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Context: Difference imaging has proven to be a powerful technique for detecting and monitoring the variability of unresolved stellar sources in M 31. Using this technique in surveys of galaxies outside the Local Group could have many interesting applications. Aims: The goal of this paper is to test difference imaging photometry on Centaurus A, the nearest giant elliptical galaxy, at a distance of 4 Mpc. Methods: We obtained deep photometric data with the Wide Field Imager at the ESO/MPG 2.2m at La Silla spread over almost two months. Applying the difference imaging photometry package DIFIMPHOT, we produced high-quality difference images and detected variable sources. The sensitivity of the current observational setup was determined through artificial residual tests. Results: In the resulting high-quality difference images, we detect 271 variable stars. We find a difference flux detection limit corresponding to m_R~24.5. Based on a simple model of the halo of Centaurus A, we estimate that a ground-based microlensing survey would detect in the order of 4 microlensing events per year due to lenses in the halo. Conclusions: Difference imaging photometry works very well at the distance of Centaurus A and promises to be a useful tool for detecting and studying variable stars in galaxies outside the local group. For microlensing surveys, a higher sensitivity is needed than achieved here, which would be possible with a large ground-based telescope or space observatory with wide-field imaging capabilities., 8 pages, 7 figures, accepted for publication in Astronomy and Astrophysics

The interstellar medium of Centaurus A (NGC 5128) has been studied extensively in recent years, using mostly molecular lines tracing low to medium density gas (500 to several 10(3) cm(-3)). The amount and distribution of the dense molecular gas was largely unknown. Here we present new millimeter data of the HCN(1-0), CS(2-1), and CS(3-2) rotational transitions towards the nearby radio galaxy Centaurus A observed with the SEST on La Silla, Chile. We obtained spectra of the HCN(1-0) emission which traces dense 10(4) cm-3 molecular gas at the center and along the prominent dust lane at offset positions +/-60 " and +/-100 ". We also obtained a few spectra of CS(2-1) and (3-2) tracing densities of similar to 10(5) cm(-3). The emission in these lines is weak and reaches a few mK at the available angular resolutions of 53 " - 36 ". At the central position, the integrated intensity ratio I(HCN)/I(CO) peaks at 0.064, and decreases to similar to 0.02 to 0.04 in the dust lane.Using the new high density tracer data, we estimate the amount, distribution and physical conditions of the dense molecular gas in the dust lane of Centaurus A. We find that Cen A and the Milky Way are comparable in their HCN(1-0) line luminosity. However, towards the nucleus the fraction of dense molecular gas measured via the line luminosity ratio L(HCN)/L(CO) as well as the star formation efficiency L-FIR/L-CO is comparable to ultra-luminous infrared galaxies (ULIRGs). Within the off-nuclear dust lane and for Cen A as a whole these quantities are between those of ULIRGs and normal and infrared luminous galaxies. This suggests that most of the FIR luminosity of Centaurus A originates in regions of very dense molecular gas and high star formation efficiency.

High signal to noise spectra of (CO)-C-12, (CO)-C-13 and (CO)-O-18 J = 1-0, (CO)-C-12 2-1 and (CO)-C-13 3-2 have been obtained toward the nucleus of Centaurus A. The (CO)-C-12 profiles reveal the presence of two superposed emission components. The narrower component (DELTA-V approximately 70-100 km s-1) is due to CO emission from the dust lane. The broader component (DELTA-V approximately 260 km s-1) represents CO emission from the cirumnuclear disk, whose presence was previously deduced from absorption line and infrared continuum observations. Analysis of the CO data suggests excitation temperatures of about 25 K in the circumnuclear disk. In both HCO+ and CO spectra, weak absorption features are seen at red-shifted velocities, which probably represent warm and dense gas falling into the nucleus.