Michele Cappellari

Abstract:

We present observations of early-type galaxies NGC 524 and 2549 with laser guide star adaptive optics (LGS AO) obtained at GEMINI North telescope using the Near-infrared Integral Field Spectrograph (NIFS) integral field unit (IFU) in the K band. The purpose of these observations is to determine high spatial resolution stellar kinematics within the nuclei of these galaxies and, in combination with previously obtained large-scale observations with the SAURON IFU, to determine the masses (M•) of the supermassive black holes (SMBH). The targeted galaxies were chosen to have central light profiles showing a core (NGC 524) and a cusp (NGC 2549), to probe the feasibility of using the galaxy centre as the natural guide source required for LGS AO. We employ an innovative technique where the focus compensation due to the changing distance to the sodium layer is made 'open loop', allowing the extended galaxy nucleus to be used only for tip-tilt correction. The data have spatial resolution of 0.23 and 0.17 arcsec full-width at half maximum (FWHM), where at least ∼40 per cent of flux comes within 0.2, showing that high quality LGS AO observations of these objects are possible. The achieved signal-to-noise ratio (S/N ∼ 50) is sufficiently high to reliably determine the shape of the line-of-sight velocity distribution. We construct axisymmetric three-integral dynamical models which are constrained with both the NIFS and SAURON data. The best-fitting models yield M• = (8.3^+2.7 -1.3) × 10⁸ M⊙ and (M/L)I = 5.8 ± 0.4 for NGC 524 and M• = (1.4 ^+0.2-1.3) × 10⁷ M⊙ and (M/L)R = 4.7 ± 0.2 for NGC 2549 (all errors are at the 3σ level). We demonstrate that the wide-field SAURON data play a crucial role in the M/L determination increasing the accuracy of M/L by a factor of at least 5, and constraining the upper limits on black hole masses. The NIFS data are crucial in constraining the lower limits of M• and in combination with the large-scale data reducing the uncertainty by a factor of 2 or more. We find that the orbital structure of NGC 524 shows significant tangential anisotropy, while at larger radii both galaxies are consistent with having almost perfectly oblate velocity ellipsoids. Tangential anisotropy in NGC 524 coincides with the size of SMBH sphere of influence and the core region in the light profile. This agrees with predictions from numerical simulations where core profiles are the result of SMBH binaries evacuating the centre nuclear regions following a galaxy merger. However, being a disc dominated fast rotating galaxy, NGC 524 has probably undergone through a more complex evolution. We test the accuracy to which M• can be measured using seeings obtained from typical LGS AO observations, and conclude that for a typical conditions and M• the expected uncertainty is of the order of 50 per cent. © 2009 RAS.

Abstract:

We measured stellar velocity dispersions σ and derived dynamical masses of nine massive (M ≈ 10¹¹ M ⊙) early-type galaxies (ETGs) from the Galaxy Mass Assembly ultra-deep Spectroscopic Survey (GMASS) sample at redshift 1.4 ≲ z ≲ 2.0. The σ are based on individual spectra for two galaxies at z ≈ 1.4 and on a stacked spectrum for seven galaxies with 1.6 < z < 2.0, with 202 hr of exposure at the ESO Very Large Telescope. We constructed detailed axisymmetric dynamical models for the objects, based on the Jeans equations, taking the observed surface brightness (from deep HST/ACS observations), point-spread function, and slit effects into account. Our dynamical masses M Jeans agree within ≲30% with virial estimates M vir = 5 × Reσ²/ G, although the latter tend to be smaller. Our M Jeans also agrees within a factor ≲2 with the M pop previously derived using stellar population models and 11 bands photometry. This confirms that the galaxies are intrinsically massive. The inferred mass-to-light ratios (M/L) U in the very age-sensitive rest-frame U band are consistent with passive evolution in the past 1 Gyr (formation redshift zf ∼3). A "bottom-light" stellar initial mass function appears to be required to ensure close agreement between M Jeans and M pop at z ∼2, as it does at z ∼0. The GMASS ETGs are on average more dense than their local counterpart. However, a few percent of local ETGs of similar dynamical masses also have comparable σ and mass surface density Σ50 inside R e. © 2009. The American Astronomical Society. All rights reserved.

Abstract:

We present mass models of a sample of 14 spiral and 14 S0 galaxies that constrain their stellar and dark matter content. For each galaxy, we derive the stellar mass distribution from near-infrared photometry under the assumptions of axisymmetry and a constant KS-band stellar mass-to-light ratio. To this we add a dark halo assumed to follow a spherically symmetric Navarro, Frenk and White profile and a correlation between concentration and dark mass within the virial radius, MDM. We solve the Jeans equations for the corresponding potential under the assumption of constant anisotropy in the meridional plane, βz. By comparing the predicted second velocity moment to observed long-slit stellar kinematics, we determine the three best-fitting parameters of the model: and βz. These simple axisymmetric Jeans models are able to accurately reproduce the wide range of observed stellar kinematics, which typically extend to ≈2-3Re or, equivalently, ≈0.5-1R25. Although our sample contains barred galaxies, we argue a posteriori that the assumption of axisymmetry does not significantly bias our results. We find a median stellar mass-to-light ratio at KS-band of with an rms scatter of 0.31. We present preliminary comparisons between this large sample of dynamically determined stellar mass-to-light ratios and the predictions of stellar population models. The stellar population models predict slightly lower mass-to-light ratios than we measure. The mass models contain a median of 15 per cent dark matter by mass within an effective radius Re (defined here as the semimajor axis of the ellipse containing half the KS-band light) and 49 per cent within the optical radius R25. Dark and stellar matter contribute equally to the mass within a sphere of radius 4.1Re or 1.0 R25. There is no evidence of any significant difference in the dark matter content of the spirals and S0s in our sample. Models without dark matter are also able to satisfactorily reproduce the observed kinematics in most cases. The improvement when a halo is added is statistically significant, however, and the stellar mass-to-light ratios of mass models with dark haloes match the independent expectations of stellar population models better. © 2009 RAS.

Abstract:

We use two-dimensional kinematic maps of simulated binary disc mergers to investigate the λR-parameter, which is a luminosity-weighted measure of projected angular momentum per unit mass. This parameter was introduced to subdivide the SAURON sample of early-type galaxies in so-called fast λR > 0.1 and slow rotators λR < 0.1. Tests on merger remnants reveal that λR is a robust indicator of the true angular momentum content in elliptical galaxies. We find the same range of λR values in our merger remnants as in the SAURON galaxies. The merger mass ratio is decisive in transforming fast rotators into slow rotators in a single binary merger, the latter being created mostly in an equal-mass merger. Slow rotators have a λR which does not vary with projection. The confusion rate with face-on fast rotators is very small. Mergers with a gas component form slow rotators with smaller ellipticities than collisionless merger remnants have, and are in much better agreement with the SAURON slow rotators. Remergers of merger remnants are slow rotators, but tend to have too high ellipticities. Fast rotators maintain the angular momentum content from the progenitor disc galaxy if merger mass ratio is high. Some SAURON galaxies have values of λR as high as our progenitor disc galaxies. © 2009 RAS.

Abstract:

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. © 2009 RAS.