M95, M96, and M105 make up the dominant members of the Leo I galactic group which also includes some fainter galaxies (NGCs 3299, 3377, 3384, 3412 , 3489 , UGC 5889). This sparse grouping of nearby galaxies has proved to be of great importance during the last decade in establishing a standard scale for the measurement of extragalactic distances. Radio observations during the 1980's revealed an enormous ring of neutral hydrogen (HI), orbiting M105, the center of the Leo I group. The cloud which spans some 650,000 light years is believed to be a primordial remnant left over from the formation of the group. Evidence that the giant ring was interacting with M96 suggested that M96, M95 and the M105 group all lie at the same distance. The close relationship of this grouping of bright spirals along with the closest elliptical galaxy to our sun (M105) has provided an unusually clean route of determining the all important and elusive "Hubble Constant". The Hubble Constant "H" gives the rate of recession of galaxies per unit distance away. The finding by Edwin Hubble that distant galaxies are receding more rapidly than closer ones was instrumental to the development of the Big Bang Theory. The refinement of the Hubble constant has great implications for cosmology and our knowledge about the ultimate fate of the universe.
A primary goal of the Hubble Space Telescope
(HST) has been to establish the extragalactic distance scale through
the identification of Cepheid variable stars in relatively nearby
galaxies which cannot be resolved by earth based telescopes. Cepheid
variables are stars that vary their brightness (pulsate) with
a period from one to fifty days.
A fortuitous peculiarity of cepheids is that their periodicity has a direct relationship with their luminosity. Therefore, astronomers can judge their distance with great precision knowing their periodicity and luminosity. A standard distance to luminosity relationship was first established for cepheids in the nearby Small Magellanic Cloud, the precise distance of which we know from parallax measurements. From the work of the HST the current estimate of the Hubble Constant is 69 +/-8 kilometers/second/Mpc.
By identifying Cepheid Variables in M95 and M96, the HST has established the distance to the Leo I group with excellent precision. In May of 1998 a type IA supernova occurred in M96. This event represented the only Type IA supernova to have appeared in a galaxy with a previously measured Hubble Space Telescope Cepheid distance. A remarkable opportunity arose to standardize the "distance to luminosity" relationship of Type IA supernovae. Since the light curves and spectra of these supernovae are consistent and their prodigious light output allows them to be observed at enormous distances, Type IA supernovae could now be used as a "standard candle" to measure the distances to far away galaxies with greater precision. The cosmological implications of this new measuring tool led to the discovery that we are living in an expanding universe that is not slowing down but actively accelerating. The acceleration began for unknown reasons when the universe was somewhat older than half its present age and continues today.
M95 is a prototypical ringed-barred system. The association of rings of HII regions and young stars with the presence of a central bar has long been known and is believed to be related to the phenomenon of "orbital resonances" (see NGC 4725). M95 possesses two rings, an inner nuclear ring and an outer ring of HII regions and young stars surrounding its stellar bar. Although M95 shows a high concentration of O and B-type stars it doesn't quite pack the overall luminosity to qualify as a starburst galaxy. A conspicuous feature of M96 is the faint outer spiral arm which encircles the galaxy with an overall diameter of about 100,000 light years. M105 is the closest normal elliptical galaxy to our sun. Investigations of M105 have revealed evidence of a supermassive black hole in its nucleus of some 50 million solar masses. M105 forms a visual trio with NGC 3389 and NGC 3384. NGC 3389 shows a redshift twice that of the other two galaxies suggesting that it is not related but actually resides much further away.