Recently, Prof. ZHAO Yinghe from Yunnan Observatories of the Chinese Academy of Sciences and his collaborators reported research progress on the properties of star formation and interstellar medium for the submillimeter galaxy (SMG) LESS 073, which lies at a redshift of z=4.755 (when the universe is only about 1/11 of the current age). Their study was published in The Astrophysical Journal.

Star formation (SF) transforms gas into stars, and thus is one of the most fundamental drivers of galaxy evolution. However, it is difficult and resource intensive to have a full description of the SF process for very distant sources, for example, SMGs, which have been discovered at submillimeter wavelengths.

In 2015, Prof. ZHAO and the collaborators proposed that several spectral lines emitting at the submillimeter band can be used to effectively derive the star formation properties, such as star formation ratio (SFR), molecular gas mass, and far-infrared color (f60/f100, i.e., the ratio of the flux density at 60 to 100 micron) of the dust continuum.

In this study, utilizing that method and using the [CII] line at 158 micron emitting by the ionic carbon, the [NII] line at 205 micron emitting by the ionic nitrogen, and the CO J=7-6 line at about 370 micron, for LESS 073, the researchers obtained its total infrared luminosity, i.e., total energy emitting at the infrared about 6,000 billion times of the solar luminosity, dust temperature about 58 Kelvin (-215 degrees Centigrade), SFR about 760 solar mass per year, and total molecular mass of 33 billion times of the solar mass, indicating that the gas will be used up within 43 million years.

Besides, they found that f60/f100 derived from the relations between the ratios of the aforementioned lines and f60/f100, is well consistent with that from the observed far-infrared spectral energy distribution of LESS 073, confirming that the line-ratio based methods advocated in 2015 can be used to practically derive f60/f100 for distant sources.

Such a large f60/f100 about 0.9 in LESS 073 indicates that there should exist intense star-forming activities, because it needs many young, massive stars to produce uv photons to heat up the dust.

By comparing the CO J=7-6 emission with other sources from the literature, at last, the researchers found that LESS 073 is among those having the highest excitation CO spectral line energy distribution, and SF-unrelated shocks should play a role in the excitation of its gas emission, which might limit the use of the CO J=7-6 to derive SFR for such kind of sources.