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. 2017 Jun 21;546(7659):510-513.
doi: 10.1038/nature22388.

A massive, dead disk galaxy in the early Universe

Sune Toft  1 , Johannes Zabl  1   2 , Johan Richard  3 , Anna Gallazzi  4 , Stefano Zibetti  4 , Moire Prescott  5 , Claudio Grillo  6 , Allison W S Man  7 , Nicholas Y Lee  1 , Carlos Gómez-Guijarro  1 , Mikkel Stockmann  1 , Georgios Magdis  1   8 , Charles L Steinhardt  1
Affiliations

A massive, dead disk galaxy in the early Universe

Sune Toft et al. Nature. .

Abstract

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. 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, 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. 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. 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. This result confirms previous indirect indications 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.

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Conflict of interest statement

The authors declare no competing financial interests.

Figures

Extended Data Figure 1
Extended Data Figure 1
Hubble color image of the lensing cluster MACS2129-0741. Indicated is the position of the X-shooter slit on the target, which has been magnified and stretched by an average factor of ~4.6 by the foreground cluster. The image is a color composite (B = F435W + F475W; G = F555W + F606W + F775W + F814W + F850LP; R = F105W + F110W + F125W + F140W + F160W) constructed from CLASH data.
Extended Data Figure 2
Extended Data Figure 2
Emission line characterization in three spatial extractions of the XSHOOTER spectrum. The top/middle/bottom panels show the full (|r| < 1.36”)/ central (|r| < 0.5”)/outer (0.5” < |r| < 1.36”) extractions, respectively. The colored lines represent spectral decomposition into nebular emission lines and stellar continuum, obtained with pPXF/GANDALF: the orchid line displays the best fitting composite model; the green line is the best fitting stellar continuum; the blue and dark red lines represent the best fitting emission lines with and without a significant detection, respectively. Shaded regions indicate spectral regions of low atmospheric transmission/high background that have been excluded from the fit.
Extended Data Figure 3
Extended Data Figure 3
Radial stellar population gradients. The full lines show azimuthally averaged radial profiles of median likelihood stellar population synthesis parameters, derived from the maps in Figure 3 in elliptical apertures following the best fitting 2D surface brightness fit. The error bars represent the standard deviation of the best fitting values in the apertures, not the absolute errors on the individual fits. The filled circles show results from the spectral fits, with absolute error bars. The dotted line shows the average sSFR profile from a sample of star forming galaxies of similar mass and redshift as MACS2129-1
Extended Data Figure 4
Extended Data Figure 4
Properties of MACS2129-1 compared to different galaxy populations. a) Stellar masses and sizes of 2Vmax/σint versus ellipticity for the two lensed z>2 cQGs MACS2129-1 and RG1M0150 compared to similar mass local galaxies. The grey histogram shows the V/σ posterior distribution from our modeling. MACS2129-1 is thus similar to local late types, (blue), while RG1M0150 is similar to local early types (red). c) The dynamical to stellar mass ratio (within re) of MACS2129 is similar to previously observed z>2 cQGs, including the strongly lensed RG1M0150, and to z~2 star-forming galaxies of similar age.
Extended Data Figure 5
Extended Data Figure 5
Correlations between lensing model parameters and derived structural parameters for MACS2129-1. Shown are the average light weighted (l.w.) magnification, the direction of maximum magnification at the position of MACS2129-1, the magnification along this axis (major. magni.) and perpendicular to it (minor magni.). These were obtained from 1979 lensing model realizations (black) sampling the full probability distribution. Also shown are correlations with the galaxy axis ratios (a/b) and position angles (PA) of MACS2129-1 derived from Galfit analysis of reconstructed source plane images for a subsample of 98 representative realizations (red).
Extended data Figure 6
Extended data Figure 6
Structural parameters. Distribution of Sersic model parameters derived from 2D surface brightness fits with Galfit, on the source plane images generated from 98 representative realizations of the lensing model. We adopt the median values of these distributions and their standard deviations as our best fitting parameters.
Extended data Figure 7
Extended data Figure 7
Variations of the magnification over MACS2129-1. Results are shown for a typical realization (middle), and for the realizations with the maximum (top) and minimum (bottom) magnifications. The columns (from left to right) show 1: Observed F160W image, 2: Magnification map, 3: Seeing convolved (FWHM=0.5”) F160W image, 4: Seeing convolved light (F160W) weighted magnification map, 5: source plane image (crosses at same position) 6: Average, light weighted magnification contributing to each spatial bin in the XSHOOTER slit (shown in the bottom row). The minor variations are caused by the galaxy 3.5 arcsec west of MACS2129-1 (see middle row).
Extended data Figure 8
Extended data Figure 8
Posterior distributions for the parameters in our dynamical modeling of the rotation and dispersion curves. The open histograms show the distributions with priors Θoff = 22 ± 10°, |Xc| <0.4″. Filled histograms with the additional prior i = 53.8 ± 2.13°, all derived from Galfit modeling.
Figure 1
Figure 1
Spectrum of MACS2129-1. Rest-frame UV-optical 2D and 1D XSHOOTER spectrum, adaptively rebinned to a constant S/N per bin, and a zoom in on the most important absorption features, binned to a resolution of 9.6Å (observed). The dashed red line shows the best-fit stellar population model. Grey/orange colored regions indicate windows of high telluric absorption/important emission lines. Also shown are color composite HST images on the image and reconstructed source plane, with the position of the slit overlaid.
Figure 2
Figure 2
Rotation and Dispersion curve for MACS2129-1. Velocity offsets and dispersions as a function of distance from the center of the galaxy, derived from pPXF fits to the individual spatial lines in the full spectrum. The grey polygon shows the best fitting thin disk model (±1σ) to the black squares. The observed dispersion is high in the center and drops off symmetrically with distance, consistent with the effect of PSF smearing of the velocity gradient, and a constant dispersion of up to ~100 km/s (grey polygon).
Figure 3
Figure 3
Stellar population maps on the reconstructed source plane. These are created from fits to the HST imaging, using the same stellar population library, as used to fit the full spectrum. The insets show 68% confidence intervals for the derived parameters. The PSF is shown in Figure 4. The younger knot in the top right corner, which may be an ongoing minor merger does not give rise to the extra light seen in Figure 4, or influence the conclusions based on azimuthally averaged profiles (see Methods).
Figure 4
Figure 4
Surface brightness and stellar mass profile for MACS2129-1. Left: Top panel shows the source plane reconstruction of WFC3/F160W band image with the PSF shown as an inset. Second panel shows the best fitting Sersic model (n=1.01−0.06+0.12). Third panel shows the residual, which may show a hint of spiral structure. Right: The 1D light and surface mass density profiles (derived in elliptical apertures, following the best fitting Sersic model) are both well represented by an n=1 exponential disk model. The increased spatial resolution due to lensing is illustrated by the (circularized) HWHM of the PSF at z=2.15, with and without lensing.
See this image and copyright information in PMC

References

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