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Register a new userThe distance to the giant elliptical galaxy NGC 5128
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M. Rejkuba
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The distance to NGC 5128, the central galaxy of the Centaurus group and the nearest giant elliptical to us, has been determined using two independent distance indicators: the Mira period-luminosity (PL) relation and the luminosity of the tip of the red giant branch (RGB). The data were taken at two different locations in the halo of NGC 5128 with the ISAAC near-IR array on ESO VLT. From more than 20 hours of observations with ISAAC a very deep Ks-band luminosity function was constructed. The tip of the RGB is detected at Ks=21.24 pm 0.05 mag. Using an empirical calibration of the K-band RGB tip magnitude, and assuming a mean metallicity of [M/H]=-0.4 dex and reddening of E(B-V)=0.11, a distance modulus of NGC 5128 of (m-M)_0=27.87 pm 0.16 was derived. The comparison of the H-band RGB tip magnitude in NGC 5128 and the Galactic Bulge implies a distance modulus of NGC 5128 of (m-M)_0=27.9 pm 0.2 in good agreement with the K-band RGB tip measurement. The population of stars above the tip of the RGB amounts to 2176 stars in the outer halo field and 6072 stars in the inner halo field. The large majority of these sources belong to the asymptotic giant branch (AGB) population in NGC 5128 with numerous long period variables. Mira variables were used to determine the distance of NGC 5128 from a period-luminosity relation calibrated using the Hipparcos parallaxes and LMC Mira period-luminosity relation in the K-band. This is the first Mira period-luminosity relation outside the Local Group. A distance modulus of 27.96 pm 0.11 was derived, adopting the LMC distance modulus of 18.50 pm 0.04. The mean of the two methods yields a distance modulus to NGC 5128 of 27.92 pm 0.19 corresponding to D=3.84 pm 0.35 Mpc.
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We present the first census of giant molecular clouds (GMCs) complete down to 10$^6 M_{odot}$ and within the inner 4 kpc of the nearest giant elliptical and powerful radio galaxy, Centaurus A. We identified 689 GMCs using CO(1--0) data with 1 spatial resolution ($sim 20$ pc) and 2 km/s velocity resolution obtained with the Atacama Large Millimeter/submillimeter Array (ALMA). The $I$(CO)-$N$(H$_2$) conversion factor based on the virial method is $X_{rm CO}$ = $(2 pm 1 )times10^{20}$ cm$^{-2}$(K km/s)$^{-1}$ for the entire molecular disk, consistent with that of the disks of spiral galaxies including the Milky Way, and $X_{rm CO}$ = $(5 pm 2)times10^{20}$ cm$^{-2}$(K km/s)$^{-1}$ for the circumnuclear disk (CND, within a galactocentric radius of 200 pc). We obtained the GMC mass spectrum distribution and find that the best-truncated power-law fit for the whole molecular disk, with index $gamma simeq -2.41 pm 0.02$ and upper cutoff mass $sim 1.3 times 10^{7} M_{odot}$, is also in agreement with that of nearby disk galaxies. A trend is found in the mass spectrum index from steep to shallow as we move to inner radii. Although the GMCs are in an elliptical galaxy, the general GMC properties in the molecular disk are as in spiral galaxies. However, in the CND, large offsets in the line-width-size scaling relations ($sim$ 0.3 dex higher than those in the GMCs in the molecular disk), a different $X_{rm CO}$ factor, and the shallowest GMC mass distribution shape ($gamma = -1.1 pm 0.2$) all suggest that there the GMCs are most strongly affected by the presence of the AGN and/or shear motions.
We present an analysis of the spatial and chemical sub-structures in a remote halo field in the nearby giant elliptical galaxy Centaurus A (NGC~5128), situated at about 38 kpc from the centre of the galaxy. The observations were taken with the Advanced Camera for Surveys instrument on board the Hubble Space Telescope, and reach down to the horizontal branch. In this relatively small 3.8 kpc by 3.8 kpc field, after correcting for Poisson noise, we do not find any statistically strong evidence for the presence of small-scale sub-structures in the stellar spatial distribution on scales greater than 100 pc. However, we do detect the presence of significant small spatial-scale inhomogeneities in the stellar median metallicity over the surveyed field. We argue that these localized chemical substructures could be associated with not-fully mixed debris from the disruption of low mass systems. NGC 5128 joins the ranks of the late-type spiral galaxies the Milky Way, for which the stellar halo appears to be dominated by small-scale spatial sub-structures, and NGC~891, where localized metallicity variations have been detected in the inner extra-planar regions. This suggests that the presence of small-scale sub-structures may be a generic property of stellar halos of large galaxies.
We discuss a new distance to NGC 5128 (Centaurus A) based on Cepheid variables observed with the Hubble Space Telescope. Twelve F555W (V) and six F814W (I) epochs of cosmic-ray-split Wide Field Planetary Camera 2 observations were obtained. A total of 56 bona-fide Cepheids were discovered, with periods ranging from 5 to ~50 days; five of these are likely Population II Cepheids of the W Virginis class, associated with the bulge or halo of NGC 5128. Based on the period and V and I-band luminosities of a sub-sample of 42 classical (Pop I) Cepheids, and adopting a Large Magellanic Cloud distance modulus and extinction of 18.50 +/- 0.10 mag and E(B-V)=0.10 mag, respectively, the true reddening-corrected distance modulus to NGC 5128 is mu_0 = 27.67 +/- 0.12 (random)+/- 0.16 (systematic) mag, corresponding to a distance of 3.42 +/- 0.18 (random) +/- 0.25 (systematic) Mpc. The random uncertainty in the distance is dominated by the error on the assumed value for the ratio of total to selective absorption, R_V, in NGC 5128, and by the possible metallicity dependence of the Cepheid Period-Luminosity relation at V and I. This represent the first determination of a Cepheid distance to an early-type galaxy.
We present ALMA CO(1-0) observations toward the dust lane of the nearest elliptical and radio galaxy, NGC 5128 (Centaurus A), with high angular resolution ($sim$ 1 arcsec, or 18 pc), including information from large to small spatial scales and total flux. We find a total molecular gas mass of 1.6$times$10$^9$ $M_odot$ and we reveal the presence of filamentary components more extended than previously seen, up to a radius of 4 kpc. We find that the global star formation rate is $sim$1 Msol yr$^{-1}$, which yields a star formation efficiency (SFE) of 0.6 Gyr$^{-1}$ (depletion time $tau =$1.5 Gyr), similar to those in disk galaxies. We show the most detailed view to date (40,pc resolution) of the relation between molecular gas and star formation within the stellar component of an elliptical galaxy, from several kpc scale to the circumnuclear region close to the powerful radio jet. Although on average the SFEs are similar to those of spiral galaxies, the circumnuclear disk (CND) presents SFEs of 0.3 Gyr$^{-1}$, lower by a factor of 4 than the outer disk. The low SFE in the CND is in contrast to the high SFEs found in the literature for the circumnuclear regions of some nearby disk galaxies with nuclear activity, probably as a result of larger shear motions and longer AGN feedback. The higher SFEs in the outer disk suggests that only central molecular gas or filaments with sufficient density and strong shear motions will remain in $sim$1 Gyr, which will later result in the compact molecular distributions and low SFEs usually seen in other giant ellipticals with cold gas.
There is a well-known discrepancy in the distance estimation for M60, a giant elliptical galaxy in Virgo: the planetary nebula luminosity function (PNLF) distance moduli for this galaxy are, on average, $~0.4$ mag smaller than the values based on the surface brightness fluctuation (SBF) in the literature. We present photometry of the resolved stars in an outer field of M60 based on deep F775W and F850LP images in the Hubble Space Telescope obtained as part of the Pure Parallel Program in the archive. Detected stars are mostly old red giants in the halo of M60. With this photometry we determine a distance to M60 using the tip of the red giant branch (TRGB). A TRGB is detected at $F850LP_{rm TRGB}=26.70pm0.06$ mag, in the luminosity function of the red giants. This value corresponds to $F814W_{0,rm TRGB}=27.13pm0.06$ mag and $QT_{rm TRGB}=27.04pm0.07$ mag, where $QT$ is a color-corrected F814W magnitude. From this we derive a distance modulus, $(m-M)_0=31.05pm0.07{rm(ran)}pm0.06{rm (sys)}$ ($d=16.23pm0.50{rm (ran)}pm0.42{rm (sys)}$ Mpc). This value is $0.3$ mag larger than the PNLF distances and $0.1$ mag smaller than the SBF distances in the previous studies, indicating that the PNLF distances to M60 in the literature have larger uncertainties than the suggested values.
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