FABRY-PEROT IMAGES OF NGC 1275 AND ITS PUZZLING HIGH-VELOCITY SYSTEM

We report the first images obtained with the Goddard Fabry-Perot imager, a very sensitive and tunable imaging system designed to achieve the high levels of performance required in the optical studies of faint emission-line extragalactic objects. A velocity sequence of calibrated narrow-band CCD images (FWHM approximately 7 angstrom) has been obtained to cover the 3000 km s-1 velocity space between the redshifted H-alpha-emission lines of NGC 1275 (the central dominant galaxy of the Perseus Cluster), its extended associated system of low-velocity (LV) filaments and the high-velocity (HV) system of knots, projected on the same line of sight in the sky. The lack of intermediate-velocity emission-line gas between the two systems leads to an upper limit of 1.5 x 10(-16) ergs cm-2 s-1 arcsec-2 (3-sigma) on stripped ionized gas due to dynamical interaction between NGC 1275 and its high-velocity companion galaxy. It also confirms the previous reports that the level of continuum light arising from stellar or nonstellar sources must be very low in otherwise bright, strongly concentrated emission-line knots with unresolved diameters of 425 h-1 pc. The two galaxies are well separated kinematically. We have measured the H-alpha-luminosities of the emission-line regions of the two systems and have derived star formation rates in order to investigate quantitatively the physical relation between the HV galaxy, NGC 1275, and the surrounding cooling flow filaments. We found that the 3' diameter region around NGC 1275 has a total observed H-alpha-luminosity of 5 x 10(41) h-2 ergs s-1, a factor of 2.5 lower than a currently popular value (based on 1983 data). On the other hand, our measurements of central regions match other published values. Implications of this new measurement are discussed in the context of cooling flows. Corrected for Galactic foreground absorption, the H-alpha-luminosity of NGC 1275 of L(G)(H-alpha) approximately 1.2 x 1.2 x 10(42) h-2 ergs s-1 is comparable to those of other strong cooling flows. A normal initial mass function (IMF) for star formation in NGC 1275 is consistent with published X-ray, UV, and optical data in this paper. In particular, the revised H-alpha-luminosity of a strong emission-line extranuclear region can be reconciled with the limit on its continuum UV flux at 1250 angstrom; therefore, the exclusion of stars more massive than 25 M. is no longer needed. Also, the hypothesis of a low-mass cutoff to the IMF at 2 M., which had been introduced to reconcile the star formation rate (SFR) derived from the H-alpha-luminosity with the mass deposition rate of X-ray gas (M(x)), is not needed when the revised value SFR approximately 10 h-2 M. yr-1 is compared with the similar value M(x) approximately 7.5-15 h-2 M. yr-1 within the 30" radius central region. In the HV galaxy, the H-alpha-luminosities of 13 regions have been measured in six narrow-band images, leading to a total observed luminosity L(H-alpha) = 7.2 x 10(40) h-2 ergs s-1 over a surface area of 877 arcsec2. The star formation rate SFR approximately 1 h-2 M. yr-1 corresponds to a total stellar mass of 12 x 10(6) h-2 M. if massive stars ionize the H-alpha-knots. The collision of gas clouds in a heterogeneous intracluster medium with the fast-moving HV galaxy has been modeled. Following the collision, the HV interstellar medium becomes violently perturbed, with giant expanding cavities sweeping considerable masses of gas. Star formation with a 2-4 h-2 % efficiency in the HV shocked disk supershells may occur under favorable conditions 1-8 Myr after the collision event.