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Register a new userThe Relationship between Stellar Mass, Gas Metallicity, and Star Formation Rate for Halpha-selected Galaxies at z~0.8 from the NewHalpha Survey
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Using a sample of 299 Ha-selected galaxies at z~0.8, we study the relationship between galaxy stellar mass, gas-phase metallicity, and star formation rate (SFR), and compare to previous results. We use deep optical spectra obtained with the IMACS spectrograph at the Magellan telescope to measure strong oxygen lines. We combine these spectra and metallicities with (1) rest-frame UV-to-optical imaging, which allows us to determine stellar masses and dust attenuation corrections, and (2) Ha narrowband imaging, which provides a robust measure of the instantaneous SFR. Our sample spans stellar masses of 10^9 to 6*10^11 solar masses, SFRs of 0.4 to 270 solar masses per year, and metal abundances of 12+log(O/H)~8.3-9.1 (~0.4-2.6 solar metallicity). The correlations that we find between the Ha-based SFR and stellar mass (i.e., the star-forming main sequence), and between the stellar mass and metallicity, are both consistent with previous z~1 studies of star-forming galaxies. We then study the relationship between the three properties using various plane-fitting techniques (Lara-Lopez et al.) and a curve-fitting projection (Mannucci et al.). In all cases, we exclude strong dependence of the M-Z relation on SFR, but are unable to distinguish between moderate and no dependence. Our results are consistent with previous mass-metallicity-SFR studies. We check whether dataset limitations may obscure a strong dependence on the SFR by using mock samples drawn from the SDSS. These experiments reveal that the adopted signal-to-noise cuts may have a significant effect on the measured dependence. Further work is needed to investigate these results, and to test whether a fundamental metallicity relation or a fundamental plane describes star-forming galaxies across cosmic time.
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We present measurements of the dust attenuation of Halpha-selected emission-line galaxies at z=0.8 from the NewHalpha narrowband survey. The analysis is based on deep follow-up spectroscopy with Magellan/IMACS, which captures the strong rest-frame optical emission lines from [OII] lambda 3727 to [OIII] lambda 5007. The spectroscopic sample used in this analysis consists of 341 confirmed Halpha emitters. We place constraints on the AGN fraction using diagnostics which can be applied at intermediate redshift. We find that at least 5% of the objects in our spectroscopic sample can be classified as AGN and 2% are composite, i.e. powered by a combination of star-formation and AGN activity. We measure the dust attenuation for individual objects from the ratios of the higher order Balmer lines. The Hbeta and Hgamma pair of lines is detected with S/N>5 in 55 individual objects and the Hbeta and Hdelta pair is detected in 50 individual objects. We also create stacked spectra to probe the attenuation in objects without individual detections. The median attenuation at Halpha based on the objects with individually detected lines is A(Halpha)=0.9+-1.0 magnitudes, in good agreement with the attenuation found in local samples of star-forming galaxies. We find that the z=0.8 galaxies occupy a similar locus of attenuation as a function of magnitude, mass and SFR as a comparison sample drawn from the SDSS DR4. Both the results from the individual z=0.8 galaxies and from the stacked spectra show consistency with the mass -- attenuation and SFR -- attenuation relations found in the local Universe, indicating that these relations are also applicable at intermediate redshift.
We report on the discovery of 28 $zapprox0.8$ metal-poor galaxies in DEEP2. These galaxies were selected for their detection of the weak [OIII]$lambda$4363 emission line, which provides a direct measure of the gas-phase metallicity. A primary goal for identifying these rare galaxies is to examine whether the fundamental metallicity relation (FMR) between stellar mass, gas metallicity, and star formation rate (SFR) holds for low stellar mass and high SFR galaxies. The FMR suggests that higher SFR galaxies have lower metallicity (at fixed stellar mass). To test this trend, we combine spectroscopic measurements of metallicity and dust-corrected SFRs, with stellar mass estimates from modeling the optical photometry. We find that these galaxies are $1.05pm0.61$ dex above the z~1 stellar mass-SFR relation, and $0.23pm0.23$ dex below the local mass-metallicity relation. Relative to the FMR, the latter offset is reduced to 0.01 dex, but significant dispersion remains (0.29 dex with 0.16 dex due to measurement uncertainties). This dispersion suggests that gas accretion, star formation and chemical enrichment have not reached equilibrium in these galaxies. This is evident by their short stellar mass doubling timescale of $approx100^{+310}_{-75}$ Myr that suggests stochastic star formation. Combining our sample with other z~1 metal-poor galaxies, we find a weak positive SFR-metallicity dependence (at fixed stellar mass) that is significant at 94.4% confidence. We interpret this positive correlation as recent star formation that has enriched the gas, but has not had time to drive the metal-enriched gas out with feedback mechanisms.
We have observed a sample of typical z=1 star forming galaxies, selected from the HiZELS survey, with the new KMOS near-infrared, multi-IFU instrument on the VLT, in order to obtain their dynamics and metallicity gradients. The majority of our galaxies have a metallicity gradient consistent with being flat or negative (i.e. higher metallicity cores than outskirts). Intriguingly, we find a trend between metallicity gradient and specific star formation rate (sSFR), such that galaxies with a high sSFR tend to have relatively metal-poor centres, a result which is strengthened when combined with datasets from the literature. This result appears to explain the discrepancies reported between different high redshift studies and varying claims for evolution. From a galaxy evolution perspective, the trend we see would mean that a galaxys sSFR is governed by the amount of metal poor gas that can be funnelled into its core, triggered either by merging or through efficient accretion. In fact merging may play a significant role as it is the starburst galaxies at all epochs, which have the more positive metallicity gradients. Our results may help to explain the origin of the fundamental metallicity relation, in which galaxies at a fixed mass are observed to have lower metallicities at higher star formation rates, especially if the metallicity is measured in an aperture encompassing only the central regions of the galaxy. Finally, we note that this study demonstrates the power of KMOS as an efficient instrument for large scale resolved galaxy surveys.
Extragalactic studies have demonstrated there is a moderately tight ($approx$0.3 dex) relationship between galaxy stellar mass ($M_{star}$) and star formation rate (SFR) that holds for star-forming galaxies at $M_{star} sim 3 times 10^8$-10$^{11}~M_{odot}$, i.e., the star formation main sequence. However, it has yet to be determined whether such a relationship extends to even lower mass galaxies, particularly at intermediate or higher redshifts. We present new results using observations for 714 narrowband H$alpha$-selected galaxies with stellar masses between $10^6$ and $10^{10}~M_{odot}$ (average of $10^{8.2}~M_{odot}$) at $z approx$ 0.07-0.5. These galaxies have sensitive UV to near-infrared photometric measurements and optical spectroscopy. The latter allows us to correct our H$alpha$ SFRs for dust attenuation using Balmer decrements. Our study reveals: (1) for low-SFR galaxies, our H$alpha$ SFRs systematically underpredict compared to FUV measurements, consistent with other studies; (2) at a given stellar mass ($approx $10$ ^{8}~M_{odot}$), log(specific SFR) evolves as $ A log(1+z) $ with $ A = 5.26 pm 0.75 $, and on average, specific SFR increases with decreasing stellar mass; (3) the SFR-$M_{star}$ relation holds for galaxies down to $sim$10$^6~M_{odot}$ ($sim$1.5 dex below previous studies), and over lookback times of up to 5 Gyr, follows a redshift-dependent relation of $log{({rm SFR})} propto alpha log(M_{star}/M_{odot}) + beta z$ with $alpha = 0.60 pm 0.01$ and $beta = 1.86 pm 0.07$; and (4) the observed dispersion in the SFR-$M_{star}$ relation at low stellar masses is $approx$0.3 dex. Accounting for survey selection effects using simulated galaxies, we estimate the true dispersion is $approx$0.5 dex.
Using a sample of dwarf galaxies observed using the VIMOS IFU on the VLT, we investigate the mass-metallicity relation (MZR) as a function of star formation rate (FMR$_{text{SFR}}$) as well as HI-gas mass (FMR$_{text{HI}}$). We combine our IFU data with a subsample of galaxies from the ALFALFA HI survey crossmatched to the Sloan Digital Sky Survey to study the FMR$_{text{SFR}}$ and FMR$_{text{HI}}$ across the stellar mass range 10$^{6.6}$ to 10$^{8.8}$ M$_odot$, with metallicities as low as 12+log(O/H) = 7.67. We find the 1$sigma$ mean scatter in the MZR to be 0.05 dex. The 1$sigma$ mean scatter in the FMR$_{text{SFR}}$ (0.02 dex) is significantly lower than that of the MZR. The FMR$_{text{SFR}}$ is not consistent between the IFU observed galaxies and the ALFALFA/SDSS galaxies for SFRs lower than 10$^{-2.4}$ M$_odot$ yr$^{-1}$, however this could be the result of limitations of our measurements in that regime. The lowest mean scatter (0.01 dex) is found in the FMR$_{text{HI}}$. We also find that the FMR$_{text{HI}}$ is consistent between the IFU observed dwarf galaxies and the ALFALFA/SDSS crossmatched sample. We introduce the fundamental metallicity luminosity counterpart to the FMR, again characterized in terms of SFR (FML$_{text{SFR}}$) and HI-gas mass (FML$_{text{HI}}$). We find that the FML$_{text{HI}}$ relation is consistent between the IFU observed dwarf galaxy sample and the larger ALFALFA/SDSS sample. However the 1$sigma$ scatter for the FML$_{text{HI}}$ relation is not improved over the FMR$_{text{HI}}$ scenario. This leads us to conclude that the FMR$_{text{HI}}$ is the best candidate for a physically motivated fundamental metallicity relation.
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