High-redshift Universe


This space mission will be able to detect emission lines powered by star formation in galaxies out to z=8 or larger. We will trace both individual dusty starburst galaxies and galaxy protoclusters with a strong dust-obscured component. The spectroscopic mode will provide an unprecedented 3D-view of the universe, mapped with a very large number of securely identified sources. The high-sensitivity spectroscopic and imaging surveys carried out by this space mission will revolutionise our understanding of galaxy formation and evolution and of the growth of large-scale structure back to the epoch of reionisation.

Thanks to the combination of its high sensitivity and of the strong sub-mm emission lines (such as [NII] 205.18mm, [CI] 170.42mm, [CII] 157.7mm, [OIII] 88.36mm, [OIII] 51.81mm, and [OI] 63.18mm), the all-sky survey will be effectively deeper than the Herschel continuum surveys with the same telescope size: spectroscopy allows us to overcome the confusion limit!

Minimum SFR as a function of redshift, for galaxies detected in lines in the 100—1000 GHz range by the ``all-sky'' survey (2 years, 90% of the sky, solid black line) and for a deep survey 6-months duration over 5% of the sky (dotted blue line). The scale on the right refers to the bolometric luminosity due to star formation, LSFR, based on the calibration by Kennicutt & Evans (2012). The solid red line, the green dot-dashed line, and the magenta dashed line show, for comparison, the IR (8--1000mm) luminosity, LIR, corresponding to the 4s detection limits (approximately 90% completeness) of the H-ATLAS survey covering 660 deg2, to the confusion limit of the CMB-S4 survey at 220 GHz expected to cover 43% of the sky (5 mJy; Abazajian et al. 2019), and to the 90% completeness limit (15 mJy) of the South Pole Telescope (SPT; Mocanu et al. 2013) survey covering 2,500 deg2, respectively. LIR is a measure of the dust-obscured SFR [Figure from Negrello et al. 2020].

Individual galaxies

The planned 2-years survey of 90% of the sky ("all-sky survey") plus the "deep" 6-months survey of 5% of the sky will provide measurements of spectroscopic redshifts, star-formation rates, dust masses, and metal content for hundreds of millions of galaxies including several thousands at the epoch of reionization (z > 6 and out to z beyond 8). This is a giant leap forward compared to any existing or forthcoming continuum surveys which, moreover, require time consuming redshift follow-up programmes that are impractical for millions of optically very faint sources.

Star formation rates (SFRs) well below those of typical galaxies will be reached over the redshift range 1<z < 3 where the cosmic SFR peaks.

Predicted integral redshift distributions of lensed (dotted lines) and unlensed (solid lines) galaxies detected in at least one line by the ``all-sky'' (black) the ``deep'' (blue) survey.

Millions of these star-forming galaxies will be strongly lensed. Brightness amplification and stretching of their sizes will make it possible to investigate (by means of follow-up observations with high-resolution instruments like ALMA, JWST, and SKA) their internal structure and dynamics on the scales of giant molecular clouds (40–100 pc) at high redshifts. This will provide direct information on the physics driving the evolution of star-forming galaxies.

Gravitational lensing will also allow us to overcome the difficulty at obtaining direct information on AGN-driven outflows at high z, a key ingredient of current galaxy evolution models. This has been demonstrated by ALMA spectroscopy of strongly lensed galaxies detected by Planck and SPT surveys (Cañameras et al. 2017, 2018; Spilker et al. 2018; Jones et al. 2019). A velocity resolution of 40–50 km/s at z=3 has been achieved, to be compared with predicted outflow velocities of order 1000 km/s (King & Pounds 2015).

Herschel-ATLAS cutout at 350mm filtered at 1’ resolution. Detected sources include local star-forming galaxies and the strongly lensed galaxy SDP.81, at z=3, observed with ALMA as part of its long baseline verification campaign.

Galaxy Protoclusters

The arcmin resolution of the telescope at submm wavelengths is ideal for detecting the cores of galaxy proto-clusters, out to the epoch of reionisation. Due to the integrated emission of member galaxies, such objects (as well as strongly lensed sources) will dominate at the highest apparent far-IR luminosities. Tens of millions of these galaxy-clusters-in-formation will be detected at z ~2–3, with a tail extending out to z=7, and thousands of detections at 6 <z <7. Their study will allow us to track the growth of the most massive halos well beyond what is possible with classical cluster surveys (mostly limited to z < 1.5 - 2), tracing the history of star formation in dense environments and teaching us how star formation and galaxy-cluster formation are related across all epochs. The obscured cosmic star-formation-rate density of the Universe will thereby be constrained. Such a survey will overcome the current lack of spectroscopic redshifts of dusty star-forming galaxies and galaxy proto-clusters, representing a quantum leap in far-IR/submm extragalactic astrophysics.

This artist’s impression depicts the formation of a galaxy cluster in the early Universe. The galaxies are vigorously forming new stars and interacting with each other and known as Far Infrared/Sub-millimeter Galaxies or Dusty Star-Forming Galaxies and revealed by this proposed mission. Credit: ESO/M. Kornmesser. Courtesy from ESO Press Release October 2014 on Dannerbauer et al. (2014).

While the cores of low-z galaxy clusters are populated by passive elliptical galaxies, above z»1.5 a large fraction of member galaxies are in the dust-obscured star-formation phase. Therefore, searches for high-z proto-clusters are most conveniently carried out at sub-mm wavelengths. The false-color image of the proto-cluster core at z = 4, obtained from I-, K-band and IRAC imaging, is from Oteo et al. (2018). At 1’ resolution, this core will appear as a very bright blob with a SFR »1.4´104 Msun/yr

Cumulative redshift distributions of proto-clusters detected in at least one line by the ‘all-sky’ survey (2 yr, 90% of the sky; solid black line) and by the deep survey (6 months, 5% of the sky; dotted blue line) as predicted by the Negrello et al. (2017) model. We expect the detection of tens of millions of proto-clusters at z » 2 and of tens of thousands of them at z »6 [Figure from Negrello et al. 2020]


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