Three billion years after the big bang (at redshift z = 2), half of the most massive galaxies were already old, quiescent systems with little to no residual star formation and extremely compact with stellar mass densities at least an order of magnitude larger than in low redshift ellipticals, their descendants. Little is known about how they formed, but their evolved, dense stellar populations suggest formation within intense, compact starbursts 1-2 Gyr earlier (at 3 < z < 6). Simulations show that gas-rich major mergers can give rise to such starbursts which produce dense remnants. Sub-millimeter selected galaxies (SMGs) are prime examples of intense, gas-rich, starbursts. With a new, representative spectroscopic sample of compact quiescent galaxies at z = 2 and a statistically wellunderstood sample of SMGs, we show that z = 3 − 6 SMGs are consistent with being the progenitors of z = 2 quiescent galaxies, matching their formation redshifts and their distributions of sizes, stellar masses and internal velocities. Assuming an evolutionary connection, their space densities also match if the mean duty cycle of SMG starbursts is 42 +40 −29 Myr (consistent with independent estimates), which indicates that the bulk of stars in these massive galaxies were formed in a major, early surge of star-formation. These results suggests a coherent picture of the formation history of the most massive galaxies in the universe, from their initial burst of violent star-formation through their appearance as high stellar-density galaxy cores and to their ultimate fate as giant ellipticals.
We measure the merger fraction of massive galaxies using the UltraVISTA/COSMOS Ks-band selected catalog, complemented with the deeper, higher resolution 3DHST+CANDELS catalog selected in the HST /WFC3 H -band, presenting the largest mass-complete photometric merger sample up to z ∼ 3. We find that selecting mergers using the H 160 -band flux ratio leads to an increasing merger fraction with redshift, while selecting mergers using the stellar mass ratio causes a diminishing redshift dependence. Defining major and minor mergers as having stellar mass ratios of 1:1 -4:1 and 4:1 -10:1 respectively, the results imply ∼1 major and 1 minor merger for an average massive (log(M ⋆ /M ⊙ ) 10.8) galaxy during z = 0.1 − 2.5. There may be an additional ∼ 0.5(0.3) major (minor) merger if we use the H-band flux ratio selection. The observed amount of major merging alone is sufficient to explain the observed number density evolution for the very massive (log(M ⋆ /M ⊙ ) 11.1) galaxies. We argue that these very massive galaxies can put on a maximum of 6% of stellar mass in addition to major and minor merging, so that their number density evolution remains consistent with observations. The observed number of major and minor mergers can increase the size of a massive quiescent galaxy by a factor of two at most. This amount of merging is enough to bring the compact quiescent galaxies formed at z > 2 to lie at 1σ below the mean of the stellar mass-size relation as measured in some works (e.g. Newman et al. 2012), but additional mechanisms are needed to fully explain the evolution, and to be consistent with works suggesting stronger evolution (e.g. van der Wel et al. 2014).
We present dynamical and structural scaling relations of quiescent galaxies at z = 2, including the dynamical mass-size relation and the first constraints on the fundamental plane (FP). The backbone of the analysis is a new, very deep VLT/X-shooter spectrum of a massive, compact, quiescent galaxy at z = 2.0389. We detect the continuum between 3700-22000Å and several strong absorption features (Balmer series, Ca H+K, G-band), from which we derive a stellar velocity dispersion of 318 ± 53 km s −1 . We perform detailed modeling of the continuum emission and line indices and derive strong simultaneous constraints on the age, metallicity, and stellar mass. The galaxy is a dusty (A V = 0.77 +0.36 −0.32 ) solar metallicity (log(Z/Z ) = 0.02 +0.20 −0.41 ) post starburst galaxy, with a mean luminosity weighted log(age/yr) of 8.9 ± 0.1. The galaxy formed the majority of its stars at z > 3 and currently has little or no ongoing star formation. We compile a sample of three other z ∼ 2 quiescent galaxies with measured velocity dispersions, two of which are also post starburst like. Their dynamical masssize relation is offset significantly less than the stellar mass-size relation from the local early type relations, which we attribute to a lower central dark matter fraction. Recent cosmological merger simulations qualitatively agree with the data, but can not fully account for the evolution in the dark matter fraction. The z ∼ 2 FP requires additional evolution beyond passive stellar aging, to be in agreement with the local FP. The structural evolution predicted by the cosmological simulations is insufficient, suggesting that additional, possibly non-homologous structural evolution is needed.
At redshift z = 2, when the Universe was just three billion years old, half of the most massive galaxies were extremely compact and had already exhausted their fuel for star formation [1][2][3][4] . It is believed that they were formed in intense nuclear starbursts and that they ultimately grew into the most massive local elliptical galaxies seen today, through mergers with minor companions 5,6 , but validating this picture requires higher-resolution observations of their centres than is currently possible. Magnification from gravitational lensing offers an opportunity to resolve the inner regions of galaxies 7 . Here we report an analysis of the stellar populations and kinematics of a lensed z = 2.1478 compact galaxy, which-surprisingly-turns out to be a fast-spinning, rotationally supported disk galaxy. Its stars must have formed in a disk, rather than in a merger-driven nuclear starburst 8 . The galaxy was probably fed by streams of cold gas, which were able to penetrate the hot halo gas until they were cut off by shock heating from the dark matter halo 9 . This result confirms previous indirect indications 10-13 that the first galaxies to cease star formation must have gone through major changes not just in their structure, but also in their kinematics, to evolve into present-day elliptical galaxies.We obtained deep spectroscopy using the XSHOOTER instrument on the Very Large Telescope (VLT) of a compact quiescent galaxy that is gravitationally lensed by the z = 0.588 cluster of galaxies MACS J2129.4−0741 (hereafter MACS2129−1; ref. 14; (Extended Data Fig. 1), and as a consequence appears 4.6 ± 0.2 times brighter and extends over 3″ on the sky. In Fig.1, we show the position of the XSHOOTER slit on a Hubble Space Telescope (HST) colour-composite image and on the reconstructed source plane. The galaxy is stretched along its major axis and we derive a spatially resolved spectrum typical of quiescent z ≈ 2 post-starburst galaxies, with a strong Balmer break and a number of strong absorption features. We fit a spectroscopic redshift of z = 2.1478 ± 0.0006 and constrain the stellar populations through modeling of the rest-frame ultraviolet-to-optical spectrum, the absorption line indices, and the spatially integrated rest-frame ultravioletto-near-infrared (NIR) colours derived from 16-band HST/Infrared Array Camera (IRAC) photometry. The best-fitting spectrum reveals a massive, old, post-starburst Table 1). We derive a velocity dispersion of σ=329±73 km s -1 from absorption lines in the spatially integrated spectrum (Fig. 1) using a penalized pixel fitting method (pPXF) 15 with the best-fitting spectral energy distribution model as a template. Interestingly, the absorption lines are tilted in the twodimensional spectrum. We extract individual rows that represent approximately 0.4-kpc bins along the major axis on the source plane. The central 11 rows have sufficient signal to noise (S/N) to detect absorption lines reliably. We fit each row with the same pPXF implementation used for the spatially integrated spec...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.