We present a detailed investigation of the X-ray and optical properties of the starburst-merger system Arp 299 (NGC 3690, Mrk 171), with an emphasis on its spectacular gaseous nebula. We analyze ROSAT and ASCA X-ray data and optical spectra and narrowband images. The X-ray nebula in Arp 299 has a diameter of approximate to 45 kpc, is elongated roughly along the H I minor axis of the merging system, has an outer (inner) temperature of 2.3 (9) x 10(6) K, a luminosity of similar to 2 x 10(41) ergs s(-1), a gas mass of 7 x 10(9) M., and a thermal energy content of similar to 10(58) ergs. An additional hard X-ray component is present with a luminosity in the 0.1-10 keV band of similar to 4 x 10(41) ergs s(-1) most likely due to X-ray binaries or possibly inverse Compton scattering. The overall X-ray spectrum is consistent with that of other starbursts (see the work of Dahlem, Weaver, & Heckman). Compared with the X-ray nebula, the optical emission-line nebula is smaller (20 kpc) but much more luminous (similar to 10(44) ergs s(-1) approximate to 4% L-bol for Arp 299). The kinematics of the emission-line nebula are complex, with the line widths increasing systematically with decreasing H alpha surface brightness to values of 500-700 km s(-1) in the faint outer filaments. The relative strengths of the low-ionization forbidden lines ([S II], [N II], and [O I]) also increase systematically as the gas surface brightness decreases and as the line widths increase. We measure high gas pressures in the inner emission-line nebula (P/k similar to few times 10(6) K cm(-3)) that decline systematically with increasing radius. We suggest that the ongoing galaxy collision has tidally redistributed the interstellar medium of the merging galaxies. The optical emission-line nebula results as this gas is photoionized by radiation that escapes from the starburst and is shock heated, accelerated, and pressurized by a "superwind" driven by the collective effect of the starburst supernovae and stellar winds. Since empirically only those galaxy mergers that contain luminous starbursts have bright X-ray nebulae, this implies that the luminous X-ray nebula in Arp 299 is also powered by the starburst outflow (rather than by the collisions of gas clouds during the merger). This outflow can be most directly traced by its X-ray emission, which is plausibly a mass-loaded flow (similar to 10(2) M. yr(-1)) of adiabatically cooling gas that carries out a substantial fraction of the energy and metals injected by the starburst at a speed close to the escape velocity from Arp 299. The mass outflow rate exceeds the star formation rate in this system. We conclude that powerful starbursts are able to heat (and possibly eject) a significant fraction of the interstellar medium in galaxies that are the products of mergers.
