ALMA explores giant molecular clouds in nearby galaxy NGC 1387

An international team of astronomers has employed the Atacama Large Millimeter/submillimeter Array (ALMA) to investigate molecular gas in a nearby galaxy known as NGC 1387. Results of the observational campaign, published Feb. 3 in Monthly Notices of the Royal Astronomical Society, deliver important insights regarding the properties of giant molecular clouds of this galaxy.

Interstellar gas reservoirs

Molecular clouds are huge complexes of interstellar gas and dust left over from the formation of galaxies, composed mostly of molecular hydrogen. Such clouds with masses greater than 100,000 solar masses are called giant molecular clouds (GMCs). In general, GMCs are 15–600 light years in diameter and are the coldest and densest parts of the interstellar medium.

Observations show that GMCs are gas reservoirs where most star formation takes place. Therefore, studying their lifecycle and characteristics is crucial to advancing our knowledge about the formation and evolution of galaxies.

Nearby galaxy with a rotating molecular gas disk

Located some 62.9 million light years away in the Fornax Cluster, NGC 1387 (also known as FCC 184) is an early-type lenticular galaxy. It has a size of about 60,000 light years and its mass is approximately 50 billion solar masses.

Previous observations have found that NGC 1387 has a total molecular gas mass of about 320 million solar masses and hosts a smooth, symmetric and regularly rotating molecular gas disk.

The obtained data indicate that the stars and the molecular gas of NGC 1387 are co-rotating, and that the center of the galaxy is a low-ionization nuclear emission-line region. The star formation rate of this galaxy is estimated to be within the range of 0.008 and 0.082 solar masses per year.

A team of astronomers led by Fu-Heng Liang of the University of Oxford, UK, decided to take a closer look at NGC 1387, aiming to get more insights into the gas content of this galaxy. For this purpose, they conducted high-resolution observations of NGC 1387 with ALMA, as part of the mm-Wave Interferometric Survey of Dark Object Masses (WISDOM) project.

Exploring a large sample of GMCs

The observations allowed Liang’s team to identify 1,285 GMCs in NGC 1387. The clouds in this sample have a mean radius of about 65 light years and a mean molecular gas mass of some 316,000 solar masses. The average surface density of the molecular gas in these clouds was estimated to be 251 solar masses/pc².

The study found that the mass spectrum (the cumulative probability distribution function of GMC mass) of the identified clouds in NGC 1387 has a slope of −1.8, which is comparable to the population of GMCs in the Milky Way’s disk. The cut-off mass of about 1.5 million solar masses indicates a lack of high-mass GMCs in NGC 1387, therefore similar to that of the outer Milky Way GMCs.

The observations also found that the internal rotation of the GMCs in NGC 1387 does not seem to arise from the large-scale circular rotation of the galaxy. In addition, massive or large GMCs at small galactocentric distances do appear to be better coupled to the large-scale galactic rotation.

According to the authors of the paper, the new results show that early-type galaxies like NGC 1387 have more diversified GMC properties than previously thought. Thus, they propose multi-molecule multitransition observations of the cold molecular gas in a larger sample of early-type galaxies to advance our knowledge in this field.

© 2026 Science X Network