Sunday, April 11, 2021

All about the Messier 94 galaxy

Messier 94 (abbreviated to M94 and also known as NGC 4736) is a spiral galaxy, located at a distance of around 15 million light years in the constellations of Canes Venatici. It was discovered on the 22nd of March 1781 by Pierre Mechain. 

The galaxy has a Low Ionization Nuclear Emission Region (LINER) nucleus. Thus, in its optical spectra low ionization emission lines can be detected.

The most striking feature of M94 as seen clearly from its optical images is that the galaxy has two ring structures. The inner ring has a diameter around 5,400 light years, while the outer ring is around 45,000 light years. The rings have formed most likely at resonance points in the disk of the galaxy.

The inner ring is an area of intense star formation, thus it is classified as a starburst region. Responsible for the starburst is gas that flows into the inner ring through an oval shaped and bar like structure (note that the galaxy does not have an actual bar). 

Observations in the ultraviolet and infrared have shown that the outer ring is not a closed structure, but a complex structure of spiral arms. Additionally, observations have confirmed that this is an active region. The outer ring contains around a quarter of the total stellar mass of the galaxy and it contributes around 10% of the new stars. If we measure star formation in units of stellar mass, then the star formation rate of the outer ring is two times higher than the inner. 

The origin of the outer disk is not known, but most likely its formation has to do with an external event. Thus, the outer disk could be the result of cannibalization of a satellite galaxy, or due to gravitational interaction with a nearby stellar system. So far none has been confirmed and there are several issues with both scenarios.

A typical spiral galaxy consists of a disk of young stars and hot gas that intersects a large bulge of older stars. But, there are spiral galaxies that do not have a bulge. In contrast, they contain a bright central structure with intense star formation, known as pseudobulge. Such a region looks like a bulge when the galaxy is observed face on. The same is the case with M94. Its pseudobulge has the form of a ring around a central oval-shaped region.

Image 1: Optical image of Messier 94 taken with the 0.5-metre telescope at Blackbird Observatory. The image was created using broadband filters.

Image 2: Composite infrared and ultraviolet image of Messier 94. The image was created using infrared data from Spitzer Space Observatory (red, blue, and green) and ultraviolet data from Galaxy Evolution Explorer (purple). With yellow color near the centre, we can see ultraviolet emission from hot young stars, together with infrared emission from gas clouds and dust near the stars.

Image 1 Credit:  R Jay Gabany (Blackbird Obs.)

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