Stellar velocity profiles and line strengths out to four effective radii in the early-type galaxies NGC 3379 and 821

We use the integral-field spectrograph SAURON to measure the stellar line-of-sight velocity distribution and absorption line strengths out to four effective radii (Re) in the early-type galaxies NGC 3379 and 821. With our newly developed observing technique, we can now probe these faint regions in g...

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Bibliographic Details
Published in:	Monthly notices of the Royal Astronomical Society 2009-09, Vol.398 (2), p.561-574
Main Authors:	Weijmans, Anne-Marie, Cappellari, Michele, Bacon, Roland, De Zeeuw, P. T., Emsellem, Eric, Falcón-Barroso, Jesús, Kuntschner, Harald, McDermid, Richard M., Van Den Bosch, Remco C. E., Van De Ven, Glenn
Format:	Article
Language:	eng
Subjects:	Astronomy Astrophysics Dark matter Earth, ocean, space Exact sciences and technology galaxies: elliptical and lenticular galaxies: elliptical and lenticular, cD galaxies: haloes galaxies: individual: NGC 3379 galaxies: individual: NGC 821 galaxies: kinematics and dynamics Kinematics Star & galaxy formation Stars & galaxies Velocity
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Summary:	We use the integral-field spectrograph SAURON to measure the stellar line-of-sight velocity distribution and absorption line strengths out to four effective radii (Re) in the early-type galaxies NGC 3379 and 821. With our newly developed observing technique, we can now probe these faint regions in galaxies that were previously not accessible with traditional long-slit spectroscopy. We make optimal use of the large field-of-view and high throughput of the spectrograph: by adding the signal of all ∼1400 lenslets into one spectrum, we obtain sufficient signal-to-noise in a few hours of observing time to reliably measure the absorption line kinematics and line strengths out to large radius. We find that the line strength gradients previously observed within 1 Re remain constant out to at least 4 Re, which puts constraints on the merger histories of these galaxies. The stellar halo populations are old and metal poor. By constructing orbit-based Schwarzschild dynamical models, we find that dark matter is necessary to explain the observed kinematics in NGC 3379 and 821, with 30–50 per cent of the total matter being dark within 4 Re. The radial anisotropy in our best-fitting halo models is less than in our models without halo, due to differences in orbital structure. The halo also has an effect on the Mg b–Vesc relation: its slope is steeper when a dark matter halo is added to the model.
ISSN:	0035-8711 1365-2966