TRACING THE OUTER HALO IN A GIANT ELLIPTICAL TO 25 R {sub eff}

We have used the Advanced Camera for Surveys and Wide Field Camera 3 cameras on board the Hubble Space Telescope to resolve stars in the halo of the nearest giant elliptical (gE) galaxy NGC 5128 out to a projected distance of 140 kpc (25 effective radii, R {sub eff}) along the major axis and 90 kpc...

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Bibliographic Details
Published in:Astrophysical journal. Letters 2014-08, Vol.791 (1)
Main Authors: Rejkuba, M., Harris, W. E., Greggio, L., Harris, G. L. H., Jerjen, H., Gonzalez, O. A.
Format: Article
Language:English
Subjects:
ASTROPHYSICS, COSMOLOGY AND ASTRONOMY
CAMERAS
COLOR
DENSITY
DIAGRAMS
GALAXIES
METALLICITY
RED GIANT STARS
SPACE
STAR CLUSTERS
TELESCOPES
VARIATIONS
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Summary:We have used the Advanced Camera for Surveys and Wide Field Camera 3 cameras on board the Hubble Space Telescope to resolve stars in the halo of the nearest giant elliptical (gE) galaxy NGC 5128 out to a projected distance of 140 kpc (25 effective radii, R {sub eff}) along the major axis and 90 kpc (16 R {sub eff}) along the minor axis. This data set provides an unprecedented radial coverage of the stellar halo properties in any gE galaxy. Color-magnitude diagrams clearly reveal the presence of the red giant branch stars belonging to the halo of NGC 5128, even in our most distant fields. The star counts demonstrate increasing flattening of the outer halo, which is elongated along the major axis of the galaxy. The V – I colors of the red giants enable us to measure the metallicity distribution in each field and so map the gradient out to ∼16 R {sub eff} from the galaxy center along the major axis. A median metallicity is obtained even for the outermost fields along both axes. We observe a smooth transition from a metal-rich ([M/H] ∼0.0) inner galaxy to lower metallicity in the outer halo, with the metallicity gradient slope along the major axis of Δ[M/H]/ΔR ≅ –0.0054 ± 0.0006 dex kpc{sup –1}. In the outer halo, beyond ∼10 R {sub eff}, the number density profile follows a power law, but also significant field-to-field metallicity and star count variations are detected. The metal-rich component dominates in all observed fields, and the median metallicity is [M/H] >–1 dex in all fields.
ISSN:2041-8205
2041-8213
DOI:10.1088/2041-8205/791/1/L2