Key Science Cases
The LST will contribute to research on a wide range of topics in the fields of astronomy and astrophysics, e.g., investigations of the chemical evolution from protostellar cores to protoplanetary disks, star formation in our Galaxy and galaxies, the evolution of galaxy clusters via the Sunyaev-Zel'dovich (SZ) effect, the search for submillimeter transients such as gamma-ray burst (GRB) reverse shocks produced at the epoch of re-ionization (EoR) via high cadence wide sky surveys, electromagnetic follow up of detected gravitational wave sources with extremely large positional uncertainty (on a scale of a few 10 sq-deg scale, even in the LIGO-VIRGO-KAGURA era), and examination of general relativity in the vicinity of supermassive black holes (SMBHs) via submillimeter VLBI. Details of these science cases will be given in the later subsection. Here, we describes some selected key science cases, which determine the major requirements for the telescope and focal plane instruments.
Charting the invisible sky
The Large Submillimeter Telescope (LST) will contribute to research on a wide range of topics in the fields of astronomy and astrophysics.
1. Exploration of cosmic star formation history and large-scale structures by uncovering galaxies using LST
One of the major scientific goals of the new telescope is to determine the complete history of star formation across cosmic time, by uncovering a statistically large number of dusty star-forming galaxies (e.g., Casey et al. 2014). By exploiting the state-of-the-art direct detector technologies, the LST, with a large FoV (0.5 degree diameter or larger), will facilitate groundbreaking exploration of an extremely large 3D volume of the universe and, also, completely new advancements in time-domain science for millimeter and submillimeter astronomy. The LST is very complementary to ALMA and can establish an unbiased census of metal-enriched high-redshift galaxies through wide-field spectroscopic and continuum surveys.
Here we focus on 3 key science cases to unveil the dust-obscured part of the cosmic SFRD history from the present-day Universe to an early phase of galaxy formation, i.e., up to the epoch of reionization (EoR).
Blind spectroscopic survey of CO, [CII], and [OIII] line emitting galaxies (CO/[CII]/[OIII] tomography)
A search for Pop-III long Gamma-ray bursts by a high-cadence continuum imaging survey
Uncovering dusty star-forming galaxies during the EoR by confusion-limited deep 1.3mm to 850μm surveys
2. Physics of clusters of galaxies and cosmology with the Sunyaev-Zel'dovich effect
Another science goal is astrophysical study with the observations of Sunyaev-Zel’dovich effect (SZE) in cluster of galaxies. The SZE is a powerful tool for detecting shocks and hot gas in the clusters, especially, violent cluster mergers. Spatial resolutions of the SZE images obtained so far are at arcmin scales except for a few cases. The large single dish telescope with small beam sizes, e.g., ~10 arcsec at 150 GHz, is hence promising and complementary tool for the high sensitivity SZE observations; the beam size just corresponds to the core scale of clusters, ~100 kpc. The following two projects have been raised in the working group as the important science case. One is high spatial resolution deep imaging of SZ clusters up to z ~ 1, which allow us to resolve its core scale with covering a cluster scale, ~100 Mpc. The imaging provides us with the mass of clusters and the structures, such as shocks produced by merging. Another is blind survey for high-z SZ clusters up to z ~ 2. The survey should be designed to obtain a large sample of high-z clusters, which are not well understood to date. Such a survey perhaps allows to understand the high-mass end of the LSS and the cluster mass function at z ~ 2, where cosmic star formation activity starts to decrease sharply toward z ~ 0, and also allows to test the Lambda-CDM cosmology.
3. Tracing gas physics in galaxy evolution
In present-day galaxies, most of their baryons are in a form of stars, like in the Milky
Way. The fraction of the interstellar medium (ISM) is from a few % for giant early-
type galaxies to at most 50 % for unevolved gaseous dwarfs(e.g. Roberts & Haynes,
1994). However, of course these galaxies have formed from a ubiquitous almost primordial
gas after the recombination of the Universe, and they must have been very gaseous in
their early phase of the evolution. One of the fundamental processes in galaxy formation
and evolution is therefore the transition from gas to stars. This gaseous side of galaxy
evolution was, despite of its importance, not focused for a long time mainly because of
the observational limitation. The LST will perfectly play a role as an ideal facility for large surveys, allowing us to study e.g., the evolution of K-S law with cosmic time, with good statistics.
4. Chemical evolution from protostellar cores to protoplanetary disks
Observations with large single-dish telescopes including the LST provide us with important and unique information on chemical processes occurring in molecular clouds, which cannot readily be obtained with large interferometers including ALMA. In view of a 10”-scale angular resolution achieved by LST, the main targets will be large scale distributions of molecules from a molecular cloud scale (a few pc) to a dense core scale (0.01 pc), which enable us to obtain high dynamic range maps of molecular abundances. It is indispensable to investigate whole chemical evolution from clouds to cores, and eventually to disks, for understanding and establishing the comprehensive view of the chemical evolution.
5. Magnetic fields and star formation
It is recently observed that magnetic fields (B-fields) in molecular clouds are quite ordered (Li et al. 2009), which implies that B-fields are dynamically important. The consequences of strong B-fields on molecular cloud shapes (Li et al. 2013; Tassis et al. 2009) and fragmentation (Li et al. 2015) are also observed. While ALMA is capable to resolve a disc and study disc structures, we need LST to study disc formation. The resolution of LST can reach kAU scale at the distance of the Gould belt and thus can efficiently map the vicinity fields at 10-kAU scale, which probes the fields close enough to the disc to study the formation environment yet far enough to stay away from the transition scale.
6. Planetary atmospheres
Highly frequency-resolved spectroscopy at submm-wave bands is a powerful tool
that may be used to probe the physical and chemical conditions of terrestrial planets, gaseous or icy planets, and comets in the solar system. On-the-fly mapping observations made by highly sensitive superconducting heterodyne detectors at submm-wave bands with LST will allow us to comprehensively determine the short-, middle-, and long-term changes in the three dimensional distributions of minor constituents driven by, as well as the photochemical reaction network, the dynamics of the planetary atmosphere and by solar activity. This study with LST will be complementary to the research with ALMA, as, for instance, it is difficult for ALMA to observe the night-side chemistry and dynamics of Venus, because the apparent diameter of the night-side (antisolar) disk ( 60′′) during interior conjunction is much greater than the resolve-out size of ALMA.
7. Mapping spectral line surveys twoard nearby galaxies
The Large Submillimeter Telescope (LST) is one of the best telescope for mapping spectral line survey toward nearby galaxies, because of its' large field of view and large collecting area. Chemical compositions revealed by the spectral mapping observations allow us to study evolutionary stages of giant molecular clouds (GMC) and dynamical effect on the GMCs. Understanding formation and evolution of the GMC is crucial to explore the evolution of galaxies.