Messier 105

Lore & Background

Messier 105 was discovered by Pierre Méchain in 1781, just a few days after he discovered the nearby galaxies Messier 95 and Messier 96. It is one of a few objects not verified by Charles Messier, so it was omitted from the editions of his catalogue during his era. The galaxy was later appended when Helen S. Hogg found a letter by Méchain locating and describing this object, which matched those aspects under its first-published name, NGC 3379.

The galaxy has a morphological classification of E1, indicating a standard elliptical galaxy with a flattening of 10%. Its major axis is aligned along a position angle of 71°, and its isophotes are near perfect ellipses, twisting no more than 5° out of alignment with changes in ellipticity of no more than 0.06. There is no fine structure such as ripples in the isophotes. Observation of giant stars in the halo reveals two general populations: a dominant metal-rich subpopulation and a weaker metal-poor group.

Messier 105 is known to have a supermassive black hole at its core with a mass estimated between 1.4×10^8 and 2×10^8 M☉. It has a weak active galactic nucleus of the LINER type with a spectral class of L2/T2, meaning no broad Hα line and intermediate emission line ratios between a LINER and an H II region. The galaxy also contains a few young stars and stellar clusters, suggesting some elliptical galaxies still form new stars, but very slowly. Along with its companion NGC 3384, it is surrounded by an enormous ring of neutral hydrogen with a radius of 200 kiloparsecs and a mass of 1.8×10^9 M☉, where star formation has been detected. Messier 105 is one of several galaxies within the M96 Group (also known as the Leo I Group), a group of galaxies in Leo that also includes M95 and M96, and is one of the richest groups in the Local Volume, dominated by not one but several galaxies.

Reader's Guide

Messier 105 holds significance as the largest elliptical galaxy in the Messier catalogue that is not part of the Virgo cluster, making it a key object for studying elliptical galaxies outside dense cluster environments. Its discovery history is notable: it was found by Pierre Méchain in 1781 but omitted from Messier's original catalogue because Messier did not verify it; it was later appended after Helen S. Hogg discovered Méchain's letter describing it. The galaxy's morphological classification of E1 and its near-perfect isophotes with minimal twisting and no fine structure provide a clean example of a standard elliptical galaxy. The presence of a supermassive black hole at its core, with a mass between 1.4×10^8 and 2×10^8 M☉, and a weak LINER-type active galactic nucleus, adds to its astrophysical interest. The detection of a few young stars and stellar clusters suggests that even elliptical galaxies can form new stars, albeit very slowly. The enormous ring of neutral hydrogen surrounding M105 and its companion NGC 3384, with a radius of 200 kiloparsecs and a mass of 1.8×10^9 M☉, where star formation has been detected, highlights the galaxy's role in a dynamic group environment. As part of the M96 Group, one of the richest groups in the Local Volume, M105 contributes to understanding galaxy interactions and evolution in a group dominated by multiple large galaxies rather than a single dominant one.

Did You Know?

• Messier 105 is the biggest elliptical galaxy in the Messier catalogue that is not in the Virgo cluster.
• The galaxy was omitted from Messier's original catalogue and later appended after Helen S. Hogg found a letter by Méchain describing it.