We present Hubble Space Telescope ( HST ) spectroscopy of the nucleus of M 31 obtained with the Space Telescope Imaging Spectrograph ( STIS ) .
Spectra that include the Ca II infrared triplet ( \lambda \simeq 8500 Å ) see only the red giant stars in the double brightness peaks P1 and P2 .
In contrast , spectra taken at \lambda \lambda \simeq 3600 – 5100 Å are sensitive to the tiny blue nucleus embedded in P2 , the lower-surface-brightness nucleus of the galaxy .
P2 has a K-type spectrum , but we find that the blue nucleus has an A-type spectrum – it shows strong Balmer absorption lines .
Hence , the blue nucleus is not blue because of AGN light but rather because it is dominated by hot stars .
We show that the spectrum is well described by A0 giant stars , A0 dwarf stars , or a 200-Myr-old , single-burst stellar population .
White dwarfs , in contrast , can not fit the blue nucleus spectrum .
Given the small likelihood for stellar collisions , recent star formation appears to be the most plausible origin of the blue nucleus .
In stellar population , size , and velocity dispersion , the blue nucleus is so different from P1 and P2 that we call it P3 and refer to the nucleus of M 31 as triple .

Because P2 and P3 have very different spectra , we can make a clean decomposition of the red and blue stars and hence measure the light distribution and kinematics of each uncontaminated by the other .
The line-of-sight velocity distributions of the red stars near P2 strengthen the support for Tremaine ’ s ( 1995 ) eccentric disk model .
Their wings indicate the presence of stars with velocities of up to ~ { } 1000 km s ^ { -1 } on the anti-P1 side of P2 .

The kinematic properties of P3 are consistent with a circular stellar disk in Keplerian rotation around a supermassive black hole .
If the P3 disk is perfectly thin , then the inclination angle i \simeq 55 ^ { \circ } is identical within the errors to the inclination of the eccentric disk models for P1 + P2 by Peiris & Tremaine ( 2003 ) and by Salow & Statler ( 2004 ) .
Both disks rotate in the same sense and are almost coplanar .
The observed velocity dispersion of P3 is largely caused by blurred rotation and has a maximum value of \sigma = 1183 \pm 201 km s ^ { -1 } .
This is much larger than the dispersion \sigma \simeq 250 km s ^ { -1 } of the red stars along the same line of sight and is the largest integrated velocity dispersion observed in any galaxy .
The rotation curve of P3 is symmetric around its center .
It reaches an observed velocity of V = 618 \pm 81 km s ^ { -1 } at radius 0 \farcs 05 = 0.19 pc , where the observed velocity dispersion is \sigma = 674 \pm 95 km s ^ { -1 } .
The corresponding circular rotation velocity at this radius is \sim 1700 km s ^ { -1 } .
We therefore confirm earlier suggestions that the central dark object interpreted as a supermassive black hole is located in P3 .

Thin disk and Schwarzschild models with intrinsic axial ratios b / a _ { < } \atop { { } ^ { \sim } } 0.26 corresponding to inclinations between 55 ^ { \circ } and 58 ^ { \circ } match the P3 observations very well .
Among these models , the best fit and the lowest black hole mass are obtained for a thin disk model with M _ { \bullet } = 1.4 \times 10 ^ { 8 } M _ { \odot } .
Allowing P3 to have some intrinsic thickness and considering possible systematic errors , the 1- \sigma confidence range becomes ( 1.1 to 2.3 ) \times 10 ^ { 8 } M _ { \odot } .
The black hole mass determined from P3 is independent of but consistent with Peiris & Tremaine ’ s mass estimate based on the eccentric disk model for P1 + P2 .
It is \sim 2 times larger than the prediction by the correlation between M _ { \bullet } and bulge velocity dispersion \sigma _ { bulge } .
Taken together with other reliable black hole mass determinations in nearby galaxies , notably the Milky Way and M 32 , this strengthens the evidence that the M _ { \bullet } – \sigma _ { bulge } relation has significant intrinsic scatter , at least at low black hole masses .

We show that any dark star cluster alternative to a black hole must have a half-mass radius _ { < } \atop { { } ^ { \sim } } 0 \farcs 03 = 0.11 pc in order to match the observations .
Based on this , M 31 becomes the third galaxy ( after NGC 4258 and our Galaxy ) in which clusters of brown dwarf stars or dead stars can be excluded on astrophysical grounds .