Galaxies with the mass of the Milky Way dominate the stellar mass density of the universe but it is uncertain how and when they were assembled. Here we study progenitors of these galaxies out to z ... Show moreGalaxies with the mass of the Milky Way dominate the stellar mass density of the universe but it is uncertain how and when they were assembled. Here we study progenitors of these galaxies out to z = 2.5, using data from the 3D-HST and CANDELS Treasury surveys. We find that galaxies with present-day stellar masses of log (M) {\ap} 10.7 built \~{}90\% of their stellar mass since z = 2.5, with most of the star formation occurring before z = 1. In marked contrast to the assembly history of massive elliptical galaxies, mass growth is not limited to large radii: the mass in the central 2 kpc of the galaxies increased by a factor of 3.2\^{}$\{$+0.8$\}$\_$\{$-0.7$\}$ between z = 2.5 and z = 1. We therefore rule out simple models in which bulges were fully assembled at high redshift and disks gradually formed around them. Instead, bulges (and black holes) likely formed in lockstep with disks, through bar instabilities, migration, or other processes. We find that after z = 1 the growth in the central regions gradually stopped and the disk continued to be built up, consistent with recent studies of the gas distributions in z \~{} 1 galaxies and the properties of many spiral galaxies today. Show less
We present an analysis of the spatial distribution of star formation in a sample of 60 visually identified galaxy merger candidates at z {\gt} 1. Our sample, drawn from the 3D-HST survey, is flux... Show moreWe present an analysis of the spatial distribution of star formation in a sample of 60 visually identified galaxy merger candidates at z {\gt} 1. Our sample, drawn from the 3D-HST survey, is flux limited and was selected to have high star formation rates based on fits of their broad-band, low spatial resolution spectral energy distributions. It includes plausible pre-merger (close pairs) and post-merger (single objects with tidal features) systems, with total stellar masses and star formation rates derived from multiwavelength photometry. Here we use near-infrared slitless spectra from 3D-HST which produce H{$α$} or [O III] emission line maps as proxies for star formation maps. This provides a first comprehensive high-resolution, empirical picture of where star formation occurred in galaxy mergers at the epoch of peak cosmic star formation rate. We find that detectable star formation can occur in one or both galaxy centres, or in tidal tails. The most common case (58 per cent) is that star formation is largely concentrated in a single, compact region, coincident with the centre of (one of) the merger components. No correlations between star formation morphology and redshift, total stellar mass or star formation rate are found. A restricted set of hydrodynamical merger simulations between similarly massive and gas-rich objects implies that star formation should be detectable in both merger components, when the gas fractions of the individual components are the same. This suggests that z {\tilde} 1.5 mergers typically occur between galaxies whose gas fractions, masses and/or star formation rates are distinctly different from one another. Show less
The assembly of galaxies can be described by the distribution of their star formation as a function of cosmic time. Thanks to the WFC3 grism on the Hubble Space Telescope (HST) it is now possible... Show moreThe assembly of galaxies can be described by the distribution of their star formation as a function of cosmic time. Thanks to the WFC3 grism on the Hubble Space Telescope (HST) it is now possible to measure this beyond the local Universe. Here we present the spatial distribution of H{$α$} emission for a sample of 54 strongly star-forming galaxies at z \~{} 1 in the 3D-HST Treasury survey. By stacking the H{$α$} emission, we find that star formation occurred in approximately exponential distributions at z \~{} 1, with a median Sérsic index of n = 1.0 {\plusmn} 0.2. The stacks are elongated with median axis ratios of b/a = 0.58 {\plusmn} 0.09 in H{$α$} consistent with (possibly thick) disks at random orientation angles. Keck spectra obtained for a subset of eight of the galaxies show clear evidence for rotation, with inclination corrected velocities of 90-330 km s$^{-1}$. The most straightforward interpretation of our results is that star formation in strongly star-forming galaxies at z \~{} 1 generally occurred in disks. The disks appear to be ''scaled-up'' versions of nearby spiral galaxies: they have EW(H{$α$}) \~{} 100 Å out to the solar orbit and they have star formation surface densities above the threshold for driving galactic scale winds. Show less
We analyze the resolved stellar populations of 473 massive star-forming galaxies at 0.7 {lt} z {lt} 1.5, with multi-wavelength broadband imaging from CANDELS and H{$α$} surface brightness profiles... Show moreWe analyze the resolved stellar populations of 473 massive star-forming galaxies at 0.7 {lt} z {lt} 1.5, with multi-wavelength broadband imaging from CANDELS and H{$α$} surface brightness profiles at the same kiloparsec resolution from 3D-HST. Together, this unique data set sheds light on how the assembled stellar mass is distributed within galaxies, and where new stars are being formed. We find the H{$α$} morphologies to resemble more closely those observed in the ACS I band than in the WFC3 H band, especially for the larger systems. We next derive a novel prescription for H{$α$} dust corrections, which accounts for extra extinction toward H II regions. The prescription leads to consistent star formation rate (SFR) estimates and reproduces the observed relation between the H{$α$}/UV luminosity ratio and visual extinction, on both a pixel-by-pixel and a galaxy-integrated level. We find the surface density of star formation to correlate with the surface density of assembled stellar mass for spatially resolved regions within galaxies, akin to the so-called ''main sequence of star formation'' established on a galaxy-integrated level. Deviations from this relation toward lower equivalent widths are found in the inner regions of galaxies. Clumps and spiral features, on the other hand, are associated with enhanced H{$α$} equivalent widths, bluer colors, and higher specific SFRs compared to the underlying disk. Their H{$α$}/UV luminosity ratio is lower than that of the underlying disk, suggesting that the ACS clump selection preferentially picks up those regions of elevated star formation activity that are the least obscured by dust. Our analysis emphasizes that monochromatic studies of galaxy structure can be severely limited by mass-to-light ratio variations due to dust and spatially inhomogeneous star formation histories. Show less
