The central part of NGC 4636 is circular and is surrounded by an elongated fainter envelope, containing a large number of globular clusters. The galaxy has an
active galactic nucleus (AGN) that has been categorised as
LINER or a type 1.9
Seyfert galaxy. The source of nuclear activity in galaxies is suggested to be a supermassive black hole that accretes material. NGC 4636 harbors a relatively small
supermassive black hole with mass , as inferred from the bulge
velocity dispersion.
Molecular gas When imaged in CO(2–1) there appear
molecular clouds in NGC 4636. Cloud 1 is not associated with detectable optical emission and is out of the dust extinction map field of view, while cloud 2 is centered on a dust absorption knot and aligned with a ridge in the optical line emission map. The faint NGC 4636
ALMA continuum is in good agreement with the expected emission from cold dust, which would indicate that the dust content of NGC 4636 is fairly centrally located. The associated total
molecular mass is . The
ultraviolet emission of NGC 4636 exhibits O vi emission, which is a tracer of gas cooling. The measured emission indicates a cooling rate of 0.3 M⊙ yr−1.
Polycyclic aromatic hydrocarbons (PAH) emission was detected at 11.3 and 17 μm, as well as [Ne ii], [Ne iii], and [S iii] lines in the center of NGC 4636 (within re/8) using the
Spitzer IRS. The far
infrared emission of the galaxy, as observed by the
Infrared Space Observatory is 50 times more than expected based on stellar emission alone. This strongly suggests that there is dust, probably accreted in a recent merger with a gas-rich galaxy.
Hα observations reveal the presence of warm (T ~ 104 K) ionized gas in the inner kpc of NGC 4636. Spectra of this gas indicate irregular motion, with a typical velocity of 150–200 km/s. Hα maps of the galaxy core show the presence of a cavity in the distribution of the ionized gas encircled by a dense shell located at a distance of ~400 pc from the center. Again, the most plausible explanation is gas expansion caused by AGN activity. In NGC 4636, the [C ii] emission extends to a radius of ~1 kpc and is centrally peaked. The velocities inferred from the [C ii] line are consistent with those measured for the Hα line. Finally, NGC 4636 has an excess of cold dust, approximately cospatial with the ionized and molecular gas. As above, this dust is expected be embedded in cold gas, to be protected against rapid sputtering. The extended dust distribution originates from the ejection of cold gas by AGN activity 10 Myr ago.
Globular clusters NGC 4636 is characterised by its large number of
globular clusters, much larger than that of galaxies with similar size located not in the centre of galaxy clusters. The total number of globular clusters within a radius of 14 arcminutes is estimated to be 4,200 ± 120 and within 7 arcminutes is estimated to be 3,500 ± 170. In comparison, globular clusters orbit around
Messier 87, the giant elliptical galaxy at the centre of the Virgo Cluster, and 150–200 lie in and around the
Milky Way. The number of globular clusters drops abruptly at 7 and 9 arcminutes, probably indicating the edge of the galaxy. The velocity dispersion of the clusters is km/s, with the velocity dispersion of the blue clusters being slightly larger. This velocity dispersion is similar to that of
Messier 60, which is, however, a brighter galaxy. Comparing the velocity dispersion of the globular clusters with the stellar one it is calculated that mass-to-light ratio is not constant, but should increase as the galactocentric distance increases, indicating the existence of an extended
dark matter halo in NGC 4636.
X-ray emission File:Chandra image of NGC 4636 with Hα+Nii contours.jpg|thumb|Chandra image of the core of NGC 4636 with superimposed contours of Hα+[N ii] emission. White crosses mark the detected CO cloud positions. NGC 4636 is one of the most luminous nearby elliptical galaxies when observed in
X-rays, with estimated X-ray flux of erg/s. A hot gas corona around the galaxy was first detected by the
Einstein Observatory. Based on the hot
interstellar medium temperature profile, the total mass of the halo was estimated to be within a radius of 35 kpc. The percentage of non-luminous matter mass is estimated to be between 50% and 80% of the total galactic mass, implying an exceptionally low
baryon fraction in NGC 4636 and the presence of a large
dark matter halo. The halo of NGC 4636 has some unique features. Observations by the
Chandra X-ray Observatory revealed symmetric, 8 kpc long structures within the halo that look like
spiral arms. The arms are about 30 percent hotter than the surrounding gas cloud. The arms form the rim of two large ellipsoid bubbles of hot gas. One more bubble-like feature has been detected about 2 kpc south of the northeastern arm. A weak radio source, elongated in the NE–SW direction, connects the NE and SW bubbles. These large bubbles are likely the result of shocks generated by the AGN jets. It is possible that the bubbles have different ages, generated by different AGN outbursts, as indicated by the presence of radio-emitting plasma in one cavity, while the others are radio-quiet. NGC 4636 has an X-ray-bright core, having a radius of ~1 kpc. The core shows a central cavity surrounded by a bright edge. and is likely generated by the jet. Thus, the X-ray and radio observations point to a scenario in which gas may be currently outflowing in the central kpc of NGC 4636. == Supernovae ==