Searcharxiv⌕ Search

arXiv subjects

M. Bureau

Publications and source records attributed to M. Bureau.

At least 55 records · Page 3Linked to original sources
arXiv2006

The SAURON project - VIII. OASIS/CFHT integral-field spectroscopy of elliptical and lenticular galaxy centres

We present high spatial resolution integral-field spectroscopy of 28 elliptical (E) and lenticular (S0) galaxies from the SAURON representative survey obtained with the OASIS spectrograph during its operation at the CFHT. These seeing-limited observations explore the central 8"x10" (typically one kiloparsec diameter) regions of these galaxies using a spatial sampling four times higher than SAURON (0.27" vs. 0.94" spatial elements), resulting in almost a factor of two improvement in the median PSF. These data allow accurate study of the central regions to complement the large-scale view provided by SAURON. Here we present the stellar and gas kinematics, stellar absorption-line strengths and nebular emission-line strengths for this sample. We also characterise the stellar velocity maps using the "kinemetry" technique, and derive maps of the luminosity-weighted stellar age, metallicity and abundance ratio via stellar population models. We give a brief review of the structures found in our maps, linking also to larger-scale structures measured with SAURON. We present two previously unreported kinematically-decoupled components (KDCs) in the centres of NGC3032 and NGC4382. We compare the intrinsic size and luminosity-weighted stellar age of all the visible KDCs in the full SAURON sample, and find two types of components: kiloparsec-scale KDCs, which are older than 8 Gyr, and are found in galaxies with little net rotation; and compact KDCs, which have intrinsic diameters of less than a few hundred parsec, show a range of stellar ages from 0.5 - 15 Gyr (with 5/6 younger than 5 Gyr), are found exclusively in fast-rotating galaxies, and are close to counter-rotating around the same axis as their host. (abridged)

astro-ph↗
arXiv2006

K-Band Observations of Boxy Bulges. I. Morphology and Surface Brightness Profiles

Kn-band images, unsharp-masked images, as well as major-axis and vertically- summed surface brightness profiles are presented for 30 edge-on spiral galaxies, most with a boxy or peanut-shaped (B/PS) bulge. Such galaxies have more complex morphologies than galaxies of other bulge types, more often showing (off-)centered X structures, secondary major-axis maxima and spiral-like structures. Those features are also observed in N-body simulations of barred discs and may trace the main bar orbit families. The surface brightness profiles of galaxies with a B/PS bulge are also more complex, with typically 3 or more clearly separated regions, including a flat intermediate region (Freeman Type II profiles). Those radial breaks offer evidence for bar-driven transfer of angular momentum and radial redistribution of material. The profiles also suggest a rapid variation of the scaleheight of the disc material, contrary to conventional wisdom but again as expected from vertical resonances and instabilities in barred discs. The steep inner region of the surface brightness profiles is often shorter than the isophotally thick part of the galaxies, itself always shorter than the flat region of the profiles. Contrary to the standard `bulge + disc' model, we thus propose that galaxies with a B/PS bulge are composed of a thin concentrated disc (a disc-like bulge) contained within a partially thick bar (the B/PS bulge) and a thin outer disc. The inner disc likely formed secularly through bar-driven processes and is responsible for the steep inner region of the surface brightness profiles, while the bar is responsible for the flat region and the thick complex morphological structures observed. Those components are strongly coupled dynamically and are formed mostly of the same (disc) material. [Abridged]

astro-ph↗
arXiv2006

The SAURON project - VII. Integral-field absorption and emission-line kinematics of 24 spiral galaxy bulges

(Abridged) We present observations of the stellar and gas kinematics for a representative sample of 24 Sa galaxies obtained with our custom-built integral-field spectrograph SAURON operating on the William Herschel Telescope. Our maps typically cover the bulge dominated region. We find a significant fraction of kinematically decoupled components (12/24), many of them displaying central velocity dispersion minima. They are mostly aligned and co-rotating with the main body of the galaxies, and are usually associated with dust discs and rings detected in unsharp-masked images. Almost all the galaxies in the sample (22/24) contain significant amounts of ionised gas which, in general, is accompanied by the presence of dust. The kinematics of the ionised gas is consistent with circular rotation in a disc co-rotating with respect to the stars. The distribution of mean misalignments between the stellar and gaseous angular momenta in the sample suggest that the gas has an internal origin. The [OIII]/Hbeta ratio is usually very low, indicative of current star formation, and shows various morphologies (ring-like structures, alignments with dust lanes or amorphous shapes). The star formation rates in the sample are comparable with that of normal disc galaxies. Low gas velocity dispersion values appear to be linked to regions of intense star formation activity. We interpret this result as stars being formed from dynamically cold gas in those regions. In the case of NGC5953, the data suggest that we are witnessing the formation of a kinematically decoupled component from cold gas being acquired during the ongoing interaction with NGC5954.

astro-ph↗
arXiv2006

SAURON observations of Sa bulges: the formation of a kinematically decoupled core in NGC5953

We present results from our ongoing effort to understand the nature and evolution of nearby galaxies using the SAURON integral-field spectrograph. In this proceeding we focus on the study of the particular case formed by the interacting galaxies NGC5953 and NGC5954. We present stellar and gas kinematics of the central regions of NGC5953. We use a simple procedure to determine the age of the stellar populations in the central regions and argue that we may be witnessing the formation of a kinematically decoupled component from cold gas being acquired during the ongoing interaction with NGC5954.

astro-ph↗
arXiv2006

Young Kinematically Decoupled Components in Early-Type Galaxies

We present results from a series of follow-up observations of a sub-sample of the representative SAURON survey elliptical (E) and lenticular (S0) galaxies using the OASIS integral-field spectrograph. These observations focus on the central 10" x 10", with roughly double the spatial resolution of the SAURON observations. This increased spatial resolution reveals a number of interesting and previously unresolved features in the measured stellar kinematics and absorption-line strengths. We find that galaxies exhibiting the youngest global stellar populations (as measured with SAURON) often contain a distinctly young central region (on scales of a few hundred parsec or less) compared to the rest of the galaxy. Moreover, these compact, young components are found to be mostly counter-rotating with respect to the rest of the galaxy. Given that there is no well-established reason for such young components to `prefer' counter- over co-rotation, this finding raises the following questions: How common are these small KDCs as a function of age? Why are there more young than old compact KDCs? Where are the equivalent co-rotating components? We explore these questions using simple simulated velocity fields and stellar population models, and find that the fading of the young component as it evolves, coupled with the fact that counter-rotating components are more easily detected in the velocity field, may help explain the observed trends.

astro-ph↗
arXiv2006

The SAURON project - VI. Line strength maps of 48 elliptical and lenticular galaxies

We present absorption line strength maps of 48 representative elliptical and lenticular galaxies obtained as part of the SAURON survey of nearby galaxies. Using high-quality spectra, spatially binned to a constant signal-to-noise, we measure four key age, metallicity and abundance ratio sensitive indices from the Lick/IDS system over a two-dimensional field extending up to approximately one effective radius. We modify the classical Fe5270 index to define a new index, Fe5270S, which maximizes the usable spatial coverage of SAURON. Maps of Hbeta, Fe5015, Mgb and Fe5270S are presented for each galaxy. We use the maps to compute average line strengths integrated over circular apertures of one-eighth effective radius, and compare the resulting relations of index versus velocity dispersion with previous long-slit work. The metal line strength maps show generally negative gradients with increasing radius roughly consistent with the morphology of the light profiles. Remarkable deviations from this general trend exist, particularly the Mgb isoindex contours appear to be flatter than the isophotes of the surface brightness for about 40% of our galaxies without significant dust features. Generally these galaxies exhibit significant rotation. We infer from this that the fast-rotating component features a higher metallicity and/or an increased Mg/Fe ratio as compared to the galaxy as a whole. The Hbeta maps are typically flat or show a mild positive outwards radial gradient, while a few galaxies show strong central peaks and/or elevated overall Hbeta-strength likely connected to recent star-formation activity. For the most prominent post-starburst galaxies even the metal line strength maps show a reversed gradient. (abridged)

astro-ph↗
arXiv2006

The stellar populations of E and S0 galaxies as seen with SAURON

We present selected results from integral-field spectroscopy of 48 early-type galaxies observed as part of the SAURON survey. Maps of the Hbeta, Fe5015, Mgb and Fe5270 indices in the Lick/IDS system were derived for each of the survey galaxies. The metal line strength maps show generally negative gradients with increasing radius roughly consistent with the morphology of the light profiles. Remarkable deviations from this general trend exist, particularly the Mgb isoindex contours appear to be flatter than the isophotes of the surface brightness for about 40% of our galaxies without significant dust features. Generally these galaxies exhibit significant rotation. We infer from this that the fast-rotating component features a higher metallicity and/or an increased Mg/Fe ratio as compared to the galaxy as a whole. We also use the line strengths maps to compute average values integrated over circular apertures of one effective radius, and derive luminosity weighted ages and metallicities. The lenticular galaxies show a wide range in age and metallicity estimates, while elliptical galaxies tend to occupy regions of older stellar populations.

astro-ph↗
arXiv2005

Discovery of Counter-Rotating Gas in the Galaxies NGC1596 and NGC3203 and the Incidence of Gas Counter-Rotation in S0 Galaxies

We have identified two new galaxies with gas counter-rotation (NGC1596 and NGC3203) and have confirmed similar behaviour in another one (NGC128), this using results from separate studies of the ionized-gas and stellar kinematics of a well-defined sample of 30 edge-on disc galaxies. Gas counter-rotators thus represent 10+/-5% of our sample, but the fraction climbs to 21+/-11% when only lenticular (S0) galaxies are considered and to 27+/-13% for S0s with detected ionized-gas only. Those fractions are consistent with but slightly higher than previous studies. A compilation from well-defined studies of S0s in the literature yields fractions of 15+/-4% and 23+/-5%, respectively. Although mainly based on circumstantial evidence, we argue that the counter-rotating gas originates primarily from minor mergers and tidally-induced transfer of material from nearby objects. Assuming isotropic accretion, twice those fractions of objects must have undergone similar processes, underlining the importance of (minor) accretion for galaxy evolution. Applications of gas counter-rotators to barred galaxy dynamics are also discussed.

astro-ph↗
arXiv2005

The SAURON project - IV. The mass-to-light ratio, the virial mass estimator and the fundamental plane of elliptical and lenticular galaxies

We investigate with unprecedented accuracy the correlations between the dynamical mass-to-light ratio M/L and other global observables of E and S0 galaxies. We construct two-integral Jeans and three-integral Schwarzschild dynamical models for a sample of 25 E/S0 galaxies with SAURON integral-field stellar kinematics. We find a tight correlation of the form (M/L)=(3.80+/-0.14)*(sigma/200 km/s)^(0.84+/-0.07) between the dynamical M/L (in the I-band) and the luminosity-weighted second moment (sigma) of the line-of-sight velocity-distribution within Re. The observed rms scatter in M/L for our sample is 18%, while the inferred intrinsic scatter is ~13%. The (M/L)-sigma relation can be included in the remarkable series of tight correlations between sigma and other galaxy global observables. The comparison of the observed correlations with the predictions of the Fundamental Plane (FP), and with simple virial estimates, shows that the `tilt' of the FP of early-type galaxies, is due to a real M/L variation, while structural and orbital non-homology have a negligible effect. The virial mass is a reliable estimator of the mass in the central regions of galaxies. The best-fitting virial relation has the form (M/L)_vir=(5.0+/-0.1)*Re*sigma^2/(L*G). The comparison of the dynamical M/L with the (M/L)_pop inferred from the analysis of the stellar population, indicates a median dark matter fraction in early-type galaxies of ~30% of the total mass inside one Re. (Abridged)

astro-ph↗
arXiv2005

A SAURON study of stars and gas in Sa bulges

We present results from our ongoing effort to understand the morphological and kinematical properties of early-type galaxies using the integral-field spectrograph SAURON. We discuss the relation between the stellar and gas morphology and kinematics in our sub-sample of 24 representative Sa spiral bulges. We focus on the frequency of kinematically decoupled components and on the presence of star formation in circumnuclear rings.

astro-ph↗
arXiv2005

Dark Matter in the Central Regions of Early Type Galaxies

We investigate the well-known correlations between the dynamical mass-to-light ratio M/L and other global observables of elliptical (E) and lenticular (S0) galaxies. We construct two-integral Jeans and three-integral Schwarzschild dynamical models for a sample of 25 E/S0 galaxies with SAURON integral-field stellar kinematics to about one effective (half-light) radius R_e. The comparison of the dynamical M/L with the (M/L)_pop inferred from the analysis of the stellar population, indicates that dark matter in early-type galaxies contributes ~30% of the total mass inside one R_e, in agreement with previous studies, with significant variations from galaxy to galaxy. Our results suggest a variation in M/L at constant (M/L)_pop, which seems to be linked to the galaxy dynamics. We speculate that fast rotating galaxies have lower dark matter fractions than the slow rotating and generally more massive ones.

astro-ph↗
arXiv2005

The SAURON project - V. Integral-field emission-line kinematics of 48 elliptical and lenticular galaxies

We present the emission-line fluxes and kinematics of 48 representative elliptical and lenticular galaxies obtained with our custom-built integral-field spectrograph SAURON. Hb, [OIII], and [NI] emission lines were measured using a new procedure that simultaneously fits both the stellar spectrum and the emission lines. Using this technique we can detect emission lines down to an equivalent width of 0.1A set by the current limitations in describing galaxy spectra with synthetic and real stellar templates, rather than by the quality of our spectra. Emission is detected in 75% of our sample galaxies, and comes in a variety of resolved spatial distributions and kinematic behaviours. The ionised-gas kinematics is rarely consistent with simple coplanar circular motions. However, the gas almost never displays completely irregular kinematics, generally showing coherent motions with smooth variations in angular momentum. In the majority of the cases the gas kinematics is decoupled from the stellar kinematics, and in half of the objects this decoupling implies a recent acquisition of gaseous material. Over the entire sample however, the distribution of the mean misalignment values between stellar and gaseous angular momenta is inconsistent with a purely external origin. Consistent with previous studies, the presence of dust features is always accompanied by gas emission while the converse is not always true. A considerable range of values for the [OIII]/Hb ratio is found both across the sample and within single galaxies. Despite the limitations of this ratio as an emission-line diagnostic, this finding suggests either that a variety of mechanisms is responsible for the gas excitation in E and S0 galaxies or that the metallicity of the interstellar material is quite heterogeneous.

astro-ph↗
arXiv2005

Three-Dimensional Bar Structure and Disc/Bulge Secular Evolution

Kn-band imaging of a sample of 30 edge-on spiral galaxies with a boxy or peanut-shaped (B/PS) bulge is discussed. Galaxies with a B/PS bulge tend to have a more complex morphology than galaxies with other bulge types, unsharp-masked images revealing structures that trace the major orbit families of three-dimensional bars. Their surface brightness profiles are also more complex, typically containing 3 or more clearly separated regions, including a shallow or flat intermediate region (Freeman Type II profiles), suggestive of bar-driven transfer of angular momentum and radial redistribution of material. The data also suggest abrupt variations of the discs' scaleheights, as expected from the vertical resonances and instabilities present in barred discs but contrary to conventional wisdom. Counter to the standard `bulge + disc' model, we thus propose that galaxies with a B/PS bulge are composed of a thin concentrated disc (a disc-like bulge) contained within a partially thick bar (the B/PS bulge), itself contained within a thin outer disc. The inner disc most likely formed through bar-driven processes while the thick bar arises from buckling instabilities. Both are strongly coupled dynamically and are formed mostly of the same (disc) material.

astro-ph↗
arXiv2005

Stellar Kinematics and Populations of Early-Type Galaxies with the SAURON and OASIS Integral-Field Spectrographs

We summarise the results and achievements of integral-field spectroscopy of early-type galaxies, observed as part of a survey using both the SAURON and OASIS spectrographs. From the perspective of integral-field spectroscopy, these otherwise smooth and featureless objects show a wealth of structure, both in their stellar kinematics and populations. We focus on the stellar content, and examine properties on both kiloparsec scales with SAURON, and scales of 100's of parsecs with OASIS. These complementary studies reveal two types of kinematically distinct components (KDCs), differing primarily in their intrinsic sizes. In previous studies, KDCs and their host galaxies have generally been found to be unremarkable in other aspects. We show that large KDCs, typical of the well-studied cases, indeed show little or no age differences with their host galaxy. The KDCs detected with the higher spatial-resolution of OASIS are intrinsically smaller and include, in contrast, a significant fraction of young stars. We speculate on the relationship between KDCs and their host galaxies, and the implications for young populations in early-type galaxies.

astro-ph↗
arXiv2005

Morphology and kinematics of the ionised gas in early-type galaxies

We present results of our ongoing study of the morphology and kinematics of the ionised gas in 48 representative nearby elliptical and lenticular galaxies using the SAURON integral-field spectrograph on the 4.2m William Herschel Telescope. Making use of a recently developed technique, emission is detected in 75% of the galaxies. The ionised-gas distributions display varied morphologies, ranging from regular gas disks to filamentary structures. Additionally, the emission-line kinematic maps show, in general, regular motions with smooth variations in kinematic position angle. In most of the galaxies, the ionised-gas kinematics is decoupled from the stellar counterpart, but only some of them present signatures of recent accretion of gaseous material. The presence of dust is very common in our sample and is usually accompanied by gas emission. Our analysis of the [OIII]/Hbeta emission-line ratios, both across the whole sample as well as within the individual galaxies, suggests that there is no unique mechanism triggering the ionisation of the gas.

astro-ph↗
arXiv2005

Bar Diagnostics in Edge-On Spiral Galaxies. III. N-Body Simulations of Disks

Present in over 45% of local spirals, boxy and peanut-shaped bulges are generally interpreted as edge-on bars and may represent a key phase in the evolution of bulges. Aiming to test such claims, the kinematic properties of self-consistent 3D N-body simulations of bar-unstable disks are studied. Using Gauss-Hermite polynomials to describe the stellar kinematics, a number of characteristic bar signatures are identified in edge-on disks: 1) a major-axis light profile with a quasi-exponential central peak and a plateau at moderate radii (Freeman Type II profile); 2) a ``double-hump'' rotation curve; 3) a sometime flat central velocity dispersion peak with a plateau at moderate radii and occasional local central minimum and secondary peak; 4) an h3-V correlation over the projected bar length. All those kinematic features are spatially correlated and can easily be understood from the orbital structure of barred disks. They thus provide a reliable and easy-to-use tool to identify edge-on bars. Interestingly, they are all produced without dissipation and are increasingly realized to be common in spirals, lending support to bar-driven evolution scenarios for bulge formation. So called ``figure-of-eight'' position-velocity diagrams are never observed, as expected for realistic orbital configurations. Although not uniquely related to triaxiality, line-of-sight velocity distributions with a high velocity tail (i.e. an h3-V correlation) appear as particularly promising tracers of bars. The stellar kinematic features identified grow in strength as the bar evolves and vary little for small inclination variations. Many can be used to trace the bar length. Comparisons with observations are encouraging and support the view that boxy and peanut-shaped bulges are simply thick bars viewed edge-on.

astro-ph↗
arXiv2004

Bar-Driven Evolution and 2D Spectroscopy of Bulges

A multi-faceted approach is described to constrain the importance of bar-driven evolution in disk galaxies, particularly bulge formation. N-body simulations are used to construct stellar kinematic bar diagnostics for edge-on systems and to quantify the expected vertical structure of bars, and they are compared to observations of 30 edge-on spirals, most with a boxy bulge. Long-slit spectra of the galaxies show characteristic double-hump rotation curves, dispersion profiles with secondary peaks and/or flat maxima, and correlated h3 and V profiles, indicating that most of them harbor edge-on bars. The presence of cold, quasi-axisymmetric central stellar disks is also suggested, presumably formed through bar-driven gaseous inflow and star formation. K-band imaging of the same galaxies spectacularly highlights radial variations of the bars' scaleheights, as expected from vertical disk instabilities. The light profiles also vary radially in shape but never approach a classic de Vaucouleurs law. Filtering of the images further isolates the specific orbit families at the origin of the boxy structure, which can be directly related to periodic orbit calculations in 3D barred potentials. Bars are thus shown to contribute substantially to the formation of both large-scale triaxial bulges and embedded central disks. Relevant results from the SAURON survey of the stellar/ionized-gas kinematics and stellar populations of spheroids are also described. Examples are used to illustrate the potential of coupling stellar kinematics and linestrengths (age and metallicity), here specifically to unravel the dynamical evolution and related chemical enrichment history of bars and bulges. [Abridged]

astro-ph↗