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Van Der Wel A.,Max Planck Institute for Astronomy | Chang Y.-Y.,Max Planck Institute for Astronomy | Bell E.F.,University of Michigan | Holden B.P.,University of California at Santa Cruz | And 17 more authors.
Astrophysical Journal Letters | Year: 2014

We determine the intrinsic, three-dimensional shape distribution of star-forming galaxies at 0 < z < 2.5, as inferred from their observed projected axis ratios. In the present-day universe, star-forming galaxies of all masses 109-1011 M are predominantly thin, nearly oblate disks, in line with previous studies. We now extend this to higher redshifts, and find that among massive galaxies (M * > 1010 M disks are the most common geometric shape at all z ≲ 2. Lower-mass galaxies at z > 1 possess a broad range of geometric shapes: the fraction of elongated (prolate) galaxies increases toward higher redshifts and lower masses. Galaxies with stellar mass 109 M (1010 M) are a mix of roughly equal numbers of elongated and disk galaxies at z1 (z2). This suggests that galaxies in this mass range do not yet have disks that are sustained over many orbital periods, implying that galaxies with present-day stellar mass comparable to that of the Milky Way typically first formed such sustained stellar disks at redshift z1.5-2. Combined with constraints on the evolution of the star formation rate density and the distribution of star formation over galaxies with different masses, our findings imply that, averaged over cosmic time, the majority of stars formed in disks. © 2014. The American Astronomical Society. All rights reserved..

Van Dokkum P.G.,Yale University | Bezanson R.,University of Arizona | Van Der Wel A.,Max Planck Institute for Astronomy | Nelson E.J.,Yale University | And 14 more authors.
Astrophysical Journal | Year: 2014

The dense interiors of massive galaxies are among the most intriguing environments in the universe. In this paper,we ask when these dense cores were formed and determine how galaxies gradually assembled around them. We select galaxies that have a stellar mass >3 × 1010 M inside r = 1 kpc out to z = 2.5, using the 3D-HST survey and data at low redshift. Remarkably, the number density of galaxies with dense cores appears to have decreased from z = 2.5 to the present. This decrease is probably mostly due to stellar mass loss and the resulting adiabatic expansion, with some contribution from merging. We infer that dense cores were mostly formed at z > 2.5, consistent with their largely quiescent stellar populations. While the cores appear to form early, the galaxies in which they reside show strong evolution: their total masses increase by a factor of 2-3 from z = 2.5 to z = 0 and their effective radii increase by a factor of 5-6. As a result, the contribution of dense cores to the total mass of the galaxies in which they reside decreases from 50% at z = 2.5 to 15% at z = 0. Because of their early formation, the contribution of dense cores to the total stellar mass budget of the universe is a strong function of redshift. The stars in cores with M 1 kpc > 3 × 1010 M ̇make up 0.1% of the stellar mass density of the universe today but 10%-20% at z 2, depending on their initial mass function. The formation of these cores required the conversion of 1011 M of gas into stars within 1 kpc, while preventing significant star formation at larger radii. © 2014. The American Astronomical Society. All rights reserved..

Van Dokkum P.G.,Yale University | Nelson E.J.,Yale University | Franx M.,Leiden University | Oesch P.,Yale University | And 12 more authors.
Astrophysical Journal | Year: 2015

In this paper we study a key phase in the formation of massive galaxies: the transition of star-forming galaxies into massive (Mstars ∼ 1011Mo), compact (re ∼ 1 kpc) quiescent galaxies, which takes place from z ∼ 3 to z ∼ 1.5. We use HST grism redshifts and extensive photometry in all five 3D-HST/CANDELS fields, more than doubling the area used previously for such studies, and combine these data with Keck MOSFIRE and NIRSPEC spectroscopy. We first confirm that a population of massive, compact, star-forming galaxies exists at z ≳ 2, using K-band spectroscopy of 25 of these objects at 2.0 < z < 2.5. They have a median [N ii]/Hα ratio of 0.6, are highly obscured with SFR(tot)/SFR(Hα) ∼10, and have a large range of observed line widths. We infer from the kinematics and spatial distribution of Hα that the galaxies have rotating disks of ionized gas that are a factor of ∼2 more extended than the stellar distribution. By combining measurements of individual galaxies, we find that the kinematics are consistent with a nearly Keplerian fall-off from Vrot ∼ 500 km s-1 at 1 kpc to Vrot ∼ 250 km s-1 at 7 kpc, and that the total mass out to this radius is dominated by the dense stellar component. Next, we study the size and mass evolution of the progenitors of compact massive galaxies. Even though individual galaxies may have had complex histories with periods of compaction and mergers, we show that the population of progenitors likely followed a simple inside-out growth track in the size-mass plane of Δ log re ∼ 0.3 Δ log Mstars. This mode of growth gradually increases the stellar mass within a fixed physical radius, and galaxies quench when they reach a stellar density or velocity dispersion threshold. As shown in other studies, the mode of growth changes after quenching, as dry mergers take the galaxies on a relatively steep track in the size-mass plane. © 2015. The American Astronomical Society. All rights reserved.

Patel S.G.,Leiden University | Fumagalli M.,Leiden University | Franx M.,Leiden University | Van Dokkum P.G.,Yale University | And 13 more authors.
Astrophysical Journal | Year: 2013

We follow the structural evolution of star-forming galaxies (SFGs) like the Milky Way by selecting progenitors to z ̃ 1.3 based on the stellar mass growth inferred from the evolution of the star-forming sequence. We select our sample from the 3D-HST survey, which utilizes spectroscopy from the HST/WFC3 G141 near-IR grism and enables precise redshift measurements for our sample of SFGs. Structural properties are obtained from Sérsic profile fits to CANDELS WFC3 imaging. The progenitors of z = 0 SFGs with stellar mass M = 1010.5 M * are typically half as massive at z ̃ 1. This late-time stellar mass growth is consistent with recent studies that employ abundance matching techniques. The descendant SFGs at z ̃ 0 have grown in half-light radius by a factor of ̃1.4 since z ̃ 1. The half-light radius grows with stellar mass as re ∝M 0.29. While most of the stellar mass is clearly assembling at large radii, the mass surface density profiles reveal ongoing mass growth also in the central regions where bulges and pseudobulges are common features in present day late-type galaxies. Some portion of this growth in the central regions is due to star formation as recent observations of Hα maps for SFGs at z ̃ 1 are found to be extended but centrally peaked. Connecting our lookback study with galactic archeology, we find the stellar mass surface density at R = 8 kpc to have increased by a factor of ̃2 since z ̃ 1, in good agreement with measurements derived for the solar neighborhood of the Milky Way. © 2013. The American Astronomical Society. All rights reserved.

Pacifici C.,Yonsei University | Pacifici C.,CNRS Paris Institute of Astrophysics | Cunha E.D.,Max Planck Institute for Astronomy | Charlot S.,CNRS Paris Institute of Astrophysics | And 14 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2014

Interpreting observations of distant galaxies in terms of constraints on physical parameters - such as stellarmass (M*), star formation rate (SFR) and dust optical depth (τV) - requires spectral synthesis modelling.We analyse the reliability of these physical parameters as determined under commonly adopted 'classical' assumptions: star formation histories assumed to be exponentially declining functions of time, a simple dust law and no emission-line contribution. Improved modelling techniques and data quality now allow us to use a more sophisticated approach, including realistic star formation histories, combined with modern prescriptions for dust attenuation and nebular emission. We present a Bayesian analysis of the spectra and multiwavelength photometry of 1048 galaxies from the 3D-HST survey in the redshift range 0.7 < z < 2.8 and in the stellar mass range 9 ≲ log (M/M) ≲ 12. We find that, using the classical spectral library, stellar masses are systematically overestimated (~0.1 dex) and SFRs are systematically underestimated (~0.6 dex) relative to our more sophisticated approach.We also find that the simultaneous fit of photometric fluxes and emission-line equivalent widths helps break a degeneracy between SFR and τV , reducing the uncertainties on these parameters. Finally, we show how the biases of classical approaches can affect the correlation between M and SFR for star-forming galaxies (the 'star-formation main sequence').We conclude that the normalization, slope and scatter of this relation strongly depend on the adopted approach and demonstrate that the classical, oversimplified approach cannot recover the true distribution of M and SFR. © 2014 The Authors.

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