ALMA reveals giant molecular clouds across Needle galaxy’s full disk

An international team of astronomers has employed the Atacama Large Millimeter/submillimeter Array (ALMA) to perform high-resolution observations of the Needle galaxy. Results of the new observational campaign, presented April 15 on the arXiv preprint server, provide more insights into the properties of molecular gas in this galaxy.

A needle of interest

NGC 4565, known as the Needle galaxy due to its narrow profile, is a prominent example of an edge-on spiral galaxy. The galaxy has a diameter of about 176,000 light years and its mass is 80 billion solar masses. The distance to NGC 4565 is estimated to be some 39 million light years.

The Needle galaxy is considered as a Milky Way analog and also shares many similarities to the Andromeda galaxy. That is why a group of astronomers led by Grace Krahm of the Ohio State University decided to take a closer look at this galaxy with ALMA, focusing on the molecular gas in its interstellar medium (ISM). Such a study could be essential for learning about the processes that govern star formation, galaxy evolution, and the cycling of gas between different phases of the ISM.

ALMA digs into the Needle’s gas

“In this paper, we present high-resolution ALMA observations of ¹²CO(2-1) and ¹³CO(2-1) that provide a detailed, high-inclination view of molecular gas in NGC 4565,” the researchers write.

ALMA observations allowed Krahm’s team to resolve giant molecular cloud-scale structures across the entire molecular disk of the Needle galaxy, including in low-density outer regions where atomic gas makes up the majority of the ISM. This helped them examine how molecular cloud properties and the vertical structure of the ISM vary with galactocentric radius and local environment.

What do ALMA images show?

In particular, the observations found that the Needle galaxy has very little molecular gas inside its ring. This is followed by a disk dominated by molecular hydrogen and a hydrogen-iodide outer disk. In terms of radial profiles, the findings point to some similarity to the Andromeda galaxy and to the spiral galaxy NGC 2775.

The collected data indicate that the stellar, carbon monoxide, and mid-infrared radial profiles of NGC 4565 extend far out with long scale lengths, reflecting the galaxy’s high mass and large size. The ¹³CO/¹²CO line ratio was found to be approximately flat over a radius of 16,300–42,400 light years, consistent with uniform optical depth and isotopologue abundance across the galaxy’s disk.

The study found that the molecular disk of the Needle galaxy is thin, with minimal vertical flaring with increasing radius. When it comes to the giant molecular clouds (GMCs), it turns out that they are preferentially aligned with the major axis of the galaxy with moderate axis ratios of about 1.5.

In general, the GMCs in the Needle galaxy have sizes, velocity dispersions, surface densities, and virial parameters that generally align with the distribution of physical properties in less inclined galaxies.

East Ring Pileup

The observations also detected a prominent star-forming complex on the ring of the Needle galaxy, which corresponds to a high density of molecular gas. The authors of the paper named this complex the “East Ring Pileup.” The obtained images indicate that the “East Ring Pileup” includes a compact region dubbed the “Jewel,” which is bright at many wavelengths and comparable in density to Local Group starburst regions like 30 Doradus.

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