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Register a new userChronos: A NIR spectroscopic galaxy survey. From the formation of galaxies to the peak of activity
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Chronos is our response to ESAs call for white papers to define the science for the future L2, L3 missions. Chronos targets the formation and evolution of galaxies, by collecting the deepest NIR spectroscopic data, from the formation of the first galaxies at z~10 to the peak of formation activity at z~1-3. The strong emission from the atmospheric background makes this type of survey impossible from a ground-based observatory. The spectra of galaxies represent the equivalent of a DNA fingerprint, containing information about the past history of star formation and chemical enrichment. The proposed survey will allow us to dissect the formation process of galaxies including the timescales of quenching triggered by star formation or AGN activity, the effect of environment, the role of infall/outflow processes, or the connection between the galaxies and their underlying dark matter haloes. To provide these data, the mission requires a 2.5m space telescope optimised for a campaign of very deep NIR spectroscopy. A combination of a high multiplex and very long integration times will result in the deepest, largest, high-quality spectroscopic dataset of galaxies from z=1 to 12, spanning the history of the Universe, from 400 million to 6 billion years after the big bang, i.e. covering the most active half of cosmic history.
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(Abridged summary) Responding to ESAs Voyage 2050 call to define the long-term plan for the future space missions that will address the astrophysics science questions during the 2035-2050 cycle, we propose a dedicated, ultra-deep spectroscopic survey in the near infrared (NIR), that will target a mass-limited sample of galaxies during two of the most fundamental epochs of cosmic evolution: the formation of the first galaxies (at z>6; cosmic dawn), and at the peak of galaxy formation activity (between redshift z=1 and 3; cosmic noon). By way of NIR observations, it is possible to study the Lyman-alpha region in the former, and the optical rest-frame in the latter, allowing us to extract fundamental observables such as gas and stellar kinematics, chemical abundances, and ages, providing a unique legacy database covering these two crucial stages of cosmic evolution. A dedicated, space-based facility will overcome the challenges faced by ground-based telescopes, no matter how large the aperture, or the reduced field of view and low multiplex factor of the best space-based instrument in the near future, namely NIRSpec at the JWST. Our project (codename Chronos) aims to produce about 1 million high quality spectra, with a high S/N in the continuum, where information about the underlying stellar populations is encoded.
We present the results of SPT-GMOS, a spectroscopic survey with the Gemini Multi-Object Spectrograph (GMOS) on Gemini South. The targets of SPT-GMOS are galaxy clusters identified in the SPT-SZ survey, a millimeter-wave survey of 2500 squ. deg. of the southern sky using the South Pole Telescope (SPT). Multi-object spectroscopic observations of 62 SPT-selected galaxy clusters were performed between January 2011 and December 2015, yielding spectra with radial velocity measurements for 2595 sources. We identify 2243 of these sources as galaxies, and 352 as stars. Of the galaxies, we identify 1579 as members of SPT-SZ galaxy clusters. The primary goal of these observations was to obtain spectra of cluster member galaxies to estimate cluster redshifts and velocity dispersions. We describe the full spectroscopic dataset and resulting data products, including galaxy redshifts, cluster redshifts and velocity dispersions, and measurements of several well-known spectral indices for each galaxy: the equivalent width, W, of [O II] 3727,3729 and H-delta, and the 4000A break strength, D4000. We use the spectral indices to classify galaxies by spectral type (i.e., passive, post-starburst, star-forming), and we match the spectra against photometric catalogs to characterize spectroscopically-observed cluster members as a function of brightness (relative to m*). Finally, we report several new measurements of redshifts for ten bright, strongly-lensed background galaxies in the cores of eight galaxy clusters. Combining the SPT-GMOS dataset with previous spectroscopic follow-up of SPT-SZ galaxy clusters results in spectroscopic measurements for >100 clusters, or ~20% of the full SPT-SZ sample.
We use the HiZELS narrow-band H-alpha survey in combination with CANDELS, UKIDSS and WIRDS near-infrared imaging, to investigate the morphologies, merger rates and sizes of a sample of H-alpha emitting galaxies in the redshift range z=0.40 - 2.23, an epoch encompassing the rise to the peak of the star formation rate density. Merger rates are estimated from space- and ground-based imaging using the M20 coefficient. To account for the increase in the specific star-formation rate (sSFR) of the star forming `main-sequence with redshift, we normalise the star-formation rate of galaxies at each epoch to the typical value derived from the H-alpha luminosity function. Once this trend in sSFR is removed we see no evidence for an increase in the number density of star-forming galaxies or the merger rate with redshift. We thus conclude that neither is the main driver of the enhanced star-formation rate density at z=1-2, with secular processes such as instabilities within efficiently fuelled, gas-rich discs or multiple minor mergers the most likely alternatives. However, we find that 40-50% of starburst galaxies, those with enhanced specific star formation at their epoch, are major mergers and this fraction is redshift independent. Finally, we find the surprising result that the typical size of a star-forming galaxy of a given mass does not evolve across the redshift range considered, suggesting a universal size-mass relation. Taken in combination, these results indicate a star-forming galaxy population that is statistically similar in physical size, merger rate and mass over the 6 Gyr covered in this study, despite the increase in typical sSFR.
We utilize the galaxy shape catalogue from the first-year data release of the Subaru Hyper Suprime-cam Survey (HSC) to study the dark matter content of galaxy groups in the Universe using weak gravitational lensing. As our lens sample, we use galaxy groups that have been spectroscopically selected from the Galaxy Mass and Assembly galaxy survey in approximately 100 sq. degrees of the sky that overlap with the HSC survey. We restrict our analysis to the 1587 groups with at least five group members. We divide these galaxy groups into six bins each of galaxy group luminosity and group member velocity dispersion and measure the coherent tangential ellipticity pattern on background HSC galaxies imprinted by weak gravitational lensing. We measure the weak lensing signal with a signal-to-noise ratio of 55 and 51 for these two different selections, respectively. We use a Bayesian halo model framework to infer the halo mass distribution of our galaxy groups binned in the two different observable properties and obtain constraints on the power-law scaling relation between mean halo masses and the two group observable properties. We obtain a 5 percent constraint on the amplitude of the scaling relation between halo mass and group luminosity with $langle Mrangle = (0.81pm0.04)times10^{14}h^{-1}M_odot$ for $L_{rm grp}=10^{11.5}h^{-2}L_odot$, and a power-law index of $alpha=1.01pm0.07$. We also obtain a 5-percent constraint on the amplitude of the scaling relation between halo mass and velocity dispersion with $langle Mrangle=(0.93pm0.05)times10^{14}h^{-1}M_odot$ for $sigma=500{,rm kms}^{-1}$ and a power-law index $alpha=1.52pm0.10$, although these scaling relations are sensitive to the exact cuts applied to the number of group members. Comparisons with similar scaling relations from the literature indicate that our results are consistent, but have significantly reduced errors.
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 HST it is now possible to measure this beyond the local Universe. Here we present the spatial distribution of Halpha emission for a sample of 54 strongly star-forming galaxies at z~1 in the 3D-HST Treasury survey. By stacking the Halpha emission we find that star formation occurred in approximately exponential distributions at z~1, with median Sersic index of n=1.0+-0.2. The stacks are elongated with median axis ratios of b/a=0.58+-0.09 in Halpha, 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 to 330 km/s. 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
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