XSMT-15m: Chinese 15m Submillimeter Telescope

• XSMT-15m is a state-of-the-art 15-meter submillimeter telescope designed for high-sensitivity and high-resolution observations of Galactic and extragalactic phenomena.
• It utilizes a multi-band continuum camera and dual-polarization heterodyne receivers to perform deep surveys and precise spectral line studies under optimal high-altitude conditions.
• The facility bridges northern hemisphere observational gaps and drives domestic innovation by complementing global observatories in submillimeter astronomy.

The Xue-shan-mu-chang 15-meter Submillimeter Telescope (XSMT-15m) is a state-of-the-art, single-dish facility designed for high-sensitivity and high-resolution submillimeter astronomical observations. Located at 4813 meters altitude in Qinghai, China, it marks the mainland’s first independently developed, world-class submillimeter platform, enabling Chinese researchers direct access to scientific frontiers previously inaccessible due to limited domestic instrumentation. The XSMT-15m combines advanced detection technologies with an optimal atmospheric site to address cutting-edge questions in extragalactic physics, Galactic ecology, astrochemistry, and time-domain astrophysics. Its construction and design reflect international best practices and establish a strategic complement to southern hemisphere capabilities.

1. Site Characteristics and Observing Conditions

The XSMT-15m will be situated at a premier high-altitude location (4813 m) in Qinghai, selected for its exceptionally thin atmosphere and very low precipitable water vapor (PWV). The atmospheric absorption profile at this altitude is notably favorable for submillimeter observations, especially at frequencies above ~300 GHz, where atmospheric transmission is typically limited by water vapor. Low PWV maximizes the transmission in all major submillimeter bands, allowing deep continuum imaging and sensitive spectroscopy across multiple frequency ranges.

The telescope’s 15-meter dish supports diffraction-limited angular resolution determined by $\theta \approx \lambda / D$. For instance, at 230 GHz ($\lambda = 1.3\,\text{mm}$), the resolution approaches $18''$, with even finer resolution accessible at shorter wavelengths. These metrics ensure the XSMT-15m offers competitive imaging performance critical for both large-scale surveys and detailed mapping.

2. Instrumentation Architecture

The XSMT-15m integrates several classes of instrumentation optimized for submillimeter science:

• Multi-band, large-format continuum camera: Based on kinetic inductance detectors (KIDs), the camera operates simultaneously across bands near 230, 345, and 460 GHz. Its array, containing more than 10,000 pixels over a wide field of view, is engineered for deep surveys of dust continuum emission. The polarimetric capability is available for mapping magnetic field orientations via polarized continuum emission.
• Heterodyne receiver suite: Incorporates dual-polarization elements covering four bands (85–115, 211–275, 275–373, and 440–510 GHz). The receivers are designed for intermediate frequency (IF) coverage in the 4–8 GHz range with sideband rejection over 15 dB, supporting high-resolution spectral line studies. This enables precise kinematic and chemical diagnostics via molecular line observations.

Continuum and spectral line capabilities are unified under stable calibration and rapid mapping speeds due to the underlying instrument technology, supporting both large-area surveys and targeted high-fidelity science.

Instrument Type	Frequency Coverage (GHz)	Pixel/Channel Count
KID-based Continuum Camera	230 / 345 / 460 (simultaneous)	>10,000 pixels
Heterodyne Receivers (Dual Polarization)	85–115, 211–275, 275–373, 440–510	Multiple IF channels, high SBR

3. Scientific Objectives

The XSMT-15m addresses an array of forefront science topics:

• Extragalactic Astronomy: Large-scale continuum surveys of nearby galaxies and Local Group members allow multi-band tracing of dust spectral energy distributions (SEDs), constraining dust temperature, emissivity index $\beta$, and ISM mass. Multi-band photometry and spectroscopy clarify the regulation of galaxy formation and evolution by interactions between gas, dust, and star formation processes.
• Galactic Structure: High-resolution dust continuum and molecular line mapping on parsec scales supports unbiased GMC surveys. By profiling transitions such as CO and [C I], the XSMT-15m elucidates the distribution, kinematics, and evolutionary processes of Galactic molecular gas and dust.
• Time-Domain Astrophysics: The XSMT-15m opens previously inaccessible submillimeter windows to time-variable phenomena, including protostellar variability, episodic accretion events, and transient sources (e.g., GRBs, supernovae, gravitational wave counterparts). Monthly—or better—cadence mapping is feasible, providing a robust platform for monitoring and discovery.
• Astrochemistry: Sensitive spectral line surveys through advanced heterodyne techniques enable the paper of physical and chemical mechanisms shaping the ISM, including molecule formation/destruction and feedback processes in diverse environments such as star-forming regions and evolved stars. This allows characterization of complex chemical networks crucial to understanding star and planet formation.

4. Synergistic Operations and Global Network Integration

XSMT-15m is designed to operate synergistically with other major observatories:

• Its wide-field survey capacities complement high-resolution interferometric arrays such as ALMA, NOEMA, and future instruments like ngEHT, particularly for follow-up and multi-scale studies.
• Coordinated programs with space observatories, including Herschel and JWST, integrate dust SED measurements and emission line diagnostics to support comprehensive multiwavelength models of galaxy evolution and star formation.
• The telescope’s coverage of northern sky regions fills a critical gap in the global submillimeter survey landscape, aligning with southern hemisphere facilities (e.g., CCAT-prime) and participating in very long baseline interferometry (VLBI) as part of emerging next-generation EHT arrays.

5. Technological and Scientific Impact on Chinese Astronomy

As the first independently developed, world-class submillimeter facility in mainland China, the XSMT-15m substantially elevates national research capability in the millimeter/submillimeter regime. The project establishes a technological and scientific asset to support domestic and international collaboration, particularly due to its complementary sky access vis-à-vis southern hemisphere projects.

The deployment of advanced superconducting detectors and dual-polarization heterodyne systems is expected to foster domestic innovation. The facility also functions as a training ground for Chinese astronomers, stimulating professional development and positioning China as a major contributor to global astronomical research.

A plausible implication is that by bridging observational gaps and advancing technological expertise, the XSMT-15m can serve as both a research catalyst and a focal point for international partnerships in submillimeter science.

6. Technical Performance Metrics and Calculation Examples

The XSMT-15m’s angular resolution is determined by the diffraction limit: $\theta \approx \frac{\lambda}{D}$ At $\lambda = 1.3\,\text{mm}$ ($230\,\text{GHz}$), for $D = 15\,\text{m}$, this yields: $$\theta \approx \frac{1.3 \times 10^{-3}\,\text{m}}{15\,\text{m}} \approx 8.7 \times 10^{-5}\,\text{radians} \approx 18''$$ Finer resolutions are achievable at shorter wavelengths.

The heterodyne instrumentation’s IF coverage (4–8 GHz) and sideband rejection (>15 dB) support rigorous line identification and velocity measurements, enabling precise astrochemical and kinematic analysis.

7. Summary and Future Prospects

By leveraging its optimal high-altitude location, advanced instrumentation, and broad scientific scope, the XSMT-15m is positioned to advance knowledge from the mass assembly of galaxies to the dynamics of the Galactic ISM, transient phenomena, and interstellar chemistry. Operating in synergy with world-class observatories, the facility bridges gaps in submillimeter coverage and contributes significantly to Chinese and northern hemisphere astronomy. The XSMT-15m is anticipated to drive both fundamental research and technological development, inaugurating a new era of submillimeter science focused on the cold, dusty, and dynamically active universe (Group et al., 17 Sep 2025).