  Context. The LIRG Zw 049.057 contains a compact obscured nucleus where a
considerable amount of the galaxy's luminosity is generated. This nucleus
contains a dusty environment that is rich in molecular gas. One approach to
probing this kind of environment and to revealing what is hidden behind the
dust is to study the rotational lines of molecules that couple well with the IR
radiation emitted by the dust. Methods. We observed Zw 049.057 with PACS and
SPIRE onboard the Herschel Space Observatory in rotational lines of H2O, H218O,
OH, 18OH, and [O I]. We modeled the unresolved core of the galaxy using a
spherically symmetric radiative transfer code. Results. We present the full
SPIRE FTS spectrum of Zw 049.057, along with relevant spectral scans in the
PACS range. We find that a minimum of two different components (nuclear and
extended) are required in order to account for the rich molecular line
spectrum. The nuclear component has a radius of 10-30 pc, a very high infrared
surface brightness (1e14 Lsun/kpc2), warm dust (Td > 100 K), and a very large
H2 column density (NH2 = 1e24-1e25 cm-2). The modeling also indicates high
nuclear H2O (5e-6) and OH (4e-6) abundances relative to H2 as well as a low
16O/18O-ratio of 50-100. We also find a prominent infall signature in the [O I]
line. We tentatively detect a 500 km/s outflow in the H2O 313->202 line.
Conclusions. The high surface brightness of the core indicates the presence of
either a buried active galactic nucleus or a very dense nuclear starburst.nThe
H2O abundance is comparable to that of other compact (ultra-)luminous infrared
galaxies such as NGC 4418 and Arp 220 - and also to hot cores in the Milky Way.
The enhancement of 18O is a possible indicator that the nucleus of Zw 049.057
is in a similar evolutionary stage as the nuclei of Arp 220 - and more advanced
than NGC 4418.
