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Register a new userThe Structural Evolution of Isolated Galaxies at Low Redshift in the IllustrisTNG Simulation
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We study the structural evolution of isolated star-forming galaxies in the Illustris TNG100-1 hydrodynamical simulation, with a focus on investigating the growth of the central core density within 2 kpc ($Sigma_{*,2kpc}$) in relation to total stellar mass ($M_*$) at z < 0.5. First, we show that several observational trends in the $Sigma_{*,2kpc}$-$M_*$ plane are qualitatively reproduced in IllustrisTNG, including the distributions of AGN, star forming galaxies, quiescent galaxies, and radial profiles of stellar age, sSFR, and metallicity. We find that galaxies with dense cores evolve parallel to the $Sigma_{*,2kpc}$-$M_*$ relation, while galaxies with diffuse cores evolve along shallower trajectories. We investigate possible drivers of rapid growth in $Sigma_{*,2kpc}$ compared to $M_*$. Both the current sSFR gradient and the BH accretion rate are indicators of past core growth, but are not predictors of future core growth. Major mergers (although rare in our sample; $sim$10%) cause steeper core growth, except for high mass ($M_*$ >$sim$ $10^{10} M_{odot}$) mergers, which are mostly dry. Disc instabilities, as measured by the fraction of mass with Toomre Q < 2, are not predictive of rapid core growth. Instead, rapid core growth results in more stable discs. The cumulative black hole feedback history sets the maximum rate of core growth, preventing rapid growth in high-mass galaxies ($M_*$ >$sim$ $10^{9.5} M_{odot}$). For massive galaxies the total specific angular momentum of accreting gas is the most important predictor of future core growth. Our results suggest that the angular momentum of accreting gas controls the slope, width and zero-point evolution of the $Sigma_{*,2kpc}$-$M_*$ relation.
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We have recently developed a post-processing framework to estimate the abundance of atomic and molecular hydrogen (HI and H2, respectively) in galaxies in large-volume cosmological simulations. Here we compare the HI and H2 content of IllustrisTNG galaxies to observations. We mostly restrict this comparison to $z approx 0$ and consider six observational metrics: the overall abundance of HI and H2, their mass functions, gas fractions as a function of stellar mass, the correlation between H2 and star formation rate, the spatial distribution of gas, and the correlation between gas content and morphology. We find generally good agreement between simulations and observations, particularly for the gas fractions and the HI mass-size relation. The H2 mass correlates with star formation rate as expected, revealing an almost constant depletion time that evolves up to z = 2 as observed. However, we also discover a number of tensions with varying degrees of significance, including an overestimate of the total neutral gas abundance at z = 0 by about a factor of two and a possible excess of satellites with no or very little neutral gas. These conclusions are robust to the modelling of the HI/H2 transition. In terms of their neutral gas properties, the IllustrisTNG simulations represent an enormous improvement over the original Illustris run. All data used in this paper are publicly available as part of the IllustrisTNG data release.
We investigate the formation history of massive disk galaxies in hydro-dynamical simulation--the IllustrisTNG, to study why massive disk galaxies survive through cosmic time. 83 galaxies in the simulation are selected with M$_{*,z=0}$ $>8times10^{10}$ M$_odot$ and kinematic bulge-to-total ratio less than $0.3$. We find that 8.4 percent of these massive disk galaxies have quiet merger histories and preserve disk morphology since formed. 54.2 percent have a significant increase in bulge components in history, then become disks again till present time. The rest 37.3 percent experience prominent mergers but survive to remain disky. While mergers and even major mergers do not always turn disk galaxies into ellipticals, we study the relations between various properties of mergers and the morphology of merger remnants. We find a strong dependence of remnant morphology on the orbit type of major mergers. Specifically, major mergers with a spiral-in falling orbit mostly lead to disk-dominant remnants, and major mergers of head-on galaxy-galaxy collision mostly form ellipticals. This dependence of remnant morphology on orbit type is much stronger than the dependence on cold gas fraction or orbital configuration of merger system as previously studied.
We analyze the optical morphologies of galaxies in the IllustrisTNG simulation at $zsim0$ with a Convolutional Neural Network trained on visual morphologies in the Sloan Digital Sky Survey. We generate mock SDSS images of a mass complete sample of $sim12,000$ galaxies in the simulation using the radiative transfer code SKIRT and include PSF and noise to match the SDSS r-band properties. The images are then processed through the exact same neural network used to estimate SDSS morphologies to classify simulated galaxies in four morphological classes (E, S0/a, Sab, Scd). The CNN model finds that $sim95%$ of the simulated galaxies fall in one the four main classes with high confidence. The mass-size relations of the simulated galaxies divided by morphological type also reproduce well the slope and the normalization of observed relations which confirms the realism of optical morphologies in the TNG suite. However, the Stellar Mass Functions decomposed into different morphologies still show significant discrepancies with observations both at the low and high mass end. We find that the high mass end of the SMF is dominated in TNG by massive disk galaxies while early-type galaxies dominate in the observations according to the CNN classifications. The present work highlights the importance of detailed comparisons between observations and simulations in comparable conditions.
We present 0.2arcsec-resolution Atacama Large Millimeter/submillimeter Array observations at 870 $mu$m in a stellar mass-selected sample of 85 massive ($M_mathrm{star}>10^{11}~M_odot$) star-forming galaxies (SFGs) at z=1.9-2.6 in the 3D-HST/CANDELS fields of UDS and GOODS-S. We measure the effective radius of the rest-frame far-infrared (FIR) emission for 62 massive SFGs. They are distributed over wide ranges of FIR size from $R_mathrm{e,FIR}=$0.4 kpc to $R_mathrm{e,FIR}=$6 kpc. The effective radius of the FIR emission is smaller by a factor of 2.3$^{+1.9}_{-1.0}$ than the effective radius of the optical emission and by a factor of 1.9$^{+1.9}_{-1.0}$ smaller than the half-mass radius. Even with taking into account potential extended components, the FIR size would change by ~10%. By combining the spatial distributions of the FIR and optical emission, we investigate how galaxies change the effective radius of the optical emission and the stellar mass within a radius of 1 kpc, $M_mathrm{1kpc}$. The compact starburst puts most of massive SFGs on the mass--size relation for quiescent galaxies (QGs) at z~2 within 300 Myr if the current star formation activity and its spatial distribution are maintained. We also find that within 300 Myr, ~38% of massive SFGs can reach the central mass of $M_mathrm{1kpc}=10^{10.5}~M_odot$, which is around the boundary between massive SFGs and QGs. These results suggest an outside-in transformation scenario in which a dense core is formed at the center of a more extended disk, likely via dissipative in-disk inflows. Synchronized observations at ALMA 870 $mu$m and JWST 3-4 $mu$m will explicitly verify this scenario.
We present the first comparison of observed stellar continuum spectra of high-redshift galaxies and mock galaxy spectra generated from hydrodynamical simulations. The mock spectra are produced from the IllustrisTNG TNG100 simulation combined with stellar population models and take into account dust attenuation and realistic observational effects (aperture effects and noise). We compare the simulated $D_n4000$ and EW(H$delta$) of galaxies with $10.5 leq log(M_ast/M_odot) leq 11.5$ at $0.6 leq z leq 1.0$ to the observed distributions from the LEGA-C survey. TNG100 globally reproduces the observed distributions of spectral indices, implying that the age distribution of galaxies in TNG100 is generally realistic. Yet there are small but significant differences. For old galaxies, TNG100 shows small $D_n4000$ when compared to LEGA-C, while LEGA-C galaxies have larger EW(H$delta$) at fixed $D_n4000$. There are several possible explanations: 1) LEGA-C galaxies have overall older ages combined with small contributions (a few percent in mass) from younger ($<1$~Gyr) stars, while TNG100 galaxies may not have such young sub-populations; 2) the spectral mismatch could be due to systematic uncertainties in the stellar population models used to convert stellar ages and metallicities to observables. In conclusion, the latest cosmological galaxy formation simulations broadly reproduce the global age distribution of galaxies at $zsim1$ and, at the same time, the high quality of the latest observed and simulated datasets help constrain stellar population synthesis models as well as the physical models underlying the simulations.
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