Testing the nature of S0 galaxies using planetary nebula kinematics in NGC 1023

We investigate the manner in which lenticular galaxies are formed by studying their stellar kinematics: an S0 formed from a fading spiral galaxy should display similar cold outer disc kinematics to its progenitor, while an S0 formed in a minor merger should be more dominated by random motions. In a pilot study, an attempt to distinguish between these scenarios, we have measured the planetary nebula (PN) kinematics of the nearby S0 system NGC 1023. Using the Planetary Nebula Spectrograph, we have detected and measured the line-of-sight velocities of 204 candidate planetary nebulae (PNe) in the field of this galaxy. Out to intermediate radii, the system displays the kinematics of a normal rotationally supported disc system. After correction of its rotational velocities for asymmetric drift, the galaxy lies just below the spiral galaxy Tully–Fisher relation, as one would expect for a fading system. However, at larger radii the kinematics undergo a gradual but major transition to random motion with little rotation. This transition does not seem to reflect a change in the viewing geometry or the presence of a distinct halo component, since the number counts of PNe follow the same simple exponential decline as the stellar continuum with the same projected disc ellipticity out to large radii. The galaxy's small companion, NGC 1023A, does not seem to be large enough to have caused the observed modification either. This combination of properties would seem to indicate a complex evolutionary history in either the transition to form an S0 or in the past life of the spiral galaxy from which the S0 formed. More data sets of this type from both spirals and S0s are needed in order to definitively determine the relationship between these types of system.     

Circumnuclear kinematics in NGC 5248: the origin of nuclear spiral arms

We present for the first time a two-dimensional velocity field of the central region of the grand-design spiral galaxy NGC 5248, at 0.9-arcsec spatial resolution. The H α velocity field is dominated by circular rotation. While no systematic streaming motions are seen in the area of the nuclear grand-design spiral or the circumnuclear ring, the amplitude of residual velocities, after subtracting a model circular velocity field, reaches 20 km s⁻¹ in projection. The rotation curve levels out at around 140 km s⁻¹, after a well-resolved and rather shallow rise. We have generated an analytical model for the nuclear spiral and fitted it to our observations to obtain estimates of the pattern speed of the spiral and the speed of sound in the central region of NGC 5248. Our results are consistent with a low pattern speed, suggesting that the nuclear spiral rotates with the same rate as the main spiral structure in NGC 5248, and thus that the spiral structure is coupled from scales of a few hundred parsecs to several kiloparsecs. We have also compared the observed structure and kinematics between the nuclear regions of NGC 5248 and M100. Several similarities and differences are discussed, including the location of the peak emission regions on major and minor axes, and the spiral arm streaming motions. We find no kinematic evidence for the presence of a nuclear bar in NGC 5248.     

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]/Hβ 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.     

Star formation rates and the kinematics of gas in the spiral arms of NGC 628

The relations between star formation rates along the spiral arms and the velocities of gas inflow into the arms in the grand‐design galaxy NGC 628 were studied. We found that the radial distribution of the average star formation rate in individual star formation regions in regular spiral arms correlates with the velocity of gas inflow into the spiral arms. Both distributions have maxima at a galactocentric distance of 4.5–5 kpc. There are no correlations between the radial distributions of the average star formation rate in star formation regions in spiral arms and outside spiral arms in the main disc. We also did not find a correlation between the radial distribution of the average star formation rate in star formation regions in spiral arms and the H I column density. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The shape of the dark matter halo in the early-type galaxy NGC 2974

We present H  i observations of the elliptical galaxy NGC 2974, obtained with the Very Large Array. These observations reveal that the previously detected H  i disc in this galaxy is in fact a ring. By studying the harmonic expansion of the velocity field along the ring, we constrain the elongation of the halo and find that the underlying gravitational potential is consistent with an axisymmetric shape.
We construct mass models of NGC 2974 by combining the H  i rotation curve with the central kinematics of the ionized gas, obtained with the integral-field spectrograph SAURON. We introduce a new way of correcting the observed velocities of the ionized gas for asymmetric drift, and hereby disentangle the random motions of the gas caused by gravitational interaction from those caused by turbulence. To reproduce the observed flat rotation curve of the H  i gas, we need to include a dark halo in our mass models. A pseudo-isothermal sphere provides the best model to fit our data, but we also tested an NFW halo and modified Newtonian dynamics, which fit the data marginally worse.
The mass-to-light ratio M / L _I increases in NGC 2974 from 4.3 M_⊙/L_⊙, _I at one effective radius to 8.5 M_⊙/L_{⊙, I} at 5  R _e. This increase of M / L already suggests the presence of dark matter: we find that within 5  R _e at least 55 per cent of the total mass is dark.     

Neutral hydrogen gas distribution and kinematics in NGC 7479

We have investigated the effects of a bar and an asymmetric spiral structure on the neutral hydrogen distribution and kinematics in the strongly barred spiral galaxy NGC 7479. The strongest 21-cm line emission at 1-kpc resolution comes from the western spiral arm which appears to be slightly inclined to the plane of the main disc. In contrast, the area within the radius of the bar is devoid of 21-cm line emission. The radio continuum emission at 21 cm follows the bar dust lanes, but beyond 3 kpc from the nucleus the radio continuum emission has a peculiar morphology, unlike that of optical and near-infrared images. We did not detect any low surface brightness gas-rich companions in the near neighbourhood of NGC 7479. This leads us to propose that the strong western spiral arm was created in a recent minor merger.     

S0 galaxies in Fornax: data and kinematics

We have obtained long-slit spectroscopy for a sample of nine S0 galaxies in the Fornax Cluster using the FORS2 spectrograph at the 8.2-m European Southern Observatory (ESO) Very Large Telescope (VLT). From these data, we have extracted the kinematic parameters, comprising the mean velocity, velocity dispersion and higher moment h ₃ and h ₄ coefficients, as a function of position along the major axes of these galaxies. Comparison with published kinematics indicates that earlier data are often limited by their lower signal-to-noise ratio and relatively poor spectral resolution. The greater depth and higher dispersion of the new data mean that we reach well beyond the bulges of these systems, probing their disc kinematics in some detail for the first time. Qualitative inspection of the results for individual galaxies shows that they are not entirely simple systems, perhaps indicating a turbulent past. None the less, we are able to derive reliable circular velocities for most of these systems, which points the way towards a study of their Tully–Fisher relation. This study, along with an analysis of the stellar populations of these systems out to large galactocentric distances, will form the bases of future papers exploiting these new high-quality data, hopefully shedding new light on the evolutionary history of these systems.     

Dynamics of the boxy elliptical galaxy NGC 1600

We use three-integral models to infer the distribution function (DF) of the boxy E3–E4 galaxy NGC 1600 from surface brightness and line-profile data on the minor and major axes. We assume axisymmetry and that the mass-to-light ratio is constant in the central ∼1 R _e. Stars in the resulting gravitational potential move mainly on regular orbits. We use an approximate third integral K from perturbation theory and write the DF as a sum of basis functions in the three integrals E , L _z and K . We then fit the projected moments of these basis functions to the kinematic observables and deprojected density, using a non-parametric algorithm. The deduced dynamical structure is radially anisotropic, with σ _θ σ _r ≈ σ _φ σ _r ≈0.7 on the major axis. Both on the minor axis and near the centre the velocity distribution is more isotropic; thus the model is flattened by equatorial radial orbits. The kinematic data are fitted without the need for a central black hole; the central mass determined previously from ground-based data therefore overestimates the actual black-hole mass. The mass-to-light ratio of the stars is M L _V =6  h ₅₀. The anisotropy structure of NGC 1600 with a radially anisotropic main body and more nearly isotropic centre is similar to that found recently in NGC 1399, 2434, 3379 and 6703, suggesting that this pattern may be common amongst massive elliptical galaxies. We discuss a possible merger origin of NGC 1600 in the light of these results.     

Ages and metallicities of globular clusters in NGC 4472

We have derived ages and metallicities from co-added spectra of 131 globular clusters associated with the giant elliptical galaxy NGC 4472. Based upon a calibration with Galactic globular clusters, we find that our sample of globular clusters in NGC 4472 span a metallicity range of approximately −1.6≤[Fe/H]≤0 dex. There is evidence of a radial metallicity gradient in the globular cluster system which is steeper than that seen in the underlying starlight. Determination of the absolute ages of the globular clusters is uncertain, but formally, the metal-poor population of globular clusters has an age of 14.5±4 Gyr and the metal-rich population is 13.8±6 Gyr old. Monte Carlo simulations indicate that the globular cluster populations present in these data are older than 6 Gyr at the 95 per cent confidence level. We find that within the uncertainties, the globular clusters are old and coeval, implying that the bimodality seen in the broadband colours primarily reflects metallicity and not age differences.     

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  1 R _e  . 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 Λ 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.     

Regularized orbit models unveiling the stellar structure and dark matter halo of the Coma elliptical NGC 4807

This is the second in a series of papers dedicated to unveiling the mass structure and orbital content of a sample of flattened early-type galaxies in the Coma cluster. The ability of our orbit libraries to reconstruct internal stellar motions and the mass composition of a typical elliptical in the sample is investigated by means of Monte Carlo simulations of isotropic rotator models. The simulations allow a determination of the optimal amount of regularization needed in the orbit superpositions. It is shown that under realistic observational conditions and with the appropriate regularization, internal velocity moments can be reconstructed to an accuracy of ≈15 per cent; the same accuracy can be achieved for the circular velocity and dark matter fraction. In contrast, the flattening of the halo remains unconstrained. Regularized orbit superpositions are applied to a first galaxy in our sample, NGC 4807, for which stellar kinematical observations extend to  3  r _eff  . The galaxy seems dark-matter dominated outside   r > 2  r _eff  . Logarithmic dark matter potentials are consistent with the data, as well as NFW profiles, mimicking logarithmic potentials over the observationally sampled radial range. In both cases, the derived stellar mass-to-light ratio ϒ agrees well with independently obtained mass-to-light ratios from stellar population analysis. The achieved accuracy is  Δϒ≈ 0.5  . Kinematically, NGC 4807 is characterized by mild radial anisotropy outside   r > 0.5  r _eff  , becoming isotropic towards the centre. Our orbit models hint at either a distinct stellar component or weak triaxiality in the outer parts of the galaxy.     

Clumpy metal concentrations in elliptical galaxies NGC 4374 and NGC 4636

We present a high spatial resolution study of metal distributions in the nearby,gas-rich elliptical galaxies NGC 4374 and NGC 4636 with the Chandra ACIS archive data.We define the hardness ratio HRFeL as the ratio of the emission in 0.65-1.4keV to that in 0.3-0.6keV and 1.4-3.5keV(after the magnesium and silicon lines are excluded),and HRcont as the ratio of the emission in 1.4-3.5keV to that in 0.3-0.6keV,so that the HRFeL and HRcont maps can be used to trace the iron abundance and gas temperature distribu...