Michael R. Merrifield

Kinematic properties of early-type galaxy haloes using planetary nebulae★

We present new planetary nebulae (PNe) positions, radial velocities and magnitudes for six early-type galaxies obtained with the Planetary Nebulae Spectrograph (PNS), along with derived two-dimensional velocity and velocity dispersion fields, and the α parameters (i.e. the number of PNe per unit luminosity). We also present new deep absorption-line long-slit kinematics for three galaxies in the sample, obtained with the FOcal Reducer and low dispersion Spectrograph (FORS2) at the Very Large Telescope (VLT). We extend this study to include additional 10 early-type galaxies with PNe radial velocity measurements available from the literature, including previous PNS studies, in order to obtain a broader description of the outer-halo kinematics in early-type galaxies. These data extend the information derived from stellar absorption-line kinematics to typically several and up to 8 effective radii. The combination of photometry, absorption-line and PNe kinematics shows (i) a good agreement between the PNe number density distribution and the stellar surface brightness in the region where the two data sets overlap; (ii) a good agreement between PNe and absorption-line kinematics; (iii) that the mean rms velocity profiles fall into two groups, with part of the galaxies characterized by slowly decreasing profiles and the remainder having steeply falling profiles; (iv) a larger variety of velocity dispersion radial profiles; (v) that twists and misalignments in the velocity fields are more frequent at large radii, including some fast rotator galaxies; (vi) that outer haloes are characterized by more complex radial profiles of the specific angular momentum-related λ_R parameter than observed within 1 R_e; (vii) that many objects are more rotationally dominated at large radii than in their central parts and (viii) that the halo kinematics are correlated with other galaxy properties, such as total B band and X-ray luminosity, isophotal shape, total stellar mass, V/σ and α parameter, with a clear separation between fast and slow rotators. Based in part on observations made with the William Herschel Telescope operated by the Isaac Newton Group in the Spanish Observatorio del Roque de los Muchachos on the island of La Palma, of the Instituto de Astrofisica de Canarias, and on observations collected at the European Southern Observatory, Chile, Program: 76.B-0788(A). E-mail: lcoccato@mpe.mpg.de

The evolution of barred spiral galaxies in the Hubble Deep Fields North and South

The frequency of barred spiral galaxies as a function of redshift contains important information on the gravitational influence of stellar disks in their dark matter halos and also may distinguish between contemporary theories for the origin of galactic bulges. In this paper we present a new quantitative method for determining the strength of barred spiral structure, and verify its robustness to redshift-dependent effects. By combining galaxy samples from the Hubble Deep Field North with newly available data from the Hubble Deep Field South, we are able to define a statistical sample of 18 objectively-defined low-inclination barred spiral systems with I814W < 23.2 mag. Analysing the proportion of barred spiral galaxies seen as a function of redshift, we find a significant decline in the barred fraction beyond redshifts z ≃ 0.5. The physical significance of this effect remains unclear, but several possibilities include dynamically hotter (or increasingly dark-matter dominated) high-redshift discs, or an enhanced efficiency in bar destruction at high redshifts. By investigating the formation of the “orthogonal” axis of Hubble’s classification tuning fork, our result complements studies of evolution in the early–late sequence, and pushes to later epochs the redshift at which the Hubble classification sequence is observed to be in place.

The Planetary Nebula Spectrograph elliptical galaxy survey: the dark matter in NGC 4494

We present new Planetary Nebula Spectrograph observations of the ordinary elliptical galaxy NGC 4494, resulting in positions and velocities of 255 planetary nebulae out to seven effective radii (25 kpc). We also present new wide-field surface photometry from MMT/Megacam, and long-slit stellar kinematics from VLT/FORS2. The spatial and kinematical distributions of the planetary nebulae agree with the field stars in the region of overlap. The mean rotation is relatively low, with a possible kinematic axis twist outside 1Re. The velocity dispersion profile declines with radius, though not very steeply, down to ∼70 km s − 1 at the last data point. We have constructed spherical dynamical models of the system, including Jeans analyses with multi-component A cold dark matter (CDM) motivated galaxies as well as logarithmic potentials. These models include special attention to orbital anisotropy, which we constrain using fourth-order velocity moments. Given several different sets of modelling methods and assumptions, we find consistent results for the mass profile within the radial range constrained by the data. Some dark matter (DM) is required by the data; our best-fitting solution has a radially anisotropic stellar halo, a plausible stellar mass-to-light ratio and a DM halo with an unexpectedly low central density. We find that this result does not substantially change with a flattened axisymmetric model. Taken together with other results for galaxy halo masses, we find suggestions for a puzzling pattern wherein most intermediate-luminosity galaxies have very low concentration haloes, while some high-mass ellipticals have very high concentrations. We discuss some possible implications of these results for DM and galaxy formation.

The rotation curve of the Milky Way to 2.5 R[SUB]0[/SUB] from the thickness of the H I layer

A new approach to calculating the H I rotation curve of the outer Milky Way is presented. This method uses the variation with galactic longitude in the angular thickness of the H I layer to solve for the distance to slices through the H I data cube. The derived rotation curve shows excellent agreement with other kinematic constraints if the local galactic constants have values of R0 = 7.9 +/- 0.8 kpc and Theta0 = 200 +/- 10 km/s. This procedure also yields the variation in thickness of the H I layer with radius. If it is assumed that this gas has a constant velocity dispersion, then application of the hydrostatic equilibrium equation implies that the mass density in the plane of the Galaxy drops approximately exponentially with radius with an e-folding length of 0.49 +/- 0.06 R0. Combining this information with the derived rotation curve then allows the construction of a full mass model for the Milky Way.

A deep kinematic survey of planetary nebulae in the Andromeda galaxy using the Planetary Nebula Spectrograph

This thesis presents a survey of compact emission-line objects in the Andromeda Galaxy (M31), performed using a novel new instrument, the Planetary Nebula Spectrograph. The final catalogue contains the positions, magnitudes and velocities for 3300 objects displaying [O III] emission at 5007 Angstroms, of which 2615 are found likely to be planetary nebulae (PNe) associated with M31. The survey area covers some 6 square degrees, taking in the whole of M31s disk out to a projected radius of 1.5 degrees, with extensions along the major and minor axes, and the Northern Spur and Southern Stream regions. The calibrated data have been checked for internal consistency and compared with other catalogues. With the exception of the very central, high surface brightness region of M31, this survey is complete to a magnitude limit of m(5007) ~ 23.75, 3.5 magnitudes into the planetary nebula luminosity function. A number of satellite and background galaxies are located within the M31 survey area and emission line objects associated with these have been identified. Analyses of the basic kinematic properties associated with each of these galaxies are presented. The PN catalogue has been analysed for non-kinematic, kinematic and dynamical properties. We have examined the planetary nebula luminosity function across M31, the spatial distribution of PNe, and the luminosity specific PN density. These analyses indicate that apart from a small change in the luminosity specific PN density there are no other non-kinematic differences between the bulge and disk PN populations suggesting that the sample of PNe is not strongly populated by objects whose progenitors are more massive stars. There is no indication of a significant halo PN population. Rotation curves for both the surveyed PNe and H II regions have been produced as well as the PN velocity dispersion profile. The H II rotation curve is seen to be in good agreement with those in the literature, while the PN rotation curve and velocity dispersion profile exhibit some peculiarities. However, under the approximation of an axisymmetric disk these are shown to be mutually consistent, but require the disk to flare with radius if the shape of its velocity ellipsoid remains invariant. The kinematic properties of photometric substructures are examined and kinematic substructures are searched for. A possible kinematic extension of the Southern Stream has been discovered. A new approach is taken in order to search for dynamical streams in the disk of the galaxy, involving an examination of the energy angular momentum plane. This also provides a new way of looking at the distribution function of a tracer population in a disk galaxy.

Dearth of dark matter or massive dark halo? Mass-shape-anisotropy degeneracies revealed by nmagic dynamical models of the elliptical galaxy NGC 3379

Recent results from the Planetary Nebula Spectrograph (PNS) survey have revealed a rapidly falling velocity dispersion profile in the nearby elliptical galaxy NGC 3379, casting doubts on whether this intermediate-luminosity galaxy has the kind of dark matter (DM) halo expected in � cold dark matter (� CDM) cosmology. We present a detailed dynamical study of this galaxy, combining ground based long-slit spectroscopy, integral-field data from the Spectrographic Areal Unit for Research on Optical Nebulae (SAURON) instrument and PNS data reaching to more than seven effective radii. We construct dynamical models with the flexible χ 2 -made-to-measure (χ 2 M2M) particle method implemented in the NMAGIC code. We fit spherical, axisymmetric and some triaxial models to the photometric and combined kinematic data in a sequence of gravitational potentials whose circular velocity curves at large radii vary between a near-Keplerian decline and the nearly flat shapes generated by massive haloes. We find that models with a range of halo masses, anisotropies, shapes and inclinations are good representations of the data. In particular, the data are consistent both with nearisotropic systems dominated by the stellar mass and with models in moderately massive haloes with strongly radially anisotropic outer parts (β 0.8 at 7Re). Formal likelihood limits would exclude (at 1σ ) the model with stars only, as well as halo models with vcirc(7Re) 250 km s −1 . All valid models fitting all the data are dynamically stable over gigayears, including the most anisotropic ones. Overall the kinematic data for NGC 3379 out to 7Re are consistent with a range of mass distributions in this galaxy. NGC 3379 may well have a DM halo as predicted by recent merger ¯

The PN.S elliptical galaxy survey : Data reduction, planetary nebula catalog, and basic dynamics for NGC 3379

We present results from Planetary Nebula Spectrograph (PN. S) observations of the elliptical galaxy NGC 3379 and a description of the data reduction pipeline. We detected 214 planetary nebulae, of which 191 are ascribed to NGC 3379 and 23 to the companion galaxy NGC 3384. Comparison with data from the literature shows that the PN. S velocities have an internal error of

The Tully–Fisher relation for S0 galaxies

We present a study of the local B- and Ks-band Tully–Fisher relation (TFR) between absolute magnitude and maximum circular speed in S0 galaxies. To make this study, we have combined kinematic data, including a new high-quality spectral data set from the Fornax cluster, with homogeneous photometry from the Third Reference Catalogue of Bright Galaxies and Two Micron All Sky Survey catalogues, to construct the largest sample of S0 galaxies ever used in a study of the TFR. Independent of environment, S0 galaxies are found to lie systematically below the TFR for nearby spirals in both optical and infrared bands. This offset can be crudely interpreted as arising from the luminosity evolution of spiral galaxies that have faded since ceasing star formation. However, we also find a large scatter in the TFR. We show that most of this scatter is intrinsic, not due to the observational uncertainties. The presence of such a large scatter means that the population of S0 galaxies cannot have formed exclusively by the above simple fading mechanism after all transforming at a single epoch. To better understand the complexity of the transformation mechanism, we have searched for correlations between the offset from the TFR and other properties of the galaxies such as their structural properties, central velocity dispersions and ages (as estimated from line indices). For the Fornax cluster data, the offset from the TFR correlates with the estimated age of the stars in the individual galaxies, in the sense and of the magnitude expected if S0 galaxies had passively faded since being converted from spirals. This correlation implies that a significant part of the scatter in the TFR arises from the different times at which galaxies began their transformation.

The origin of the cosmic soft X-ray background: optical identification of an extremely deep ROSAT survey

We present the results of the deepest optically identified X-ray survey yet made. The X-ray survey was obtained with the ROSAT position-sensitive proportional counter (PSPC) and reaches a flux limit of 1.6 x 10(-15) erg cm(-2) s(-1) (0.5-2.0 keV). Above a flux limit of 2 x 10(-15) erg cm(-2) s(-1) we define a complete sample of 70 sources, of which 59 are identified. For a further five sources we have tentative identifications and for a further four the X-ray error boxes are blank to R = 23 mag. At brighter fluxes (greater than or equal to 10(-14) erg cm(-2) s(-1)) we confirm the results of previous less deep X-ray surveys, with 84 per cent of our sources being QSOs. However, at the faint flux limit the survey is dominated by a population of galaxies with narrow emission lines (NELGs). In addition, at intermediate fluxes we find a small number of groups and clusters of galaxies at redshifts generally >0.3. Most of these groups are poor systems of low X-ray Luminosity and the number that we find is consistent with a zero evolutionary scenario, unlike the situation for high-luminosity clusters at the same redshift. To a flux limit of 2 x 10(-15) erg cm(-2) s(-1), QSOs contribute > 31 per cent of the cosmic soft X-ray background (XRB), groups/clusters contribute similar to 10 per cent and NELGs contribute similar to 8 per cent. However, the QSO differential source count slope below 10(-14) erg cm(-2) s(-1) is similar to-1.4, severely sub-Euclidean, as is the (poorly defined) group/cluster slope, whereas the differential NELG slope is close to Euclidean (similar to-2.4). If the NELG source counts continue to rise at that slope, all of the remaining cosmic soft XRB will be explained by a flux limit of similar to 1-2 x 10(-16) erg cm(-2) s(-1), with NELGs contributing about one quarter of the XRB. The average NELG X-ray spectrum is harder than that of the QSOs, and similar to that of the remaining unresolved cosmic XRB, suggesting that NELGs will also be substantial contributors to the XRB at higher energies. The observed NELGs lie in the redshift range 0.1-0.6 and have M-R = -20 to -23, approximately 3 mag more luminous than typical field galaxies. They have predominantly blue colours, and some are definitely spirals, but the presence of some ellipticals cannot yet be ruled out. Many are in interacting or disturbed systems. The NELGs have optical spectra similar to those of the majority of the field galaxy population at a similar redshift and may simply be the more luminous members of the emission-line field galaxy population. Based on optical line ratios and X-ray/optical flux ratios, the NELGs, both as a sample and within individual galaxies, appear to be a mixture of starburst galaxies and true active galactic nuclei (AGN).

A search for counter-rotating stars in S0 galaxies

We have obtained high signal-to-noise spectra along the major axes of 28 S0 galaxies in order to search for the presence of disc stars on retrograde orbits. Full line-of-sight velocity distributions are extracted from the data, and the velocity distributions are modelled as arising from the superposition of populations of stars on prograde and retrograde orbits. We find no new cases in which a significant fraction of disc stars lie on retrograde orbits; an identical analysis of NGC 4550 does reveal the previously known counter-rotating stellar disc in this system. Upper limits determined for each object indicate that no more than similar to 5 per cent of the observed disc star light could arise from counter-rotating stellar components. These results suggest that previously discovered disc galaxies with counter-rotating stars are exceptional, and that (at 95 per cent confidence) less than 10 per cent of S0 galaxies contain significant counter-rotating populations. The most likely value for the fraction of such S0 galaxies lies closer to 1 per cent. This result contrasts with the prevalence of counter-rotating gas in these systems; combining our new observations with existing data, we find that 24+/-8 per cent (1 sigma error) of the gas discs in S0 galaxies counter-rotate relative to their stellar components.