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We present Hubble Space Telescope (HST ) spectroscopy of the nucleus of M31 obtained with the Space Telescope Imaging Spectrograph (STIS). Spectra that include the Ca ii infrared triplet (k ' 8500 8) see only the red giant stars in the double brightness peaks P1 and P2. In contrast, spectra taken at k ' 3600 5100 8 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 blue not 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, cannot 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 M31 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 eccentric disk model. Their wings indicate the presence of stars with velocities of up to 1000 km s À1 on the anti-P1 side of P2.The kinematics 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 ' 55 is identical within the errors to the inclination of the eccentric disk models for P1+P2 by Peiris & Tremaine and by Salow & Statler. 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 ¼ 1183 AE 201 km s À1 . This is much larger than the dispersion ' 250 km s À1 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 ¼ 618 AE 81 km s À1 at radius 0B05 ¼ 0:19 pc, where the observed velocity dispersion is ¼ 674 AE 95 km s À1 . The corresponding circular rotation velocity at this radius is $1700 km s À1 . 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 P 0:26 corresponding to inclinations between 55 and 58 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 ¼ 1:4 ; 10 8 M . Allowing P3 to have some intrinsic thickness and considering possible systematic errors, the 1 confi...

We present evidence that high-velocity cloud (HVC) Complex C is a low-metallicity gas cloud that is plunging toward the disk and beginning to interact with the ambient gas that surrounds the Milky Way. This evidence begins with a new high-resolution (7 km s −1 FWHM) echelle spectrum of 3C 351 obtained with the Space Telescope Imaging Spectrograph (STIS). 3C 351 lies behind the low-latitude edge of Complex C, and the new spectrum provides accurate measurements of O I, Si II, Al II, Fe II, and Si III absorption lines at the velocity of Complex C; N I, S II, Si IV, and C IV are not detected at 3σ significance in Complex C proper. However, Si IV and C IV as well as O I, Al II, Si II and Si III absorption lines are clearly present at somewhat higher velocities associated with a "high-velocity ridge" (HVR) of 21cm emission. This high-velocity ridge has a similar morphology to, and is roughly centered on, Complex 10 Throughout this paper we express abundances with the usual logarithmic notation, [X/Y] = log(X/Y) − log(X/Y) ⊙ , and we indicate the overall metallicity with the variable Z.

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The Space Telescope Imaging Spectrograph (STIS) instrument was installed on the Hubble Space Telescope (HST) during the second servicing mission, in 1997 February. Four bands cover the wavelength range of 115-1000 nm, with spectral resolving powers between 26 and 200,000. Camera modes are used for target acquisition and deep imaging. Correction for HST's spherical aberration and astigmatism is included. The 115-170 nm range is covered by a CsI MAMA (Multianode Microchannel Array) detector and the 165-310 nm range by a Cs 2 Te MAMA, each with a format of pixels, while the 305-555 and 550-1000 nm ranges are 2048 # 2048 covered by a single CCD with a format of pixels. The multiplexing advantage of using these two-1024 # 1024 dimensional detectors compared with the pixel detectors of the first-generation spectrographs is 1 or 2 1 # 512 orders of magnitude, depending on the mode used. The relationship between the scientific goals and the instrument specifications and design is discussed.