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Stellar metallicity distribution functions (MDF) have been measured for resolved stellar populations in the outer halos of many galaxies in nearby groups. Among them, the MDF of NGC 5128, the central giant elliptical in the Centaurus group, provides essential constraints for theories of massive galaxy formation and hierarchical assembly. To investigate the formation and chemical evolution history of the outer halo of giant elliptical galaxies, we examine the chemical properties of three zoom-in high resolution cosmological hydrodynamical simulations of an NGC 5128-like giant elliptical galaxy and compare their outer halo MDFs to the observed one of NGC 5128. Even though the simulated galaxies have different merging histories and age distributions, all predicted MDFs are in good qualitative agreement with the observed one. The median metallicity of the simulated galaxies is on average $\rm [M/H]=-0.41 \pm 0.06$ compared to the observed value of $\rm [M/H]=-0.38 \pm 0.02$ for NGC 5128, and the dispersion in metallicity is $\sim 0.77$ dex for both observed and simulated galaxies. We investigate the origin of the stars ending up in the outer halo field of simulated galaxies and show that most have an `accreted' origin, formed in other small galaxies and later accreted in mergers. Only $\sim 15$ percent of the stars are formed `in situ' within the main progenitor of galaxy and radially migrate outwards. We show that the contribution of metal-rich in situ stars is sub-dominant in the outer halos of our simulated galaxies, but can be prominent in the inner regions., 16 pages, 9 figures, Accepted for publication in ApJ

We report the detection, using observations of the CO(2-1) line performed with the Atacama Pathfinder EXperiment (APEX), of molecular gas in the region of the outer filament of Centaurus A, a complex region known to show various signatures of an interaction between the radio jet, an HI cloud, and ionised gas filaments. We detect CO(2-1) at all observed locations, which were selected to represent regions with very different physical conditions. The H_2 masses of the detections range between 0.2 x 10^6 and 1.1 x 10^6 \msun, for conservative choices of the CO to H_2 conversion factor. Surprisingly, the stronger detections are not coincident with the HI cloud, but instead are in the region of the ionised filaments. We also find variations in the widths of the CO(2-1) lines throughout the region, with broader lines in the region of the ionised gas, i.e. where the jet--cloud interaction is strongest, and with narrow profiles in the HI cloud. This may indicate that the molecular gas in the region of the ionised gas has the momentum of the jet-cloud interaction encoded in it, in the same way as the ionised gas does. These molecular clouds may therefore be the result of very efficient cooling of the down-stream gas photo- or shock-ionised by the interaction. On the other hand, the molecular clouds with narrower profiles, which are closer to or inside the HI cloud, could be pre-existing cold H_2 cores which manage to survive the effects of the passing jet., Comment: Accepted for A&A Letters; 4 pages, 2 figures; Only minor changes wrt v1

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.

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.

Shells are faint are-like stellar structures, which have been observed around early type galaxies and are thought to be the result of an interaction. PII gas has recently been detected in shells, a surprising result in view of the theoretical predictions that most of the gas should decouple from stars and fall into the nucleus in such interactions. Here we report the first detection of molecular gas (CO) in shells, found 15 kpc away from the center of NGC 5128 (CenA), a giant elliptical galaxy that harbors an active nucleus (AGN). The ratio between CO and HI emission in the shells is the same as that found in the central regions, which is unexpected given the metallicity gradient usually observed in galaxies. We propose that the dynamics of the gas can be understood within the standard picture of shell formation if one takes into account that the interstellar medium is clumpy and hence not highly dissipative. The observed metal enrichment could be due to star formation induced by the AGN jet in the shells. Furthermore our observations provide evidence that molecular gas in mergers may be spread out far from the nuclear regions.

Determining the internal kinematics of a galaxy from planetary nebulae (PN) is usually a two-step process in which the candidate PN are first identified in a target galaxy and then, in a follow-up run, spectra are obtained. We have implemented a new technique in which two dispersed images at the wavelength of the [O III] emission line at 5007 Angstrom are combined ro yield positions, magnitudes and velocities of the PN population in a single step. A reduction in observing time of about a factor of 2 is attainable. We present here the proof-of-principle results.

Determining the internal kinematics of a galaxy from planetary nebulae (PN) is usually a two-step process in which the candidate PN are first identified in a target galaxy and then, in a follow-up run, spectra are obtained. We have implemented a new technique in which two dispersed images at the wavelength of the [O III] emission line at 5007 Angstrom are combined ro yield positions, magnitudes and velocities of the PN population in a single step. A reduction in observing time of about a factor of 2 is attainable. We present here the proof-of-principle results.

The location of the gas responsible for the absorption-line system toward the nucleus of Centaurus A is a puzzle. It is generally accepted that the line features close to the systemic velocity originate in the disk. The redshifted line features in particular, however, are usually thought to be due to gas close to the nucleus or even falling toward it. We present new (CO)-C-12 (1-0), HCO+ (1-0) and HCN (1-0) absorption-line measurements, as well as an alternative interpretation of the line system. Previous papers have demonstrated that the distribution of line emission of the inner molecular and outer atomic hydrogen disk can be accounted for by a system of tilted rings with varying inclination. Using the same model, but assuming that corotating absorbing gas is located at high altitudes above the disk, one can account for all major features of the absorption-line system. In this model, the absorption takes place in high-altitude clouds that are up to about 160 pc above the molecular disk of Centaurus A at radii between 1.7 and 1.9 kpc, accounting for the two strongest central line features. In our model, the redshifted line features are due to gas associated with disk material up to about 300 pc above the disk at radii of 0.4-0.6 kpc orbiting in the non-spherically symmetric potential of the Centaurus A galaxy. In this model, the systemic velocity is at 546 km s(-1), which is the velocity of the sharp spectral feature about 6 km s(-1) to the blue of the deepest absorption line. Our new model provides a natural explanation for the general structure of the complex absorption-line system, based on a tilted-ring model that already explains the disk line emission. In this model, no significant absorbing gas component closer than 200 pc to the nonthermal radio continuum nucleus is required, and the peculiar velocity structure of the absorption-line system is due to the kinematics of the molecular gas disk.

The light element abundance pattern from many planetary nebulae (PNe) covering the upper 4 mag. of the [O III] luminosity function was observed with ESO VLT FORS1 multi-slit. Spectra of 51 PNe over the wavelength range 3500-7500 Angstrom were obtained in three fields at 4, 8 and 17 kpc, for a distance of 3.8 Mpc. Emission line ratios are entirely typical of PN such as in the Milky Way. The temperature sensitive [O III]4363A line was weakly detected in 10 PNe, both [O II] and [O III] lines were detected in 30 PNe, and only the bright [O III]5007A line in 7 PN. Cloudy photoionization models were run to match the spectra by a spherical, constant density nebula ionized by a black body central star. He, N, O and Ne abundances with respect to H were determined and, for brighter PNe, S and Ar; central star luminosities and temperatures are also derived. For 40 PNe with Cloudy models, from the upper 2 mag. of the luminosity function, the most reliably estimated element, oxygen, has a mean 12+log(O/H) of 8.52. No obvious radial gradient is apparent in O/H over a range 2-20 kpc. Comparison of the PN abundances with the stellar population, from the spectra of the integrated starlight on the multi-slits and photometric studies, suggests [Fe/H]=-0.4 and [O/Fe]=0.25. The masses of the PN central stars in NGC 5128 from model tracks imply an epoch of formation more recent than for the minority young population from colour-magnitude studies. The PNe progenitors may belong to the young tail of a recent, minor, star formation episode or derive from other evolutionary channels.[Abridged], 28 pages, 12 tables, 11 figures. Accepted for publication in Astronomy & Astrophysics

In this Letter, we present for the first time images and kinematics of neutral hydrogen, which is unambiguously associated with the optical shells of an E/SO galaxy, NGC 5128. Using the VLA, we have mapped diffuse H I (21 cm) emission out to approximately 15 kpc (15') from the nucleus of the galaxy to a limiting column density of 10(20) cm-2. These observations reveal the presence of 1.5 x 10(8) M. of H I, 10'-15' NE, N, and SW of the galaxy. The H I distribution follows the curvature of the outermost ''diffuse'' shells, and is slightly displaced, shifted 1' to the outside of the shells. The H I shows a smooth gradient in velocity over approximately 450 km s-1, nearly the same line-of-sight velocity range observed in the dust lane. Our results suggest that the H I might be a partial ring, rotating with the same sense as the main body of the galaxy. We discuss merger, accretion, and internal shell formation models and describe how the observed properties of the H I challenge existing theories of shell formation. We also find a morphological and kinematical connection between the NE cloud of H I and the outer optical filaments of Cen A, identifying the H I as the source of gas being ionized. Finally, we discuss the implications of these results on the study of high-z radio galaxies with extended emission-line regions.

In this Letter, we present for the first time images and kinematics of neutral hydrogen, which is unambiguously associated with the optical shells of an E/SO galaxy, NGC 5128. Using the VLA, we have mapped diffuse H I (21 cm) emission out to approximately 15 kpc (15') from the nucleus of the galaxy to a limiting column density of 10(20) cm-2. These observations reveal the presence of 1.5 x 10(8) M. of H I, 10'-15' NE, N, and SW of the galaxy. The H I distribution follows the curvature of the outermost ''diffuse'' shells, and is slightly displaced, shifted 1' to the outside of the shells. The H I shows a smooth gradient in velocity over approximately 450 km s-1, nearly the same line-of-sight velocity range observed in the dust lane. Our results suggest that the H I might be a partial ring, rotating with the same sense as the main body of the galaxy. We discuss merger, accretion, and internal shell formation models and describe how the observed properties of the H I challenge existing theories of shell formation.We also find a morphological and kinematical connection between the NE cloud of H I and the outer optical filaments of Cen A, identifying the H I as the source of gas being ionized. Finally, we discuss the implications of these results on the study of high-z radio galaxies with extended emission-line regions.

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.

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.

This review attempts to place the observations of extragalactic planetary nebulae in the context of galactic dynamics. From this point of view only the radial velocities of the PNe is important. We have built a specialised instrument to detect PNe in distant galaxies and measure their radial velocities in one step. This is explained in some detail, along with classical techniques for obtaining kinematic information. The review includes a vision of possible future developments in the field.

This review attempts to place the observations of extragalactic planetary nebulae in the context of galactic dynamics. From this point of view only the radial velocities of the PNe is important. We have built a specialised instrument to detect PNe in distant galaxies and measure their radial velocities in one step. This is explained in some detail, along with classical techniques for obtaining kinematic information. The review includes a vision of possible future developments in the field.

This review attempts to place the observations of extragalactic planetary nebulae in the context of galactic dynamics. From this point of view only the radial velocities of the PNe is important. We have built a specialised instrument to detect PNe in distant galaxies and measure their radial velocities in one step. This is explained in some detail, along with classical techniques for obtaining kinematic information. The review includes a vision of possible future developments in the field.

Shells are faint arc-like stellar structures, which have been observed around early type galaxies and are thought to be the result of an interaction. HI gas has recently been detected in shells, a surprising result in view of the theoretical predictions that most of the gas should decouple from stars and fall into the nucleus in such interactions. Here we report the first detection of molecular gas (CO) in shells, found 15kpc away from the center of NGC5128 (CenA), a giant elliptical galaxy that harbors an active nucleus (AGN). The ratio between CO and HI emission in the shells is the same as that found in the central regions, which is unexpected given the metallicity gradient usually observed in galaxies. We propose that the dynamics of the gas can be understood within the standard picture of shell formation if one takes into account that the interstellar medium is clumpy and hence not highly dissipative. The observed metal enrichment could be due to star formation induced by the AGN jet in the shells. Furthermore our observations provide evidence that molecular gas in mergers may be spread out far from the nuclear regions., Accepted for publication in Astronomy & Astrophysics Letters, (Vol. 356), 4 pages + 1 color figure

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 $\pm60^{\prime \prime}$ and $\pm100^{\prime \prime}$. We also obtained a few spectra of CS(2-1) and (3-2) tracing densities of $\sim10^5$ cm$^{-3}$. The emission in these lines is weak and reaches a few mK at the available angular resolutions of 54$^{\prime \prime}$ - 36$^{\prime \prime}$. At the central position, the integrated intensity ratio I(HCN)/I(CO) peaks at 0.064, and decreases to $\sim$0.02 to 0.04 in the dust lane., Comment: 7 pages, 5 figures, accepted by A.&A. 04. May 2000

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.

In this Letter, we present for the first time images and kinematics of neutral hydrogen, which is unambiguously associated with the optical shells of an E/SO galaxy, NGC 5128. Using the VLA, we have mapped diffuse H I (21 cm) emission out to approximately 15 kpc (15') from the nucleus of the galaxy to a limiting column density of 10(20) cm-2. These observations reveal the presence of 1.5 x 10(8) M. of H I, 10'-15' NE, N, and SW of the galaxy. The H I distribution follows the curvature of the outermost ''diffuse'' shells, and is slightly displaced, shifted 1' to the outside of the shells. The H I shows a smooth gradient in velocity over approximately 450 km s-1, nearly the same line-of-sight velocity range observed in the dust lane. Our results suggest that the H I might be a partial ring, rotating with the same sense as the main body of the galaxy. We discuss merger, accretion, and internal shell formation models and describe how the observed properties of the H I challenge existing theories of shell formation. We also find a morphological and kinematical connection between the NE cloud of H I and the outer optical filaments of Cen A, identifying the H I as the source of gas being ionized. Finally, we discuss the implications of these results on the study of high-z radio galaxies with extended emission-line regions.