Photometric Major Axis Research Articles

Keck Integral-field Spectroscopy of M87 Reveals an Intrinsically Triaxial Galaxy and a Revised Black Hole Mass

The three-dimensional intrinsic shape of a galaxy and the mass of the central supermassive black hole provide key insight into the galaxy’s growth history over cosmic time. Standard assumptions of a spherical or axisymmetric shape can be simplistic and can bias the black hole mass inferred from the motions of stars within a galaxy. Here, we present spatially resolved stellar kinematics of M87 over a two-dimensional 250″ × 300″ contiguous field covering a radial range of 50 pc–12 kpc from integral-field spectroscopic observations at the Keck II Telescope. From about 5 kpc and outward, we detect a prominent 25 km s−1 rotational pattern, in which the kinematic axis (connecting the maximal receding and approaching velocities) is 40° misaligned with the photometric major axis of M87. The rotational amplitude and misalignment angle both decrease in the inner 5 kpc. Such misaligned and twisted velocity fields are a hallmark of triaxiality, indicating that M87 is not an axisymmetrically shaped galaxy. Triaxial Schwarzschild orbit modeling with more than 4000 observational constraints enabled us to determine simultaneously the shape and mass parameters. The models incorporate a radially declining profile for the stellar mass-to-light ratio suggested by stellar population studies. We find that M87 is strongly triaxial, with ratios of p = 0.845 for the middle-to-long principal axes and q = 0.722 for the short-to-long principal axes, and determine the black hole mass to be , where the second error indicates the systematic uncertainty associated with the distance to M87.

The present-day globular cluster kinematics of lenticular galaxies from the E-MOSAICS simulations and their relation to the galaxy assembly histories

ABSTRACT We study the present-day rotational velocity (Vrot) and velocity dispersion (σ) profiles of the globular cluster (GC) systems in a sample of 50 lenticular (S0) galaxies from the E-MOSAICS galaxy formation simulations. We find that $82{{\ \rm per\ cent}}$ of the galaxies have GCs that are rotating along the photometric major axis of the galaxy (aligned), while the remaining $18{{\ \rm per\ cent}}$ of the galaxies do not (misaligned). This is generally consistent with the observations from the SLUGGS survey. For the aligned galaxies, classified as peaked and outwardly decreasing ($49{{\ \rm per\ cent}}$), flat ($24{{\ \rm per\ cent}}$), and increasing ($27{{\ \rm per\ cent}}$) based on the Vrot/σ profiles out to large radii, we do not find any clear correlation between these present-day Vrot/σ profiles of the GCs and the past merger histories of the S0 galaxies, unlike in previous simulations of galaxy stars. For just over half of the misaligned galaxies, we find that the GC misalignment is the result of a major merger within the last $10\, \mathrm{Gyr}$ so that the ex-situ GCs are misaligned by an angle between 0° (co-rotation) and 180° (counter-rotation), with respect to the in situ GCs, depending on the orbital configuration of the merging galaxies. For the remaining misaligned galaxies, we suggest that the in situ metal-poor GCs, formed at early times, have undergone more frequent kinematic perturbations than the in situ metal-rich GCs. We also find that the GCs accreted early and the in situ GCs are predominantly located within 0.2 virial radii (R200) from the centre of galaxies in 3D phase-space diagrams.

A Novel Framework for Modeling Weakly Lensing Shear Using Kinematics and Imaging at Moderate Redshift

Abstract Kinematic weak lensing describes the distortion of a galaxy’s projected velocity field due to lensing shear, an effect recently reported for the first time by Gurri et al. based on a sample of 18 galaxies at z ∼ 0.1. In this paper, we develop a new formalism that combines the shape information from imaging surveys with the kinematic information from resolved spectroscopy to better constrain the lensing distortion of source galaxies and to potentially address systematic errors that affect conventional weak-lensing analyses. Using a Bayesian forward model applied to mock galaxy observations, we model distortions in the source galaxy’s velocity field simultaneously with the apparent shear-induced offset between the kinematic and photometric major axes. We show that this combination dramatically reduces the statistical uncertainty on the inferred shear, yielding statistical error gains of a factor of 2–6 compared to kinematics alone. While we have not accounted for errors from intrinsic kinematic irregularities, our approach opens kinematic lensing studies to higher redshifts where resolved spectroscopy is more challenging. For example, we show that ground-based integral-field spectroscopy of background galaxies at z ∼ 0.7 can deliver gravitational shear measurements with signal-to-noise ratio of ∼1 per source galaxy at 1 arcminute separations from a galaxy cluster at z ∼ 0.3. This suggests that even modest samples observed with existing instruments could deliver improved galaxy cluster mass measurements and well-sampled probes of their halo mass profiles to large radii.

Searching for Local Counterparts of High-redshift Poststarburst Galaxies in Integral Field Unit Spectroscopic Surveys of Nearby Galaxies

Abstract Searching in the MaNGA IFU survey, I identify nine galaxies that have strong Balmer absorption lines and weak nebular emission lines measured from the spectra integrated over the entire IFU. The spectral features measured from the bulk of the stellar light make these galaxies local analogs of high-redshift spectroscopically selected poststarburst galaxies and thus proxies to understand the mechanisms producing poststarburst galaxies at high redshifts. I present the distributions of absorption line indices and emission line strengths, as well as the stellar kinematics of these local poststarburst galaxies. Almost all local poststarburst galaxies have central compact emission line regions at the central <1 kpc, mostly powered by weak star formation activities. The age-sensitive absorption line indices EW(Hδ) and D n 4000 indicate that the stellar populations at the outskirts are older. Toy stellar population synthesis models suggest that the galaxies as a whole are experiencing a rapid decline of star formation with residual star formation activities at the centers. These features indicate that most poststarburst galaxies are the aftermath of highly dissipative processes that drive gas into centers, invoke centrally concentrated star formation, and then quench the galaxies. Meanwhile, when measurable, poststarburst galaxies have the directions of maximum stellar velocity gradients aligned with photometric major axes, which suggest against major mergers being the principal driving mechanism, while gas-rich minor mergers are plausible. While directly obtaining the same quality of spatially resolved spectra of high-redshift poststarburst galaxies is very difficult, finding proper local counterparts provides an alternative to understand quenching processes in the distant universe.

The SLUGGS survey: combining stars, globular clusters, and planetary nebulae to understand the assembly history of early-type galaxies from their large radii kinematics

ABSTRACT We investigate the kinematic properties of nine nearby early-type galaxies with evidence of a disc-like component. Three of these galaxies are located in the field, five in the group, and only one in the cluster environment. By combining the kinematics of the stars with those of the globular clusters (GCs) and planetary nebulae (PNe), we probe the outer regions of our galaxies out to $\sim 4\!-\!6\, R_{\mathrm{e}}$. Six galaxies have PNe and red GCs that show good kinematic alignment with the stars, whose rotation occurs along the photometric major-axis of the galaxies, suggesting that both the PNe and red GCs are good tracers of the underlying stellar population beyond that traced by the stars. Additionally, the blue GCs also show rotation that is overall consistent with that of the red GCs in these six galaxies. The remaining three galaxies show kinematic twists and misalignment of the PNe and GCs with respect to the underlying stars, suggesting recent galaxy interactions. From the comparison with simulations, we propose that all six aligned galaxies that show similar dispersion-dominated kinematics at large radii ($\gt 2\!-\!3\, R_{\mathrm{e}}$) had similar late (z ≲ 1) assembly histories characterized by mini mergers (mass-ratio < 1:10). The different Vrot/σ profiles are then the result of an early (z > 1) minor merger (1:10 < mass-ratio < 1:4) for the four galaxies with peaked and decreasing Vrot/σ profiles and of a late minor merger for the two galaxies with flat Vrot/σ profiles. The three misaligned galaxies likely formed through multiple late minor mergers that enhanced their velocity dispersion at all radii, or a late major merger that spun-up both the GC subpopulations at large radii. Therefore, lenticular galaxies can have complex merger histories that shape their characteristic kinematic profile shapes.

The MASSIVE Survey XIV—Stellar Velocity Profiles and Kinematic Misalignments from 200 pc to 20 kpc in Massive Early-type Galaxies

Abstract We use high spatial resolution stellar velocity maps from the Gemini integral-field spectrograph (IFS) and wide-field velocity maps from the McDonald Mitchell IFS to study the stellar velocity profiles and kinematic misalignments from ∼200 pc to ∼20 kpc in 20 early-type galaxies (ETGs) with stellar mass M * > 1011.7 M ⊙ in the MASSIVE survey. While 80% of the galaxies have low spins (λ < 0.1) and low rotational velocities (<50 km s−1) in both the central region and the main body, we find a diverse range of velocity features and misalignment angles. For the 18 galaxies with measurable central kinematic axes, 10 have well aligned kinematic axis and photometric major axis, and the other eight galaxies have misalignment angles that are distributed quite evenly from 15° to the maximal value of 90°. There is a strong correlation between central kinematic misalignment and galaxy spin, where all four galaxies with significant spins have well aligned kinematic and photometric axes, but only 43% of the low-spin galaxies are well aligned. The central and main-body kinematic axes within a galaxy are not always aligned. When the two kinematic axes are aligned (∼60% of the cases), they are either also aligned with the photometric major axis or orthogonal to it. We find 13 galaxies to also exhibit noticeable local kinematic twists, and one galaxy to have a counterrotating core. A diverse assembly history consisting of multiple gas-poor mergers of a variety of progenitor mass ratios and orbits is likely to be needed to account for the predominance of low spins and the wide range of central and main-body velocity features reported here for local massive ETGs.

The ultra-diffuse galaxy NGC 1052-DF2 with MUSE

The so-called ultra-diffuse galaxy NGC 1052-DF2 was announced to be a galaxy lacking dark matter based on a spectroscopic study of its constituent globular clusters. Here we present the first spectroscopic analysis of the stellar body of this galaxy using the MUSE integral-field spectrograph at the (ESO) Very Large Telescope. The MUSE datacube simultaneously provides DF2’s stellar velocity field and systemic velocities for seven globular clusters (GCs). We further discovered three planetary nebulae (PNe) that are likely part of this galaxy. While five of the clusters had velocities measured in the literature, we were able to confirm the membership of two more candidates through precise radial velocity measurements, which increases the measured specific frequency of GCs in DF2. The mean velocity of the diffuse stellar body, 1792.9+1.4−1.8 km s−1, is consistent with the mean globular cluster velocity. We detect a weak but significant velocity gradient within the stellar body, with a kinematic axis close to the photometric major axis, making it a prolate-like rotator. We estimate a velocity dispersion from the clusters and PNe of σint = 10.6−2.3+3.9 km s−1. The velocity dispersion σDF2⋆(Re) for the stellar body within one effective radius is 10.8+3.2−4.0 km s−1. Considering various sources of systemic uncertainties, this central value varies between 5 and 13 km s−1, and we conservatively report a 95% confidence upper limit to the dispersion within one Re of 21 km s−1. We provide updated mass estimates based on these dispersions corresponding to the different distances to NGC 1052-DF2 that have been reported in the recent literature.

The MASSIVE Survey – X. Misalignment between kinematic and photometric axes and intrinsic shapes of massive early-type galaxies

We use spatially resolved two-dimensional stellar velocity maps over a $107"\times 107"$ field of view to investigate the kinematic features of 90 early-type galaxies above stellar mass $10^{11.5}M_\odot$ in the MASSIVE survey. We measure the misalignment angle $\Psi$ between the kinematic and photometric axes and identify local features such as velocity twists and kinematically distinct components. We find 46% of the sample to be well aligned ($\Psi < 15^{\circ}$), 33% misaligned, and 21% without detectable rotation (non-rotators). Only 24% of the sample are fast rotators, the majority of which (91%) are aligned, whereas 57% of the slow rotators are misaligned with a nearly flat distribution of $\Psi$ from $15^{\circ}$ to $90^{\circ}$. 11 galaxies have $\Psi \gtrsim 60^{\circ}$ and thus exhibit minor-axis ("prolate") rotation in which the rotation is preferentially around the photometric major axis. Kinematic misalignments occur more frequently for lower galaxy spin or denser galaxy environments. Using the observed misalignment and ellipticity distributions, we infer the intrinsic shape distribution of our sample and find that MASSIVE slow rotators are consistent with being mildly triaxial, with mean axis ratios of $b/a=0.88$ and $c/a=0.65$. In terms of local kinematic features, 51% of the sample exhibit kinematic twists of larger than $20^{\circ}$, and 2 galaxies have kinematically distinct components. The frequency of misalignment and the broad distribution of $\Psi$ reported here suggest that the most massive early-type galaxies are mildly triaxial, and that formation processes resulting in kinematically misaligned slow rotators such as gas-poor mergers occur frequently in this mass range.

Spectroscopic decomposition of NGC 3521: unveiling the properties of the bulge and disc

We study the kinematics and the stellar populations of the bulge and disc of the spiral galaxy NGC 3521. At each position in the field of view, we separate the contributions of the bulge and the disc from the total observed spectrum and study their kinematics, age, and metallicities independently. Their properties are clearly distinct: the bulge rotates more slowly, has a higher velocity dispersion, and is less luminous than the disc. We identify three main populations of stars in NGC 3521: old ($\geq7$ Gyr), intermediate ($\approx$ 3 Gyr), and young ($\leq$1 Gyr). The mass and light of NGC 3521 are dominated by the intermediate stellar population. The youngest population contributes mostly to the disc component and its contribution increases with radius. We also study the luminosity-weighed properties of the stars in NGC 3521. Along the photometric major axis, we find: i) no age gradient for the stars in the bulge, and a negative age gradient for the stars in the disc; ii) negative metallicity gradients and sub-solar $\alpha$-enhancement for both the bulge and the disc. We propose the following picture for the formation of NGC 3521: initial formation a long time ago ($\geq 7$ Gyr), followed by a second burst of star formation or a merger ($\approx$ 3 Gyrs ago), which contributed predominantly to the mass-build up of the bulge. Recently ($\leq 1$ Gyr), the disc of NGC 3521 experienced an additional episode of star formation that started in the innermost regions.

Sloshing in its cD halo: MUSE kinematics of the central galaxy NGC 3311 in the Hydra I cluster

Context.Early-type galaxies (ETGs) show a strong size evolution with redshift. This evolution is explained by fast “in-situ” star formation at high-zfollowed by a late mass assembly mostly driven by minor mergers that deposit stars primarily in the outer halo.Aims.We aim to identify the main structural components of the Hydra I cD galaxy NGC 3311 to investigate the connection between the central galaxy and the surrounding stellar halo.Methods.We produce maps of the line-of-sight velocity distribution (LOSVD) moments from a mosaic of MUSE pointings covering NGC 3311 out to 25 kpc. Combining deep photometric and spectroscopic data, we model the LOSVD maps using a finite mixture distribution, including four non-concentric components that are nearly isothermal spheroids, with different line-of-sight systemic velocitiesV, velocity dispersionsσ, and small (constant) values of the higher order Gauss-Hermite momentsh3andh4.Results.The kinemetry analysis indicates that NGC 3311 is classified as a slow rotator, although the galaxy shows a line-of-sight velocity gradient along the photometric major axis. The comparison of the correlations betweenh3andh4withV/σwith simulated galaxies indicates that NGC 3311 assembled mainly through dry mergers. Theσprofile rises to ≃ 400 km s-1at 20 kpc, a significant fraction (0.55) of the Hydra I cluster velocity dispersion, indicating that stars there were stripped from progenitors orbiting in the cluster core. The finite mixture distribution modeling supports three inner components related to the central galaxy and a fourth component with large effective radius (51 kpc) and velocity dispersion (327 km s-1) consistent with a cD envelope. We find that the cD envelope is offset from the center of NGC 3311 both spatially (8.6 kpc) and in velocity (ΔV= 204 km s-1), but coincides with the cluster core X-ray isophotes and the mean velocity of core galaxies. Also, the envelope contributes to the broad wings of the LOSVD measured by largeh4values within 10 kpc.Conclusions.The cD envelope of NGC 3311 is dynamically associated with the cluster core, which in Hydra I is in addition displaced from the cluster center, presumably due to a recent subcluster merger.

The origin of double-peaked narrow lines in active galactic nuclei – III. Feedback from biconical AGN outflows

We apply an analytic Markov Chain Monte Carlo model to a sample of 18 AGN-driven biconical outflows that we identified from a sample of active galaxies with double-peaked narrow emission lines at z < 0.1 in the Sloan Digital Sky Survey. We find that 8/18 are best described as asymmetric bicones, 8/18 are nested bicones, and 2/18 are symmetric bicones. From the geometry and kinematics of the models, we find that these moderate-luminosity AGN outflows are large and energetic. The biconical outflows axes are randomly oriented with respect to the photometric major axis of the galaxy, implying a randomly oriented and clumpier torus to collimate the outflow, but the torus also allows some radiation to escape equatorially. We find that 16/18 (89%) outflows are energetic enough to drive a two-staged feedback process in their host galaxies. All of these outflows geometrically intersect the photometric major axis of the galaxy, and 23% of outflow host galaxies are significantly redder or have significantly lower specific star formation rates when compared to a matched sample of active galaxies.

Dusty spirals versus gas kinematics in the inner kiloparsec of four low-luminosity active galactic nuclei

We used the Gemini Multi-Object Spectrograph Integral Field Unit to map the gas distribution, excitation and kinematics within the inner kiloparsec of four nearby low-luminosity active galaxies: NGC3982, NGC4501, NGC2787 and NGC4450. The observations cover the spectral range 5600-7000{\AA} at a velocity resolution of 120km/s and spatial resolution ranging from 50 to 70pc at the galaxies. Extended emission in H{\alpha}, [NII]{\lambda}{\lambda}6548,6583, [SII]{\lambda}{\lambda}6716,6730 over most of the field-of-view is observed for all galaxies, while only NGC3982 shows [OI]{\lambda}6300 extended emission. The H{\alpha} equivalent widths combined with the [NII]/H{\alpha} line ratios reveal that NGC3982 and NGC4450 harbor Seyfert nuclei surrounded by regions with LINER excitation, while NGC2787 and NGC4501 harbor LINER nuclei. NGC3982 shows a partial ring of recent star-formation at 500pc from the nucleus, while in NGC4501 a region at 500pc west of the nucleus shows LINER excitation but has been interpreted as an aging HII region with the gas excitation dominated by shocks from supernovae. The line-of-sight velocity field of the gas shows a rotation pattern for all galaxies, with deviations from pure disk rotation observed in NGC3982, NGC4501 and NGC4450. For NGC4501 and NGC4450, many of these deviations are spatially coincident with dust structures seen in optical continuum images, leading to the interpretation that the deviations are due to shocks in the gas traced by the dust. A speculation is that these shocks lead to loss of angular momentum, allowing the gas to be transferred inwards to feed the AGN. In the case of NGC2787, instead of deviations in the rotation field, we see a misalignment of 40{^\circ} between the orientation of the line of nodes of the gas rotation and the photometric major axis of the galaxy. Evidence of compact nuclear outflows are seen in NGC4501 and NGC4450.

RESOLVING GAS FLOWS IN THE ULTRALUMINOUS STARBURST IRAS 23365+3604 WITH KECK LGSAO/OSIRIS*

ABSTRACT Keck OSIRIS/LGSAO observations of the ultraluminous galaxy IRAS 23365+3604 resolve a circumnuclear bar (or irregular disk) of semimajor axis 0.″42 (520 pc) in Paα emission. The line-of-sight velocity of the ionized gas increases from the northeast toward the southwest; this gradient is perpendicular to the photometric major axis of the infrared emission. Two pairs of bends in the zero-velocity line are detected. The inner bend provides evidence for gas inflow onto the circumnuclear disk/bar structure. We interpret the gas kinematics on kiloparsec scales in relation to the molecular gas disk and multiphase outflow discovered previously. In particular, the fast component of the ouflow (detected previously in line wings) is not detected, adding support to the conjecture that the fast wind originates well beyond the nucleus. These data directly show the dynamics of gas inflow and outflow in the central kiloparsec of a late-stage, gas-rich merger and demonstrate the potential of integral field spectroscopy to improve our understanding of the role of gas flows during the growth phase of bulges and supermassive black holes.

The SLUGGS survey: globular cluster kinematics in a ‘double sigma’ galaxy – NGC 4473

NGC 4473 is a so--called double sigma (2$\sigma$) galaxy, i.e. a galaxy with rare, double peaks in its 2D stellar velocity dispersion. Here, we present the globular cluster (GC) kinematics in NGC 4473 out to $\sim10\,R_e$ (effective radii) using data from combined HST/ACS and Subaru/Suprime--Cam imaging and Keck/DEIMOS spectroscopy. We find that the 2$\sigma$ nature of NGC 4473 persists up to 3 $R_e$, though it becomes misaligned to the photometric major axis. We also observe a significant offset between the stellar and GC rotation amplitudes. This offset can be understood as a co--addition of counter--rotating stars producing little net stellar rotation. We identify a sharp radial transition in the GC kinematics at $\sim4\,R_e$ suggesting a well defined kinematically distinct halo. In the inner region ($<4\,R_e$), the blue GCs rotate along the photometric major axis, but in an opposite direction to the galaxy stars and red GCs. In the outer region ($>4\,R_e$), the red GCs rotate in an opposite direction compared to the inner region red GCs, along the photometric major axis, while the blue GCs rotate along an axis intermediate between the major and minor photometric axes. We also find a kinematically distinct population of very red GCs in the inner region with elevated rotation amplitude and velocity dispersion. The multiple kinematic components in NGC 4473 highlight the complex formation and evolutionary history of this 2$\sigma$ galaxy, as well as a distinct transition between the inner and outer components.

Photometry and dynamics of the minor mergers AM 1228-260 and AM 2058-381

We investigate interaction effects on the dynamics and morphology of the galaxy pairs AM 2058-381 and AM 1228-260. This work is based on r′ images and long-slit spectra obtained with the Gemini Multi-Object Spectrograph at the Gemini South Telescope. The luminosity ratio between the main (AM 2058A) and secondary (AM 2058B) components of the first pair is a factor of ∼ 5, while for the other pair, the main (AM 1228A) component is 20 times more luminous than the secondary (AM 1228B). The four galaxies have pseudo-bulges, with a Sérsic index n < 2. Their observed radial velocities profiles (RVPs) present several irregularities. The receding side of the RVP of AM 2058A is displaced with respect to the velocity field model, while there is a strong evidence that AM 2058B is a tumbling body, rotating along its major axis. The RVPs for AM 1228A indicate a misalignment between the kinematic and photometric major axes. The RVP for AM 1228B is quite perturbed, very likely due to the interaction with AM 1228A. NFW halo parameters for AM 2058A are similar to those of the Milky Way and M 31. The halo mass of AM 1228A is roughly 10 per cent that of AM 2058A. The mass-to-light (M/L) of AM 2058 agrees with the mean value derived for late-type spirals, while the low M/L for AM 1228A may be due to the intense star formation ongoing in this galaxy.

A kinematically distinct core and minor-axis rotation: the MUSE perspective on M87

Abstract We present evidence for the presence of a low-amplitude kinematically distinct component in the giant early-type galaxy M87, via data sets obtained with the SAURON and MUSE integral-field spectroscopic units. The MUSE velocity field reveals a strong twist of ∼140° within the central 30 arcsec connecting outwards such a kinematically distinct core to a prolate-like rotation around the large-scale photometric major axis of the galaxy. The existence of these kinematic features within the apparently round central regions of M87 implies a non-axisymmetric and complex shape for this galaxy, which could be further constrained using the presented kinematics. The associated orbital structure should be interpreted together with other tracers of the gravitational potential probed at larger scales (e.g. globular clusters, ultra-compact dwarfs, planetary nebulae): it would offer an insight in the assembly history of one of the brightest galaxies in the Virgo cluster. These data also demonstrate the potential of the MUSE spectrograph to uncover low-amplitude spectral signatures.

The SLUGGS survey: globular cluster system kinematics and substructure in NGC 4365

We present a kinematic analysis of the globular cluster (GC) system of the giant elliptical galaxy NGC 4365 and find several distinct kinematic substructures. This analysis is carried out using radial velocities for 269 GCs, obtained with the DEIMOS instrument on the Keck II telescope as part of the SAGES Legacy Unifying Globulars and Galaxies Survey (SLUGGS). We find that each of the three (formerly identified) GC colour subpopulations reveal distinct rotation properties. The rotation of the green GC subpopulation is consistent with the bulk of NGC 4365's stellar light, which `rolls' about the photometric major axis. The blue and red GC subpopulations show `normal' rotation about the minor axis. We also find that the red GC subpopulation is rotationally dominated beyond 2.5 arcmin (~17 kpc) and that the root mean squared velocity of the green subpopulation declines sharply with radius suggesting a possible bias towards radial orbits relative to the other GC subpopulations. Additionally, we find a population of low velocity GCs that form a linear structure running from the SW to the NE across NGC 4365 which aligns with the recently reported stellar stream towards NGC 4342. These low velocity GCs have g'-i' colours consistent with the overall NGC 4365 GC system but have velocities consistent with the systemic velocity of NGC 4342. We discuss the possible formation scenarios for the three GC subpopulations as well as the possible origin of the low velocity GC population.

The kinematics and spatial distribution of stellar populations in E+A galaxies

We have used the Gemini Multi–Object Spectragraph (GMOS) instrument on the 8.1–m Gemini–South Telescope to obtain spatially resolved two–colour imaging and integral field unit (IFU) spectroscopy of a sample of 10 nearby (z= 0.04–0.20)‘E+A’ galaxies selected from the Two Degree Field Galaxy Redshift Survey. These galaxies have been selected to lie in a variety of environments from isolated systems to rich clusters. Surface brightness profiles measured using our imaging data show the isophotal profiles of our sample are generally r1/4–like, consistent with a sample dominated by early–type galaxies. Only one galaxy in our sample has an obvious exponential (‘disc–like’) component in the isophotal profile. This is further underscored by all galaxies having early Hubble–type morphological classifications, and showing a behaviour in the central velocity dispersion–absolute magnitude plane that is consistent with the Faber–Jackson relation, once the transitory brightening that occurs in the E+A phase is corrected for. In addition, two–thirds of our sample shows clear evidence of either ongoing or recent tidal interactions/mergers, as evidenced by the presence of tidal tails and disturbed morphologies. While all the galaxies in our sample have total integrated colours that are relatively blue (in keeping with their E+A status), they show a diversity of colour gradients, possessing central core regions that are either redder, bluer or indistinct in colour relative to their outer regions. The E+A spectra are well fitted by that of a young stellar population, the light from which is so dominant that it is impossible to quantify the presence of the underlying old stellar population. Consistent with other recent findings, there is little evidence for radial gradients in the Balmer absorption line equivalent widths over the central few kiloparsecs (<4 kpc), although we are unable to search for the previously reported radial gradients at larger galactocentric radii due to the limited spatial extent of our IFU data. Kinematically, the most striking property is the significant and unambiguous rotation that is seen in all our E+A galaxies, with it being generally aligned close to the photometric major axis. This is contrary to the findings of Norton et al., who found little or no evidence for rotation in a very similar sample of nearby E+A galaxies. We also clearly demonstrate that our E+A galaxies are, in all but one case, consistent with being ‘fast rotators’, based on their internal angular momentum per unit mass measured as a function of radius and ellipticity. We argue that the combination of disturbed morphologies and significant rotation in these galaxies supports their production via gas–rich galaxy mergers and interactions. The large fraction of fast rotators argues against equal mass mergers being the dominant progenitor to the E+A population.

The molecular polar disc in NGC 2768

We present CO(1-0) and CO(2-1) maps of the molecular polar disc in the elliptical galaxy NGC 2768 obtained at the IRAM Plateau de Bure Interferometer. The maps have a resolution of 2.6 x 2.3 and 1.2 x 1.2 for the CO(1-0) and CO(2-1) lines, respectively. The CO maps complete the unique picture of the interstellar medium (ISM) of NGC 2768; the dust, molecular gas, ionised gas and neutral hydrogen (HI) trace the recent acquisition of cold and cool gas over two orders of magnitude in radii (and much more in density). In agreement with the other ISM components, the CO distribution extends nearly perpendicularly to the photometric major axis of the galaxy. Velocity maps of the CO show a rotating polar disc or ring in the inner kiloparsec. This cool gas could lead to kinematic substructure formation within NGC 2768. However, the stellar velocity field and H-beta absorption linestrength maps from the optical integral-field spectrograph SAURON give no indication of a young and dynamically cold stellar population coincident with the molecular polar disc. Very recent or weak star formation, undetectable in linestrengths, nevertheless remains a possibility and could be at the origin of some of the ionised gas observed. Millimetre continuum emission was also detected in NGC 2768, now one of only a few low-luminosity active galactic nuclei with observed millimetre continuum emission.

Spatial distribution of dust in the shell elliptical NGC 5982

Aims. Shells in Ellipticals are peculiar faint sharp edged features that are thought to be formed by galaxy mergers. We determine the shell and dust distributions, and colours of a well-resolved shell and the underlying galaxy in NGC 5982, and compare the spatial distributions of the dust and gas phases, in order to investigate the origin of shells and dust in NGC 5982. Methods. We use Spitzer data in the wavelength range from 3.6 to 160 mu m and HST/ACS optical data. Surface photometry, galaxy models and residual images are determined using IRAF task ELLIPSE and GALPHOT task ELLIPFIT. After subtracting the galaxy models, residual images are used to identify the shells. Excess emission is identified by subtracting a scaled stellar photosphere emission template. Colours [3.6]-[4.5], [3.6]-[8.0] and [5.8]-[8.0] of the underlying galaxy and [5.8]-[8.0] of the excess emission are obtained using techniques similar to those of Pahre et al. The [3.6]-[4.5] and V -I colours of a well-resolved shell are determined using techniques of Sikkema et al. Results. Shells are visible in the 3.6 mu m image, fainter at 4.5 mu m and undetected at 5.8 mu m. We find two new shells, the outermost identified so far in NGC 5982. All shells apart from shell 24 are on the photometric major axis. Shell 24 is the only one for which reliable colours are obtained. It has colours [3.6]-[4.5] and V - I bluer than the underlying galaxy. Excess emission at 4.5, 5.8 and 8 mu m is widely extended. The ratio of excess to total emission decreases towards the centre, which could be related to a lower mass loss rate from AGB stars. The [3.6]-[4.5] colour indicates a major contribution from late-type (K2-M0III) stars towards the centre. Dust traced by 24 mu m emission follows the stellar spatial distribution, supporting a circumnuclear origin. We find extended 160 mu m emission from cold dust, possibly forming a disk inclined to the principal axes, with a mass of a few 10(5) M-circle dot. The warmer big grains and/or smaller grains traced by the 70 mu m emission are confined to a small region elongated along the minor axis and shifted a few kpc from the centre. Conclusions. We detect for the first time shells from mid-infrared data. The very different distributions of dust, warm gas and HI gas together with the presence of shells and a kinematically decoupled core suggest a minor merger in NGC 5982.