsamples in intervals of both M_BH and L/L_Edd. From these composite spectra we measure emission-line flux ratios that are sensitive to the metallicity of the broad line region (BLR); specifically, NV/CIV, NV/HeII, (SiIV+OIV])/CIV, and AlIII/CIV. We find that there is a significant correlation between M_BH and Z_BLR as inferred from all four metallicity-sensitive emission-line flux ratios. This result strongly suggests that the observed L_AGN-Z_AGN relation is mostly a consequence of the M_BH-Z_AGN relation. The relation between M_BH and Z_BLR is likely a consequence of both the M_BH-M_bul relation and of the mass-metallicity relation in the host galaxy. We also find that L/L_Edd correlates with the emission line flux ratios involving NV (more specifically, NV/CIV and NV/HeII), while it does not correlate with the other two metallicity sensitive emission line flux ratios, i.e., (SiIV+OIV])/CIV and AlIII/CIV. These correlations indicate that the emission-line flux ratios involving NV depend on both metallicity and relative abundance of nitrogen. We suggest that the relation between L/L_Edd and those line ratios involving nitrogen, is caused by a delay of the black hole accretion rate relative to the onset of nuclear star formation of about 10^8 years, which is the timescale required for the nitrogen enrichment.
We study the mass-metallicity relation of galaxies in pairs and in isolation taken from the SDSS-DR4 using the stellar masses and oxygen abundances derived by Tremonti et al. (2004). Close galaxy pairs, defined by projected separation r_p < 25kpc/h and radial velocity Delta_V < 350 km/s, are morphologically classified according to the strength of the interaction signs. We find that only for pairs showing signs of strong interactions, the mass-metallicity relation differs significantly from that of galaxies in isolation. In such pairs, the mean gas-phase oxygen abundances of galaxies with low stellar masses (Mstar ~< 10^9 Msun/h) exhibit an excess of 0.2 dex. Conversely, at larger masses (Mstar >~ 10^10 Msun/h) galaxies have a systematically lower metallicity, although with a smaller difference (-0.05 dex). Similar trends are obtained if g-band magnitudes are used instead of stellar masses. In minor interactions, we find that the less massive member is systematically enriched, while a galaxy in interaction with a comparable stellar mass companion shows a metallicity decrement with respect to galaxies in isolation. We argue that metal-rich starbursts triggered by a more massive component, and inflows of low metallicity gas induced by comparable or less massive companion galaxies, provide a natural scenario to explain our findings.
We study the shape of the gas-phase mass-metallicity relation (MZR) of a combined sample of present-day dwarf and high-mass star-forming galaxies using IZI, a Bayesian formalism for measuring chemical abundances presented in Blanc et al. 2015. We observe a characteristic stellar mass scale at $M_* simeq 10^{9.5}$M$_{odot}$, above which the ISM undergoes a sharp increase in its level of chemical enrichment. In the $10^{6}-10^{9.5}$M$_{odot}$ range the MZR follows a shallow power-law ($Zpropto M^{alpha}_*$) with slope $alpha=0.14pm0.08$. At approaching $M_* simeq 10^{9.5}$M$_{odot}$ the MZR steepens significantly, showing a slope of $alpha=0.37pm0.08$ in the $10^{9.5}-10^{10.5}$M$_{odot}$ range, and a flattening towards a constant metallicity at higher stellar masses. This behavior is qualitatively different from results in the literature that show a single power-law MZR towards the low mass end. We thoroughly explore systematic uncertainties in our measurement, and show that the shape of the MZR is not induced by sample selection, aperture effects, a changing N/O abundance, the adopted methodology used to construct the MZR, secondary dependencies on star formation activity, nor diffuse ionized gas (DIG) contamination, but rather on differences in the method used to measure abundances. High resolution hydrodynamical simulations can qualitatively reproduce our result, and suggest a transition in the ability of galaxies to retain their metals for stellar masses above this threshold. The MZR characteristic mass scale also coincides with a transition in the scale height and clumpiness of cold gas disks, and a typical gas fraction below which the efficiency of star formation feedback for driving outflows is expected to decrease sharply.
Dwarf galaxies generally follow a mass-metallicity (MZ) relation, where more massive objects retain a larger fraction of heavy elements. Young tidal dwarf galaxies (TDGs), born in the tidal tails produced by interacting gas-rich galaxies, have been thought to not follow the MZ relation, because they inherit the metallicity of the more massive parent galaxies. We present chemical evolution models to investigate if TDGs that formed at very high redshifts, where the metallicity of their parent galaxy was very low, can produce the observed MZ relation. Assuming that galaxy interactions were more frequent in the denser high-redshift universe, TDGs could constitute an important contribution to the dwarf galaxy population. The survey of chemical evolution models of TDGs presented here captures for the first time an initial mass function (IMF) of stars that is dependent on both the star formation rate and the gas metallicity via the integrated galactic IMF (IGIMF) theory. As TDGs form in the tidal debris of interacting galaxies, the pre-enrichment of the gas, an underlying pre-existing stellar population, infall, and mass dependent outflows are considered. The models of young TDGs that are created in strongly pre-enriched tidal arms with a pre-existing stellar population can explain the measured abundance ratios of observed TDGs. The same chemical evolution models for TDGs, that form out of gas with initially very low metallicity, naturally build up the observed MZ relation. The modelled chemical composition of ancient TDGs is therefore consistent with the observed MZ relation of satellite galaxies.
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