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Register a new userThe VLT LEGA-C Spectroscopic Survey: The Physics of Galaxies at a Lookback Time of 7 Gyr
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The Large Early Galaxy Census (LEGA-C) is a Public Spectroscopic Survey of $sim3200$ $K$-band selected galaxies at redshifts $z=0.6-1.0$ with stellar masses M_star > 1e10M_sun, conducted with VIMOS on ESOs Very Large Telescope. The survey is embedded in the COSMOS field ($R.A. = 10h00$; $Dec.=+2deg$). The 20-hour long integrations produce high-$S/N$ continuum spectra that reveal ages, metallicities and velocity dispersions of the stellar populations. LEGA-Cs unique combination of sample size and depth will enable us for the first time to map the stellar content at large look-back time, across galaxies of different types and star-formation activity. Observations started in December 2014 and are planned to be completed by mid 2018, with early data releases of the spectra and value-added products. In this paper we present the science case, the observing strategy, an overview of the data reduction process and data products, and a first look at the relationship between galaxy structure and spectral properties, as it existed 7 Gyr ago.
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We present a comparison of the observed, spatially integrated stellar and ionized gas velocity dispersions of $sim1000$ massive ($log M_{star}/M_{odot}gtrsim,10.3$) galaxies in the Large Early Galaxy Astrophysics Census (LEGA-C) survey at $0.6lesssim,zlesssim1.0$. The high $S/Nsim20{rmAA^{-1}}$ afforded by 20 hour VLT/VIMOS spectra allows for joint modeling of the stellar continuum and emission lines in all galaxies, spanning the full range of galaxy colors and morphologies. These observed integrated velocity dispersions (denoted as $sigma_{g, int}$ and $sigma_{star, int}$) are related to the intrinsic velocity dispersions of ionized gas or stars, but also include rotational motions through beam smearing and spectral extraction. We find good average agreement between observed velocity dispersions, with $langlelog(sigma_{g, int}/sigma_{star, int})rangle=-0.003$. This result does not depend strongly on stellar population, structural properties, or alignment with respect to the slit. However, in all regimes we find significant scatter between $sigma_{g, int}$ and $sigma_{star, int}$, with an overall scatter of 0.13 dex of which 0.05 dex is due to observational uncertainties. For an individual galaxy, the scatter between $sigma_{g, int}$ and $sigma_{star, int}$ translates to an additional uncertainty of $sim0.24rm{dex}$ on dynamical mass derived from $sigma_{g, int}$, on top of measurement errors and uncertainties from Virial constant or size estimates. We measure the $zsim0.8$ stellar mass Faber-Jackson relation and demonstrate that emission line widths can be used to measure scaling relations. However, these relations will exhibit increased scatter and slopes that are artificially steepened by selecting on subsets of galaxies with progressively brighter emission lines.
We explore the connection between the kinematics, structures and stellar populations of massive galaxies at $0.6<z<1.0$ using the Fundamental Plane (FP). Combining stellar kinematic data from the Large Early Galaxy Astrophysics Census (LEGA-C) survey with structural parameters measured from deep Hubble Space Telescope imaging, we obtain a sample of 1419 massive ($log(M_*/M_odot) >10.5$) galaxies that span a wide range in morphology, star formation activity and environment, and therefore is representative of the massive galaxy population at $zsim0.8$. We find that quiescent and star-forming galaxies occupy the parameter space of the $g$-band FP differently and thus have different distributions in the dynamical mass-to-light ratio ($M_{rm dyn}/L_g$), largely owing to differences in the stellar age and recent star formation history, and, to a lesser extent, the effects of dust attenuation. In contrast, we show that both star-forming and quiescent galaxies lie on the same mass FP at $zsim 0.8$, with a comparable level of intrinsic scatter about the plane. We examine the variation in $M_{rm dyn}/M_*$ through the thickness of the mass FP, finding no significant residual correlations with stellar population properties, Sersic index, or galaxy overdensity. Our results suggest that, at fixed size and velocity dispersion, the variations in $M_{rm dyn}/L_g$ of massive galaxies reflect an approximately equal contribution of variations in $M_*/L_g$, and variations in the dark matter fraction or initial mass function.
A decade of study has established that the molecular gas properties of star-forming galaxies follow coherent scaling relations out to z~3, suggesting remarkable regularity of the interplay between molecular gas, star formation, and stellar growth. Passive galaxies, however, are expected to be gas-poor and therefore faint, and thus little is known about molecular gas in passive galaxies beyond the local universe. Here we present deep Atacama Large Millimeter/submillimeter Array (ALMA) observations of CO(2-1) emission in 8 massive (Mstar ~ 10^11 Msol) galaxies at z~0.7 selected to lie a factor of 3-10 below the star-forming sequence at this redshift, drawn from the Large Early Galaxy Astrophysics Census (LEGA-C) survey. We significantly detect half the sample, finding molecular gas fractions <~0.1. We show that the molecular and stellar rotational axes are broadly consistent, arguing that the molecular gas was not accreted after the galaxies became quiescent. We find that scaling relations extrapolated from the star-forming population over-predict both the gas fraction and gas depletion time for passive objects, suggesting the existence of either a break or large increase in scatter in these relations at low specific star formation rate. Finally, we show that the gas fractions of the passive galaxies we have observed at intermediate redshifts are naturally consistent with evolution into local massive early-type galaxies by continued low-level star formation, with no need for further gas accretion or dynamical stabilization of the gas reservoirs in the intervening 6 billion years.
We investigate the change in mean stellar population age and metallicity ([Z/H]) scaling relations for quiescent galaxies from intermediate redshift ($0.60leq zleq0.76$) using the LEGA-C Survey, to low redshift ($0.014leq zleq0.10$) using the SAMI Galaxy Survey. We find that, similarly to their low-redshift counterparts, the stellar metallicity of quiescent galaxies at $0.60leq zleq 0.76$ closely correlates with $M_*/R_mathrm{e}$ (a proxy for the gravitational potential or escape velocity), in that galaxies with deeper potential wells are more metal-rich. This supports the hypothesis that the relation arises due to the gravitational potential regulating the retention of metals, by determining the escape velocity required by metal-rich stellar and supernova ejecta to escape the system and avoid being recycled into later stellar generations. On the other hand, we find no correlation between stellar age and $M_*/R_mathrm{e}^2$ (stellar mass surface density $Sigma$) in the LEGA-C sample, despite this being a strong relation at low redshift. We consider this change in the age--$Sigma$ relation in the context of the redshift evolution of the star-forming and quiescent populations in the mass--size plane, and find our results can be explained as a consequence of galaxies forming more compactly at higher redshifts, and remaining compact throughout their evolution. Furthermore, galaxies appear to quench at a characteristic surface density that decreases with decreasing redshift. The $zsim 0$ age--$Sigma$ relation is therefore a result of building up the quiescent and star-forming populations with galaxies that formed at a range of redshifts and so a range of surface densities.
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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