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Register a new userMetal-poor, Strongly Star-Forming Galaxies in the DEEP2 Survey: The Relationship between Stellar Mass, Temperature-based Metallicity, and Star Formation Rate
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Chun Ly
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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.
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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.
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$0.5 dex.
We study the relations between gas-phase metallicity ($Z$), local stellar mass surface density ($Sigma_*$), and the local star formation surface density ($Sigma_{rm SFR}$) in a sample of 1120 star-forming galaxies from the MaNGA survey. At fixed $Sigma_{*}$ the local metallicity increases as decreasing of $Sigma_{rm SFR}$ or vice versa for metallicity calibrators of N2 and O3N2. Alternatively, at fixed $Sigma_{rm SFR}$ metallicity increases as increasing of $Sigma_{*}$, but at high mass region, the trend is flatter. However, the dependence of metallicity on $Sigma_{rm SFR}$ is nearly disappeared for N2O2 and N2S2 calibrators. We investigate the local metallicity against $Sigma_{rm SFR}$ with different metallicity calibrators, and find negative/positive correlations depending on the choice of the calibrator. We demonstrate that the O32 ratio (or ionization parameter) is probably dependent on star formation rate at fixed local stellar mass surface density. Additional, the shape of $Sigma_*$ -- $Z$ -- $Sigma_{rm SFR}$ (FMR) depends on metallicity calibrator and stellar mass range. Since the large discrepancy between the empirical fitting-based (N2, O3N2) to electronic temperature metallicity and the photoionization model-dependent (N2O2, N2S2) metallicity calibrations, we conclude that the selection of metallicity calibration affects the existence of FMR on $Sigma_{rm SFR}$.
We constrain the mass distribution in nearby, star-forming galaxies with the Star Formation Reference Survey (SFRS), a galaxy sample constructed to be representative of all known combinations of star formation rate (SFR), dust temperature, and specific star formation rate (sSFR) that exist in the Local Universe. An innovative two-dimensional bulge/disk decomposition of the 2MASS/$K_{s}$-band images of the SFRS galaxies yields global luminosity and stellar mass functions, along with separate mass functions for their bulges and disks. These accurate mass functions cover the full range from dwarf galaxies to large spirals, and are representative of star-forming galaxies selected based on their infra-red luminosity, unbiased by AGN content and environment. We measure an integrated luminosity density $j$ = 1.72 $pm$ 0.93 $times$ 10$^{9}$ L$_{odot}$ $h^{-1}$ Mpc$^{-3}$ and a total stellar mass density $rho_{M}$ = 4.61 $pm$ 2.40 $times$ 10$^{8}$ M$_{odot}$ $h^{-1}$ Mpc$^{-3}$. While the stellar mass of the emph{average} star-forming galaxy is equally distributed between its sub-components, disks globally dominate the mass density budget by a ratio 4:1 with respect to bulges. In particular, our functions suggest that recent star formation happened primarily in massive systems, where they have yielded a disk stellar mass density larger than that of bulges by more than 1 dex. Our results constitute a reference benchmark for models addressing the assembly of stellar mass on the bulges and disks of local ($z = 0$) star-forming galaxies.
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I. Y. Izotova Astronomical Observatory ofn Taras Shevchenko National University of Kyiv
, Kyiv
2018
We use the data for the Hbeta emission-line, far-ultraviolet (FUV) and mid-infrared 22 micron continuum luminosities to estimate star formation rates <SFR> averaged over the galaxy lifetime for a sample of about 14000 bursting compact star-forming galaxies (CSFGs) selected from the Data Release 12 (DR12) of the Sloan Digital Sky Survey (SDSS). The average coefficient linking <SFR> and the star formation rate SFR_0 derived from the Hbeta luminosity at zero starburst age is found to be 0.04. We compare <SFR>s with some commonly used SFRs which are derived adopting a continuous star formation during a period of ~100 Myr, and find that the latter ones are 2-3 times higher. It is shown that the relations between SFRs derived using a geometric mean of two star-formation indicators in the UV and IR ranges and reduced to zero starburst age have considerably lower dispersion compared to those with single star-formation indicators. We suggest that our relations for <SFR> determination are more appropriate for CSFGs because they take into account a proper temporal evolution of their luminosities. On the other hand, we show that commonly used SFR relations can be applied for approximate estimation within a factor of ~2 of the <SFR> averaged over the lifetime of the bursting compact galaxy.
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