The Properties of the Massive Star-forming Galaxies with an Outside-in Assembly Mode

Kong

Pan

2018

ApJ

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Previous findings show that the existence of dense cores or bulges is the prerequisite for quenching a galaxy, leading to a proposed two-step quenching scenario: compaction and quenching. In this scenario, galaxies first grow their cores to a stellar mass surface density threshold and subsequently quenching occurs, suggesting that galaxies evolve from extended star-forming galaxies (eS-FGs), through compact star-forming galaxies (cSFGs), to quenched population. In this work, we aim at examining the possible evolutionary link between eSFGs and cSFGs by identifying the trends in star formation rate (SFR), gas-phase metallicity and HI content, since one would naturally expect that galaxies evolve along the track of cold gas consumption and metal enhancement. We select a volumelimited sample of 15,933 galaxies with stellar mass above 10 9.5 M ⊙ and redshift of 0.02 < z < 0.05 from the NASA-Sloan-Atlas catalog within the ALFALFA footprint. The cSFGs on average exhibit similar or slightly higher SFRs of ∼0.06 dex and significantly higher gas-phase metallicity (up to 0.2 dex at low mass) with respect to the eSFGs, while the cSFGs dominate the galaxy population of the most intense star formation activities. More importantly, overall the median HI content and gas depletion time of cSFGs are about half of eSFGs. Our result supports the compaction and quenching scenario that galaxies evolve and grow their cores along the track of cold gas consumption and metal enhancement. The environments of eSFGs and cSFGs are indistinguishable, suggesting that the compaction process is independent of any environmental effects at least for low-redshift universe.5 This definition of compact SF galaxies differs from previous works, where compact is an absolute term to identify the smallest galaxies (e.g. van Dokkum et al. 2015). 6 http://www.nsatlas.org/ 7

Section: The Sfr and Gas-phase Metallicitymentioning

confidence: 97%

Connecting Compact Star-forming and Extended Star-forming Galaxies at Low Redshift: Implications for Galaxy Compaction and Quenching

Kong

Pan

2018

ApJ

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“…The three diagnostic parameters have long been used to indicate the recent SFHs on different timescales (e.g. Worthey & Ottaviani 1997;Balogh et al 1999;Kauffmann et al 2003;Li et al 2015;Wang et al 2017Wang et al , 2018. In SF galaxies, the Hα emission mainly comes from the recombination of gas ionized by photons from extremely massive stars (>15M ⊙ ), which is therefore expected to trace the SFHs within the lifetime of these massive stars (∼5 Myr).…”

Section: The Diagnostic Observational Parameters Of the Recent Sfhsmentioning

confidence: 99%

“…The Hα emission is a good tracer of the SFR averaged within the last 5 Myr, the EW(Hδ A ) traces the star formation within the last roughly ∼1 Gyr, and the 4000Å break is sensitive to the lightweighted stellar age within 2 Gyr (e.g. Balogh et al 1999;Kauffmann et al 2003;Li et al 2015;Wang et al 2017Wang et al , 2018. These three parameters can be directly measured from galaxy spectra, and each being measured at a single wavelength they are all, in principle, insensitive to dust attenuation, although dust effects can still enter if different components of the system suffer different extinctions (this is explored in Section 2.6 below).…”

Section: Introductionmentioning

confidence: 99%

The Variability of the Star Formation Rate in Galaxies. I. Star Formation Histories Traced by EW(Hα) and EW(Hδ_A)

Lilly

2020

ApJ

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To investigate the variability of the star formation rate (SFR) of galaxies, we define a star formation change parameter, SFR 5Myr /SFR 800Myr which is the ratio of the SFR averaged within the last 5 Myr to the SFR averaged within the last 800 Myr. We show that this parameter can be determined from a combination of Hα emission and Hδ absorption, plus the 4000Å break, with an uncertainty of ∼0.07 dex for star-forming galaxies. We then apply this estimator to MaNGA galaxies, both globally within R e and within radial annuli. We find that the global SFR 5Myr /SFR 800Myr , which indicates by how much a galaxy has changed its specific SFR (sSFR) is nearly independent of its sSFR, i.e. of its the position relative to the star formation main sequence (SFMS) as defined by SFR 800Myr . Also, at any sSFR, there are as many galaxies increasing their sSFR as decreasing it, as required if the dispersion in the SFMS is to stay the same. The SFR 5Myr /SFR 800Myr of the overall galaxy population is very close to that expected for the evolving Main Sequence. Both of these provide a reassuring check on the validity of our calibration of the estimator. We find that galaxies with higher global SFR 5Myr /SFR 800Myr appear to have higher SFR 5Myr /SFR 800Myr at all galactic radii, i.e. that galaxies with a recent temporal enhancement in overall SFR have enhanced star formation at all galactic radii. The dispersion of the SFR 5Myr /SFR 800Myr at a given relative galactic radius and a given stellar mass decreases with the (indirectly inferred) gas depletion time: locations with short gas depletion time appear to undergo bigger variations in their star-formation rates on Gyr or less timescales. In Wang et al. (2019) we showed that the dispersion in star-formation rate surface densities Σ SFR in the galaxy population appears to be inversely correlated with the inferred gas depletion timescale and interpreted this in terms of the dynamical response of a gas-regulator system to changes in the gas inflow rate. In this paper, we can now prove directly with SFR 5Myr /SFR 800Myr that these effects are indeed due to genuine temporal variations in the SFR of individual galaxies on timescales between 10 7 and 10 9 years rather than possibly reflecting intrinsic, non-temporal, differences between different galaxies.

“…Furthermore, we also find that the residuals for concentration index C and gravitational potential Φ are nearly zero, meaning that the local metallicity may be independent on the galaxy concentration and average gravitational potential. Recently, Wang et al (2017) have demonstrated that the local metallicity also depends on the assembly modes of galaxies. In the sense that higher Σ * (denser) regions, located in the inner regions of galaxies with an "outsidein" assembly mode, have higher metallicity and lower D n 4000 than those galaxies with an "inside-out" mode.…”

Section: 2mentioning

confidence: 99%

What Determines the Local Metallicity of Galaxies: Global Stellar Mass, Local Stellar Mass Surface Density, or Star Formation Rate?

Gao

Kong

et al. 2018

ApJ

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The metallicity and its relationship with other galactic properties is a fundamental probe of the evolution of galaxies. In this work, we select about 750,000 star-forming spatial pixels from 1122 blue galaxies in the MaNGA survey to investigate the global stellar mass -local stellar mass surface density -gas-phase metallicity (M * -Σ * -Z ) relation. At a fixed M * , the metallicity increases steeply with increasing Σ * . Similarly, at a fixed Σ * , the metallicity increases strongly with increasing M * at low mass end, while this trend becomes less obvious at high mass end. We find the metallicity to be more strongly correlated to Σ * than to M * . Furthermore, we construct a tight (0.07 dex scatter) M * -Σ * -Z relation, which reduces the scatter in the Σ * -Z relation by about 30% for galaxies with 7.8 < log(M * /M ) < 11.0, while the reduction of scatter is much weaker for high-mass galaxies. This result suggests that, especially for low-mass galaxies, the M * -Σ * -Z relation is largely more fundamental than the M * -Z and Σ * -Z relations, meaning that both M * and Σ * play important roles in shaping the local metallicity. We also find that the local metallicity is probably independent on the local star formation rate surface density at a fixed M * and Σ * . Our results are consistent with the scenario that the local metallicities in galaxies are shaped by the combination of the local stars formed in the history and the metal loss caused by galactic winds.