Showing papers by "Jesús Falcón-Barroso published in 2009"

The SAURON project XVI: On the Sources of Ionisation for the Gas in Elliptical and Lenticular Galaxies

Abstract: Following our study on the incidence, morphology and kinematics of the ionised gas in early-type galaxies we now address the question of what is powering the observed nebular emission. To constrain the likely sources of gas excitation, we resort to a variety of ancillary data, draw from complementary information on the gas kinematics, stellar populations and galactic potential from the SAURON data, and use the SAURON-specific diagnostic diagram juxtaposing the [OIII]/Hb and [NI]/Hb line ratios. We find a tight correlation between the stellar surface brightness and the flux of the Hb recombination line across our sample, which points to a diffuse and old stellar source as the main contributor of ionising photons in early-type galaxies, with post-asymptotic giant branch (pAGB) stars being still the best candidate based on ionising-balance arguments. Other ionising sources such as a central AGN, OB-stars, shocks or the interaction between the hot and warm phases of the interestellar medium are found to play only a limited or localised role in powering the diffuse nebular emission observed in our sample galaxies. These results lead us to investigate the relative importance of stellar and AGN photoionisation in explaining the ionised-gas emission observed in early-type galaxies by the Sloan Digital Sky Survey (SDSS). By simulating how our sample galaxies would appear if placed at further distance and targeted by the SDSS, we conclude that only in very few, if any, of the SDSS early-type galaxies that display modest values for the equivalent width of the [OIII] line (less than ~2.4\AA) and LINER-like [OIII]/Hb values, the nebular emission is truly powered by an AGN.

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

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 NGC 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 orbitbased Schwarzschild dynamical models we find that dark matter is necessary to explain the observed kinematics in NGC 3379 and NGC 821, with 30 - 50 per cent of the total matter being dark within 4 Re. The radial anisotropy in our best-fit 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.

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

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 NGC 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 NGC 821, with 30 - 50 per cent of the total matter being dark within 4 Re. The radial anisotropy in our best-fit halo models is less than in our models without halo, due to differences in orbital structure. The halo also has an effect on the Mgb - Vesc relation: its slope is steeper when a dark matter halo is added to the model.

The SAURON Project – XIV. No escape from Vesc: a global and local parameter in early-type galaxy evolution

Abstract: We present the results of an investigation of the local escape velocity (V-esc) - line strength index relationship for 48 early-type galaxies from the SAURON sample, the first such study based on a large sample of galaxies with both detailed integral field observations and extensive dynamical modelling. Values of V-esc are computed using multi-Gaussian expansion (MGE) photometric fitting and axisymmetric, anisotropic Jeans' dynamical modelling simultaneously on Hubble Space Telescope and ground-based images. We determine line strengths and escape velocities at multiple radii within each galaxy, allowing an investigation of the correlation within individual galaxies as well as amongst galaxies. We find a tight correlation between V-esc and the line-strength indices. For Mgb, we find that this correlation exists not only between different galaxies but also inside individual galaxies - it is both a local and global correlation. The Mgb-V-esc relation has the form: log(Mgb/4 A) = (0.32 +/- 0.03) log(V-esc/500 km s-1) - (0.031 +/- 0.007) with an rms scatter Sigma = 0.033. The relation within individual galaxies has the same slope and offset as the global relation to a good level of agreement, though there is significant intrinsic scatter in the local gradients. We transform our line strength index measurements to the single stellar population (SSP) equivalent ages (t), metallicity ([Z/H]) and enhancement ([alpha/Fe]) and carry out a principal component analysis of our SSP and V-esc data. We find that in this four-dimensional parameter space the galaxies in our sample are to a good approximation confined to a plane, given by log (V (esc)/500 km s -1) = 0.85 [Z/H] + 0.43 log (t/Gyr) - 0.29. It is surprising that a combination of age and metallicity is conserved; this may indicate a 'conspiracy' between age and metallicity or a weakness in the SSP models. How the connection between stellar populations and the gravitational potential, both locally and globally, is preserved as galaxies assemble hierarchically may provide an important constraint on modelling.

The SAURON project - XIII. SAURON-GALEX study of early-type galaxies: the ultraviolet colour-magnitude relations and Fundamental Planes

Abstract: We present Galaxy Evolution Explorer (GALEX) far (FUV) and near (NUV) ultraviolet imaging of 34 nearby early-type galaxies from the SAURON representative sample of 48 E/S0 galaxies, all of which have ground-based optical imaging from the MDM Observatory. The surface brightness profiles of nine galaxies (≈ 26 per cent) show regions with blue UV−optical colours sug

The SAURON Project - XIV. No escape from V$_{\mathrm{esc}}$: a global and local parameter in early-type galaxy evolution

Abstract: We present the results of an investigation of the local escape velocity (Vesc) - line strength index relationship for 48 early type galaxies from the SAURON sample, the first such study based on a large sample of galaxies with both detailed integral field observations and extensive dynamical modelling. Values of Vesc are computed using Multi Gaussian Expansion (MGE) photometric fitting and axisymmetric, anisotropic Jeans' dynamical modelling simultaneously on HST and ground-based images. We determine line strengths and escape velocities at multiple radii within each galaxy, allowing an investigation of the correlation within individual galaxies as well as amongst galaxies. We find a tight correlation between Vesc and the line-strength indices. For Mgb we find that this correlation exists not only between different galaxies but also inside individual galaxies - it is both a local and global correlation. The relation within individual galaxies has the same slope and offset as the global relation to a good level of agreement, though there is significant intrinsic scatter in the local gradients. We transform our line strength index measurements to the single stellar population (SSP) equivalent age (t), metallicity ([Z/H]) and enhancement ([$\alpha$/Fe]) and carry out a principal component analysis of our SSP and Vesc data. We find that in this four-dimensional parameter space the galaxies in our sample are to a good approximation confined to a plane, given by $\log \mathrm({V}_{\mathrm{esc}}/500\mathrm{km/s}) = 0.85 \mathrm{[Z/H]} + 0.43 \log (\mathrm{t}/\mathrm{Gyrs})$ - 0.20. It is surprising that it is a combination of age and metallicity that is conserved; this may indicate a 'conspiracy' between age and metallicity or a weakness in the SSP models.

The nature of late-type spiral galaxies: structural parameters, optical and near-infrared colour profiles and dust extinction

Abstract: We analyse V-and H-band surface photometry of a sample of 18 Sb-Sd galaxies. Combining high-resolution Hubble Space Telescope (HST) images with ground-based near-infrared observations, we extract photometric profiles, which cover the whole disc and provide the highest possible resolution. This is the first photometric study of late-type spirals for which the stellar kinematics have been measured. For 10 out of the 18 galaxies, HST data in both F160W (H) and F606W (V) are available, and, for those, we present colour maps and radial colour profiles at the resolution of the HST. Colours vary significantly from galaxy to galaxy, but tend to be highly homogeneous within each galaxy, with smooth and flat colour profiles. Some of the colour maps show jumps in the inner regions, likely due to dust. We determine extinction maps in an almost model-independent way using the V-H colour map and the SAURON Mgb absorption line map of Ganda et al. The maps show that AV ranges from 0 to 2 mag, in the centre from 0 to 1.5 mag, in agreement with the models of Tuffs et al. We describe the surface brightness profiles as the superposition of an exponential disc and a Sersic bulge. The bulges are small (0.1-2.5 kpc), and show a shape parameter n ranging from approximate to 0.7 to 3, with a mean value smaller than two: well below the value for the classical de Vaucouleurs bulges. Most galaxies (16 out of 18) show a central light excess above the Sersic fit to the bulge, which can be interpreted as a nuclear cluster, as shown by previous studies. We provide zero-order estimates for the magnitude of these components. We discuss the correlations among the structural galaxy parameters and with other relevant quantities, such as Hubble type and stellar velocity dispersion. We compare these results with a recent paper by Graham & Worley, who present a summary of most of the NIR surface photometry of spirals in the literature. For both early-and late-type spirals, bulge luminosity strongly correlates with central velocity dispersion; at constant velocity dispersion, later type bulges are larger and less dense, and have lower Sersic n-values.

Scaling relations in early-type galaxies from integral-field stellar kinematics

Abstract: Early-type galaxies (ETGs) satisfy a now classic scaling relation Re ∝ σ1.2eI-0.8e, the Fundamental Plane (FP; Djorgovski & Davis 1987; Dressler et al. 1987), between their size, stellar velocity dispersion and mean surface brightness. A significant effort has been devoted in the past twenty years to try to understand why the coefficients of the relation are not the ones predicted by the virial theorem Re ∝ σ2eI-1e.

MILES SSP Models

Abstract: We present SEDs for single-age, single-metallicity stellar populations (SSPs) covering the full optical spectral range at resolution (FWHM = 2.3A). These SEDs can be regarded as our base models, as we combine scaled-solar isochrones with an empirical stellar spectral library (MILES), which follows the chemical evolution pattern of the solar neighbourhood. The models rely as much as possible on empirical ingredients as also employ extensive photometric libraries. Thanks to the unprecedented parameter coverage of the MILES library we synthesize SSP SEDs from intermediate- to very-old age regimes, and the metallicity from super-solar to [M/H] = −2.3, all for a suite of IMF shapes and slopes. We propose a new Line Index System (LIS), based on flux-calibrated spectra, to avoid the intrinsic uncertainties associated with the Lick/IDS system and provide more appropriate, uniform, spectral resolution.

The nature of late-type spiral galaxies: structural parameters, optical and near-infrared colour profiles, and dust extinction

Abstract: We analyse V and H-band surface photometry of a sample of 18 Sb-Sd galaxies. Combining high resolution HST images with ground-based NIR observations, we extract photometric profiles, which cover the whole disk and provide the highest possible resolution. This is the first photometric study of late-type spirals for which the stellar kinematics have been measured. For 10 out of the 18 galaxies, HST data in both F160W (H) and F606W (V) are available, and, for those, we present colour maps and radial colour profiles at the resolution of the Hubble Space Telescope. Colours vary significantly from galaxy to galaxy, but tend to be highly homogeneous within each galaxy, with smooth and flat colour profiles. Some of the colour maps show jumps in the inner regions, likely due to dust. We determine extinction-maps in an almost model-independent way using the V-H colour map and the SAURON Mg b absorption line map of Ganda et al. (2007). The maps show that A_V ranges from 0 to 2 mag, in the center from 0 to 1.5 mag, in agreement with the models of Tuffs et al. (2004). We describe the surface brightness profiles as the superposition of an exponential disk and a Sersic bulge. The bulges are small (0.1-2.5 kpc), and show a shape parameter n ranging from ~ 0.7 to 3, with a mean value smaller than two: well below the value for the 'classical' de Vaucouleurs bulges. Most galaxies (16 out of 18) show a central light excess above the Sersic fit to the bulge, which can be interpreted as a nuclear cluster, as shown by previous studies. We provide zero-order estimates for the magnitude of these components. We discuss the correlations among the structural galaxy parameters and with other relevant quantities (abridged).

Scaling relations in early-type galaxies from integral-field stellar kinematics

Abstract: We study the origin of the scaling relations of early-type galaxies (ETGs) by constructing detailed models of the stellar dynamics for the K-band selected, volume-limited ATLAS3D sample of 263 nearby ETGs, spanning a large range of masses and stellar velocity dispersions (60 < sigma < 350 km/s).

The SAURON project - XIII. SAURON-GALEX study of early-type galaxies: the ultraviolet colour-magnitude relations and Fundamental Planes

Abstract: We present GALEX far (FUV) and near (NUV) ultraviolet imaging of 34 nearby early-type galaxies from the SAURON representative sample of 48 E/S0 galaxies, all of which have ground-based optical imaging from the MDM Observatory. The surface brightness profiles of nine galaxies show regions with blue UV-optical colours suggesting recent star formation. Five of these show blue integrated UV-optical colours that set them aside in the NUV integrated colour-magnitude relation. They also have other properties confirming they have had recent star formation, in particular H_beta absorption higher than expected for a quiescent population and a higher CO detection rate. NUV-blue galaxies are generally drawn from the lower stellar velocity dispersion and thus lower dynamical mass part of the sample. We have also constructed the first UV Fundamental Planes and show that NUV blue galaxies bias the slopes and increase the scatters. If they are eliminated the fits get closer to expectations from the virial theorem. Although our analysis is based on a limited sample, it seems that a dominant fraction of the tilt and scatter of the UV Fundamental Planes is due to the presence of young stars in preferentially low-mass early-type galaxies. Interestingly, the UV-optical radial colour profiles reveal a variety of behaviours, with many galaxies showing signs of recent star formation, a central UV-upturn phenomenon, smooth but large-scale age and metallicity gradients, and in many cases a combination of these. In addition, FUV-NUV and FUV-V colours even bluer than those normally associated with UV-upturn galaxies are observed at the centre of some quiescent galaxies. Four out of the five UV-upturn galaxies are slow rotators. These objects should thus pose interesting challenges to stellar evolutionary models of the UV-upturn.

A SAURON view of double-barred galaxies

Abstract: 1. Double bars and the σ-hollows Double bars might be the key mechanisms to transport gas to the very central regions of galaxies, so double-barred galaxies are key objects to better understand the galaxy formation and evolution scenarios. In order to disentangle the role of double bars in the galaxy build up, we are performing a whole kinematical and stellar population analysis of these objects from high S/N spectroscopic data. Four double-barred galaxies were observed with the integral-field spectrograph SAURON at the William Herschel Telescope (La Palma, Spain). The stellar kinematical analysis revealed the presence of the σ-hollows, i.e. decreases of the velocity dispersion values at the ends of the inner bars (de Lorenzo-Caceres et al. 2008). These σ-hollows seem to be the result of the contrast between the high σ of the bulge and the low σ of the inner bar, meaning that inner bars are cold, thin structures. Fig. 1 shows the intensity and σ maps for the four objects. We have investigated the stellar populations that shape these hollows, comparing the results with the same analysis for the bulge and the rest of the galaxy, within the SAURON field-of-view.

Stellar Velocity Profiles and Line-Strengths out to Four Effective Radii in the Early-Type Galaxy NGC 3379

Abstract: We describe a new technique to measure stellar kinematics and line-strengths at large radii in nearby galaxies. Using the integral-field spectrograph SAURON as a 'photon-collector', we obtain spectra out to four effective radii (Re) in the early-type galaxy NGC 3379. By fitting orbit-based models to the extracted stellar velocity profile, we find that ~40% of the total mass within 5 Re is dark. The measured absorption line-strengths reveal a radial gradient with constant slope out to 4 Re.