Showing papers by "S. L. Lu published in 2020"

Redshift evolution of the Fundamental Plane relation in the IllustrisTNG simulation

Abstract: We investigate the fundamental plane (FP) evolution of early-type galaxies in the IllustrisTNG-100 simulation (TNG100) from redshift $z=0$ to $z=2$. We find that a tight plane relation already exists as early as $z=2$. Its scatter stays as low as $\sim 0.08$ dex across this redshift range. Both slope parameters $b$ and $c$ (where $R \propto \sigma^b I^c$ with $R$, $\sigma$, and $I$ being the typical size, velocity dispersion, and surface brightness) of the plane evolve mildly since $z=2$, roughly consistent with observations. The FP residual $\rm Res$ ($\equiv\,a\,+\,b\log \sigma\,+\,c\log I\,-\,\log R$, where $a$ is the zero point of the FP) is found to strongly correlate with stellar age, indicating that stellar age can be used as a crucial fourth parameter of the FP. However, we find that $4c+b+2=\delta$, where $\delta \sim 0.8$ for FPs in TNG, rather than zero as is typically inferred from observations. This implies that a tight power-law relation between the dynamical mass-to-light ratio $M_{\rm dyn}/L$ and the dynamical mass $M_{\rm dyn}$ (where $M_{\rm dyn}\equiv 5\sigma^2R/G$, with $G$ being the gravitational constant) is not present in the TNG100 simulation. Recovering such a relation requires proper mixing between dark matter and baryons, as well as star formation occurring with correct efficiencies at the right mass scales. This represents a powerful constraint on the numerical models, which has to be satisfied in future hydrodynamical simulations.

Star Formation in Massive Galaxies at Redshift z ∼ 0.5

Abstract: It is believed that massive galaxies have quenched their star formation because of active galactic nucleus feedback. However, recent studies have shown that some massive galaxies are still forming stars. We analyze the morphology of star formation regions for galaxies of stellar mass larger than 10$^{11.3}$ M$_{\\odot}$ at around redshift $z_r=0.5$ using $u-z$ color images. We find that about $20\\%$ of the massive galaxies are star-forming (SF) galaxies, and most of them ($\\sim 85\\%$) have asymmetric structures induced by recent mergers. Moreover, for these asymmetric galaxies, we find that the asymmetry of the SF regions becomes larger for bluer galaxies. Using the Illustris simulation, we can qualitatively reproduce the observed relation between asymmetry parameter and color. Furthermore, using the merger trees in the simulation, we find a correlation between the color of the main branch galaxies at $z_r=0.5$ and the sum of the Star Formation Rates (SFRs) of the recently accreted galaxies, which implies that star formation of the accreted galaxies has contributed to the observed star formation of the massive (host) galaxies (ex situ star formation). Furthermore, we find two blue and symmetric galaxies, candidates for massive blue disks, in our observed sample, which indicates that about $\\sim 10\\%$ of massive SF galaxies are forming stars in the normal mode of disk star formation (in situ star formation). With the simulation, we find that the disk galaxies at $z_r \\approx 0.5$ should have experienced few major mergers during the last 4.3 Gyrs.

Star Formation in Massive Galaxies at Redshift $z \sim 0.5$

Abstract: It is believed that massive galaxies have quenched their star formation because of active galactic nucleus feedback. However, recent studies have shown that some massive galaxies are still forming stars. We analyze the morphology of star formation regions for galaxies of stellar mass larger than 10$^{11.3}$ M$_{\odot}$ at around redshift $z_r=0.5$ using $u-z$ color images. We find that about $20\%$ of the massive galaxies are star-forming (SF) galaxies, and most of them ($\sim 85\%$) have asymmetric structures induced by recent mergers. Moreover, for these asymmetric galaxies, we find that the asymmetry of the SF regions becomes larger for bluer galaxies. Using the Illustris simulation, we can qualitatively reproduce the observed relation between asymmetry parameter and color. Furthermore, using the merger trees in the simulation, we find a correlation between the color of the main branch galaxies at $z_r=0.5$ and the sum of the Star Formation Rates (SFRs) of the recently accreted galaxies, which implies that star formation of the accreted galaxies has contributed to the observed star formation of the massive (host) galaxies (ex situ star formation). Furthermore, we find two blue and symmetric galaxies, candidates for massive blue disks, in our observed sample, which indicates that about $\sim 10\%$ of massive SF galaxies are forming stars in the normal mode of disk star formation (in situ star formation). With the simulation, we find that the disk galaxies at $z_r \approx 0.5$ should have experienced few major mergers during the last 4.3 Gyrs.

SDSS-IV MaNGA: stellar population correlates with stellar root-mean-square velocity V rms gradients or total-density-profile slopes at fixed effective velocity dispersion σ e

Abstract: Galaxy properties are known to correlate most tightly with the galaxy effective stellar velocity dispersion $\sigma_{\rm e}$. Here we look for {\em additional} trends at fixed $\sigma_{\rm e}$ using 1339 galaxies ($M_\ast \gtrsim 6\times10^9$ M$_\odot$) with different morphologies in the MaNGA (DR14) sample with integral-field spectroscopy data. We focus on the gradients ($\gamma_{\rm rms} \equiv \sigma(R_{\rm e}/4)/\sigma_{\rm e}$) of the stellar root-mean-square velocity ($V_{\rm rms} \equiv \sqrt{V^2 + \sigma^2}$), which we show traces the total mass density gradient $\gamma_{\rm tot}$ derived from dynamical models and, more weakly, the bulge fraction. We confirm that $\gamma_{\rm rms}$ increases with $\sigma_{\rm e}$, age and metallicity. We additionally find that these correlations still exist at fixed $\sigma_{\rm e}$, where galaxies with larger $\gamma_{\rm rms}$ are found to be older and more metal-rich. It means that mass density gradients contain information of the stellar population which is not fully accounted for by $\sigma_{\rm e}$. This result puts an extra constraint on our understanding of galaxy quenching. We compare our results with galaxies in the IllustrisTNG hydrodynamical simulations and find that, at fixed $\sigma_{\rm e}$, similar trends exist with age, the bulge fraction, and the total mass density slope but, unlike observations, no correlation with metallicity can be detected in the simulations.