arXiv subjects

M. Bureau

Publications and source records attributed to M. Bureau.

87 records · Page 5Linked to original sources

arXiv2001

Ram Pressure and Anomalous Shell Formation in HoII

Neutral hydrogen VLA D-array observations of the dwarf irregular galaxy HoII, a prototype galaxy for studies of shell formation, are presented. The large-scale HI morphology is reminiscent of ram pressure and is unlikely caused by interactions. A case is made for intragroup gas in poor and compact groups like the M81 group, to which HoII belongs. Numerous shortcomings of the supernova explosions and stellar winds scenario to create the shells in HoII are highlighted, and it is suggested that ram pressure may be able to reconcile the observations available.

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arXiv2001

The SAURON project. II. Sample and early results

Early results are reported from the SAURON survey of the kinematics and stellar populations of a representative sample of nearby E, S0 and Sa galaxies. The survey is aimed at determining the intrinsic shape of the galaxies, their orbital structure, the mass-to-light ratio as a function of radius, the age and metallicity of the stellar populations, and the frequency of kinematically decoupled cores and nuclear black holes. The construction of the representative sample is described, and its properties are illustrated. A comparison with long-slit spectroscopic data establishes that the SAURON measurements are comparable to, or better than, the highest-quality determinations. Comparisons are presented for NGC 3384 and NGC 4365 where stellar velocities and velocity dispersions are determined to a precision of 6 km/s, and the h3 and h4 parameters of the line-of-sight velocity distribution to a precision of better than 0.02. Extraction of accurate gas emission-line intensities, velocities and line widths from the datacubes is illustrated for NGC 5813. Comparisons with published line-strengths for NGC 3384 and NGC 5813 reveal uncertainties of < 0.1 A on the measurements of the Hbeta, Mgb and Fe5270 indices. Integral-field mapping uniquely connects measurements of the kinematics and stellar populations to the galaxy morphology. The maps presented here illustrate the rich stellar kinematics, gaseous kinematics, and line-strength distributions of early-type galaxies. The results include the discovery of a thin, edge-on, disk in NGC 3623, confirm the axisymmetric shape of the central region of M32, illustrate the LINER nucleus and surrounding counter-rotating star-forming ring in NGC 7742, and suggest a uniform stellar population in the decoupled core galaxy NGC 5813.

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arXiv2001

SAURON: An Innovative Look at Early-Type Galaxies

A summary of the SAURON project and its current status is presented. SAURON is a panoramic integral-field spectrograph designed to study the stellar kinematics, gaseous kinematics, and stellar populations of spheroids. Here, the sample of galaxies and its properties are described. The instrument is detailed and its capabilities illustrated through observational examples. These includes results on the structure of central stellar disks, the kinematics and ionization state of gaseous disks, and the stellar populations of galaxies with decoupled cores.

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arXiv2001

Kinemetry: quantifying kinematic maps

We describe a new technique, kinemetry, to quantify kinematic maps of early-type galaxies in an efficient way. We present the first applications to velocity fields obtained with the integral-field spectrograph SAURON.

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arXiv2001

Ionized Gas in Spheroids: The SAURON Survey

Early results are reported from the SAURON survey of the kinematics and stellar populations of nearby cluster and field E, S0, and Sa galaxies. We present maps of the Hbeta and [OIII] emission-line distribution and kinematics for NGC 5813 and NGC 7742.

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arXiv2001

The Large-Scale Structure and Environment of HoII

Neutral hydrogen VLA D-array observations of the dwarf irregular galaxy HoII, a prototype galaxy for studies of shell formation and self-propagating star formation, are presented. The large-scale HI morphology is reminiscent of ram pressure and is unlikely caused by interactions. A case is made for intragroup gas in poor and compact groups similar to the M81 group, to which HoII belongs. Numerous shortcomings of the supernova explosions and stellar winds scenario to create the shells in HoII are highlighted, and it is suggested that ram pressure may be able to reconcile the numerous observations available.

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arXiv2001

The SAURON project. I. The panoramic integral-field spectrograph

A new integral-field spectrograph, SAURON, is described. It is based on the Tiger principle, and uses a lenslet array. SAURON has a large field of view and high throughput, and allows simultaneous sky subtraction. Its design is optimized for studies of the stellar kinematics, gas kinematics, and line-strength distributions of nearby early-type galaxies. The instrument design and specifications are described, as well as the extensive analysis software which was developed to obtain fully calibrated spectra, and the associated kinematic and line-strength measurements. A companion paper reports on the first results obtained with SAURON on the William Herschel Telescope.

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arXiv2000

Galaxy mapping with the SAURON integral-field spectrograph: The star formation history of NGC 4365

We report the first wide-field mapping of the kinematics and stellar populations in the E3 galaxy NGC 4365. The velocity maps extend previous long-slit work. They show two independent kinematic subsystems: the central 300 x 700 pc rotates about the projected minor axis, and the main body of the galaxy, 3 x 4 kpc, rotates almost at right angles to this. The line-strength maps show that the metallicity of the stellar population decreases from a central value greater than solar, to one-half solar at a radius of 2 kpc. The decoupled core and main body of the galaxy have the same luminosity-weighted age, of ~14 Gyr, and the same elevated magnesium-to-iron ratio. The two kinematically distinct components have thus shared a common star formation history. We infer that the galaxy underwent a sequence of mergers associated with dissipative star formation that ended >12 Gyr ago. The misalignment between the photometric and kinematic axes of the main body is unambiguous evidence of triaxiality. The similarity of the stellar populations in the two components suggests that the observed kinematic structure has not changed substantially in 12 Gyr.

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arXiv2000

SAURON Observations of Disks in Early-Type Galaxies

We briefly describe the SAURON project, aimed at determining the intrinsic shape and internal dynamics of spheroids. We focus here on the ability of SAURON to identify gaseous and stellar disks and to measure their morphology and kinematics. We illustrate some of our results with complete maps of NGC3377, NGC3623, and NGC4365.

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arXiv1999

The Shape and Figure Rotation of NGC 2915's Dark Halo

NGC 2915 is a blue compact dwarf galaxy with a very extended HI disk showing a short central bar and extended spiral arms, both reaching far beyond the optical component. We use Tremaine & Weinberg (1984) method to measure the pattern speed of the bar from HI radio synthesis data. Our measurements yield a pattern speed of 0.21+/-0.06 km/s/arcsec (8.0+/-2.4 km/s/kpc for D=5.3 Mpc), in disagreement with the general view that corotation in barred disks lies just outside the end of the bar, but consistent with recent models of barred galaxies with dense dark matter halos. Our adopted bar semi-length puts corotation at more than 1.7 bar radii. The existence of the pattern is also problematic. Because NGC 2915 is isolated, interactions cannot account for the structure observed in the HI disk. We also demonstrate that the low observed disk surface density and the location of the pseudo-rings make it unlikely that swing amplification or bar-driven spiral arms could explain the bar and spiral pattern. Based on the similarity of the dark matter and HI surface density profiles, we discuss the possibility of dark matter distributed in a disk and following closely the HI distribution. The disk then becomes unstable and can naturally form a bar and spiral pattern. However, this explanation is hard to reconcile with some properties of NGC 2915. We also consider the effect of a massive and extended triaxial dark matter halo with a rotating figure. The existence of such halos is supported by CDM simulations showing strongly triaxial dark halos with slow figure rotation. The observed structure of the HI disk can then arise through forcing by the rotating triaxial figure. We associate the measured pattern speed in NGC 2915 with the figure rotation of its dark halo.

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arXiv1999

Bar Diagnostics in Edge-On Spiral Galaxies. II. Hydrodynamical Simulations

We develop diagnostics based on gas kinematics to identify the presence of a bar in an edge-on spiral galaxy and determine its orientation. We use position-velocity diagrams (PVDs) obtained by projecting edge-on two-dimensional hydrodynamical simulations of the gas flow in a barred galaxy potential. We show that when a nuclear spiral is formed, the presence of a gap in the PVDs, between the signature of the nuclear spiral and that of the outer parts of the disk, reliably indicates the presence of a bar. This gap is due to the presence of shocks and inflows in the simulations, leading to a depletion of the gas in the outer bar region. If no nuclear spiral signature is present in a PVD, only indirect arguments can be used to argue for the presence of a bar. The shape of the signature of the nuclear spiral, and to a lesser extent that of the outer bar region, allows to determine the orientation of the bar with respect to the line-of-sight. The presence of dust can also help to discriminate between viewing angles on either side of the bar. Simulations covering a large fraction of parameter space constrain the bar properties and mass distribution of observed galaxies. The strongest constraint comes from the presence or absence of the signature of a nuclear spiral in the PVD.

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arXiv1999

The Nature of Boxy/Peanut-Shaped Bulges in Spiral Galaxies

We present a systematic observational study of the relationship between bars and boxy/peanut-shaped (B/PS) bulges. We first review and discuss proposed mechanisms for their formation, focussing on accretion and bar-buckling scenarios. Using new methods relying on the kinematics of edge-on disks, we then look for bars in a large sample of edge-on spiral galaxies with a B/PS bulge and in a smaller control sample of edge-on spirals with more spheroidal bulges. We present position-velocity diagrams of the ionised gas obtained from optical long-slit spectroscopy. We show that almost all B/PS bulges are due to a thick bar viewed edge-on, while only a few extreme cases may be due to the accretion of external material. This strongly supports the bar-buckling mechanism for the formation of B/PS bulges. None of the galaxies in the control sample shows evidence for a bar, which suggests conversely that bars are generally B/PS. We consider the effects of dust in the disk of the galaxies, but conclude that it does not significantly affect our results. Unusual emission line ratios correlating with kinematical structures are observed in many objects, and we argue that this is consistent with the presence of strong bars in the disk of the galaxies. As expected from N-body simulations, the boxy--peanut transition appears to be related to the viewing angle, but more work is required to derive the precise orientation of the bars in the bulges. The reliable identification of bars in edge-on spiral galaxies opens up for the first time the possibility of studying observationally their vertical structure.

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arXiv1999

Bar Diagnostics in Edge-On Spiral Galaxies. I. The Periodic Orbits Approach

We develop diagnostics to detect the presence and orientation of a bar in an edge-on disk, using its kinematical signature in the position-velocity diagram (PVD) of a spiral galaxy observed edge-on. Using a well-studied barred spiral galaxy mass model, we briefly review the orbital properties of two-dimensional non-axisymmetric disks and identify the main families of periodic orbits. We use those families as building blocks to model real galaxies and calculate the PVDs obtained for various realistic combinations of periodic orbit families and for a number of viewing angles with respect to the bar. We show that the global structure of the PVD is a reliable bar diagnostic in edge-on disks. Specifically, the presence of a gap between the signatures of the families of periodic orbits in the PVD follows directly from the non-homogeneous distribution of the orbits in a barred galaxy. Similarly, material in the two so-called forbidden quadrants of the PVD results from the elongated shape of the orbits. We show how the shape of the signatures of the dominant x1 and x2 families of periodic orbits in the PVD can be used efficiently to determine the viewing angle with respect to the bar and, to a lesser extent, to constrain the mass distribution of an observed galaxy. We also address the limitations of the models when interpreting observational data.

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arXiv1999

Bars and Boxy/Peanut-Shaped Bulges: An Observational Point of View

Prompted by work on the buckling instability in barred spiral galaxies, much effort has been devoted lately to the study of boxy/peanut-shaped (B/PS) bulges. Here, we present new bar diagnostics for edge-on spiral galaxies based on periodic orbits calculations and hydrodynamical simulations. Both approaches provide reliable ways to identify bars and their orientations in edge-on systems. We also present the results of an observational search for bars in a large sample of edge-on spirals with and without B/PS bulges. We show that most B/PS bulges are due to the presence of a thick bar viewed edge-on while only a few may be due to accretion. This strongly supports the bar-buckling mechanism for the formation of B/PS bulges.

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arXiv1998

FCC 35 and its HI Companion: Multi-Wavelength Observations and Interpretation

The Fornax cluster galaxy FCC 35 shows an unusual multiply-peaked integrated HI profile (Bureau, Mould & Staveley-Smith 1996). We have now observed FCC 35 with the Australia Telescope Compact Array (ATCA) and have found a compact HI source with M_{HI} = 2.2 x 10^{8} M_{sun}, and a spatially overlapping complex of HI gas with the same mass. By combining optical observations with the HI data, we are able to identify FCC 35 as a young compact source of star formation with a nearby intergalactic HI cloud which is devoid of stars. We classify FCC 35 as a blue compact dwarf (BCD) or HII galaxy, having large amounts of neutral hydrogen, very blue colors ((U-V) = 0.1), and a low metallicity spectrum with strong narrow emission lines. Together with the presence of the HI cloud, this suggests that FCC 35 is the result of a recent interaction within the Fornax cluster.

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