The molecular gas content of the advanced S+E merger NGC 4441

Evidence for an extended decoupled nuclear disc?

¹ Astronomisches Institut der Ruhr-Universität Bochum, 44780 Bochum, Germany e-mail: eva.juette@astro.rub.de
² Astron, 7990AA Dwingeloo, The Netherlands
³ Onsala Space Observatory, Chalmers University of Technology, 439 92 Onsala, Sweden

Received: 29 February 2008
Accepted: 28 August 2009

Abstract

Context. Despite their importance to galaxy evolution, mergers between a spiral and an elliptical (S+E mergers) have been poorly studied so far. NGC 4441 is a nearby candidate of an advanced remnant of such a merger, showing typical tidal structures such as an optical tail and two shells as well as two H i tails.

Aims. Studying the molecular gas content provides clues about the impact of a recent merger event on the star formation. Simulations of S+E mergers yield conflicting predictions about both the strength and the extent of an induced starburst. Thus, observations of the amount and the distribution of the molecular gas, the raw material of star formation, are needed to understand the influence of the merger on the star formation history.

Methods. ¹²CO and ¹³CO (1-0) and (2-1) observations were obtained using the Onsala Space Observatory 20 m and IRAM 30 m telescope as well as the Plateau de Bure interferometer. These data allow us to carry out a basic analysis of the molecular gas properties, such as estimates of the molecular gas mass, temperature, and density and the star formation efficiency.

Results. The CO observations detect an extended molecular gas reservoir out to ~4 kpc, with a total molecular gas mass of  . Furthermore, high resolution imaging shows a central molecular gas feature, which is probably a rotating disc hosting most of the molecular gas ( ). This nuclear disc has a different sense of rotation to the large-scale H i structure, indicating a kinematically decoupled core. We modeled the state of the interstellar medium with the radiative transfer code RADEX, using the ratios of the ¹²CO to ¹³CO line strenghts. The results are consistent with a diffuse ( cm^-3) molecular medium with no significant indications of cold, dense cores of ongoing star formation. This is in agreement with the moderate star formation rate of measured for NGC 4441. Assuming a constant star formation rate, the gas depletion time is  10⁸ yr. NGC 4441 might be a nearby candidate early-type galaxy with a dominant A-star population, a so-called E+A galaxy, which is in a post-starburst phase several 10⁸ yr after a merger event.