We show that the stellar specific angular momentum j , mass M , and bulge fraction of normal galaxies of all morphological types are consistent with a simple model based on a linear superposition of ...independent disks and bulges. In this model, disks and bulges follow scaling relations of the form and with but offset from each other by a factor of 8 2 over the mass range . Separate fits for disks and bulges alone give and , respectively. This model correctly predicts that galaxies follow a curved 2D surface in the 3D space of , , and . We find no statistically significant indication that galaxies with classical and pseudo bulges follow different relations in this space, although some differences are permitted within the observed scatter and the inherent uncertainties in decomposing galaxies into disks and bulges. As a byproduct of this analysis, we show that the j -M scaling relations for disk-dominated galaxies from several previous studies are in excellent agreement with each other. In addition, we resolve some conflicting claims about the dependence of the j -M scaling relations. The results presented here reinforce and extend our earlier suggestion that the distribution of galaxies with different in the j -M diagram constitutes an objective, physically motivated alternative to subjective classification schemes such as the Hubble sequence.
The ultra-diffuse galaxy NGC1052-DF2 has a very low velocity dispersion, indicating that it has little or no dark matter. Here we report the discovery of a second galaxy in this class, residing in ...the same group. NGC1052-DF4 closely resembles NGC1052-DF2 in terms of its size, surface brightness, and morphology; has a similar distance of ; and also has a population of luminous globular clusters extending out to from the center of the galaxy. Accurate radial velocities of the diffuse galaxy light and seven of the globular clusters were obtained with the Low Resolution Imaging Spectrograph on the Keck I telescope. The velocity of the diffuse light is identical to the median velocity of the clusters, , and close to the central velocity of the NGC 1052 group. The rms spread of the globular cluster velocities is very small at . Taking observational uncertainties into account we determine an intrinsic velocity dispersion of , consistent with the expected value from the stars alone ( ) and lower than expected from a standard NFW halo ( ). We conclude that NGC1052-DF2 is not an isolated case but that a class of such objects exists. The origin of these large, faint galaxies with an excess of luminous globular clusters and an apparent lack of dark matter is, at present, not understood.
There is good evidence that the centers of massive early-type galaxies have a bottom-heavy stellar initial mass function (IMF) compared to that of the Milky Way. Here we study the radial variation of ...the IMF within such galaxies, using a combination of high-quality Keck spectroscopy and a new suite of stellar population synthesis models that cover a wide range in metallicity. As in the previous studies in this series, the models are fitted directly to the spectra and treat all elemental abundance ratios as free parameters. Using newly obtained spectroscopy for six galaxies, including deep data extending to ∼ 1 R e for the galaxies NGC 1407, NGC 1600, and NGC 2695, we find that the IMF varies strongly with galactocentric radius. For all six galaxies the IMF is bottom-heavy in the central regions, with average mass-to-light ratio "mismatch" parameter ( M L ) ( M L ) MW 2.5 at R = 0. The IMF rapidly becomes more bottom-light with increasing radius, flattening off near the Milky Way value ( 1.1 ) at R > 0.4 R e . A consequence is that the luminosity-weighted average IMF depends on the measurement aperture: within R = R e we find 〈 〉 L = 1.3 - 1.5 , consistent with recent lensing and dynamical results from SLACS and ATLAS 3 D . Our results are also consistent with several earlier studies that were based on analyses of radial gradients of line indices. The observed IMF gradients support galaxy formation models in which the central regions of massive galaxies had a different formation history than their outer parts. Finally, we make use of the high signal-to-noise central spectra of NGC 1407 and NGC 2695 to demonstrate how we can disentangle IMF effects and abundance effects.
The velocity dispersion of the ultra diffuse galaxy NGC1052-DF2 was found to be km s−1, much lower than expected from the stellar mass-halo mass relation and nearly identical to the expected value ...from the stellar mass alone. This result was based on the radial velocities of 10 luminous globular clusters that were assumed to be associated with the galaxy. A more precise measurement is possible from high-resolution spectroscopy of the diffuse stellar light. Here we present an integrated spectrum of the diffuse light of NGC1052-DF2 obtained with the Keck Cosmic Web Imager (KCWI), with an instrumental resolution of instr 12 km s−1. The systemic velocity of the galaxy is vsys = 1805 1.1 km s−1, in very good agreement with the average velocity of the globular clusters ( km s−1). There is no evidence for rotation within the KCWI field of view. We find a stellar velocity dispersion of km s−1, consistent with the dispersion that was derived from the globular clusters. The implied dynamical mass within the half-light radius r1/2 = 2.7 kpc is Mdyn = (1.3 0.8) × 108 M , similar to the stellar mass within that radius (Mstars = (1.0 0.2) × 108 M ). With this confirmation of the low velocity dispersion of NGC1052-DF2, the most urgent question is whether this "missing dark matter problem" is unique to this galaxy or applies more widely.
ABSTRACT
We study ultra-diffuse galaxies (UDGs) in zoom in cosmological simulations, seeking the origin of UDGs in the field versus galaxy groups. We find that while field UDGs arise from dwarfs in a ...characteristic mass range by multiple episodes of supernova feedback (Di Cintio et al.), group UDGs may also form by tidal puffing up and they become quiescent by ram-pressure stripping. The field and group UDGs share similar properties, independent of distance from the group centre. Their dark-matter haloes have ordinary spin parameters and centrally dominant dark-matter cores. Their stellar components tend to have a prolate shape with a Sérsic index n ∼ 1 but no significant rotation. Ram pressure removes the gas from the group UDGs when they are at pericentre, quenching star formation in them and making them redder. This generates a colour/star-formation-rate gradient with distance from the centre of the dense environment, as observed in clusters. We find that ∼20 per cent of the field UDGs that fall into a massive halo survive as satellite UDGs. In addition, normal field dwarfs on highly eccentric orbits can become UDGs near pericentre due to tidal puffing up, contributing about half of the group-UDG population. We interpret our findings using simple toy models, showing that gas stripping is mostly due to ram pressure rather than tides. We estimate that the energy deposited by tides in the bound component of a satellite over one orbit can cause significant puffing up provided that the orbit is sufficiently eccentric. We caution that while the simulations produce UDGs that match the observations, they under-produce the more compact dwarfs in the same mass range, possibly because of the high threshold for star formation or the strong feedback.
Motivated by a new wave of kinematical tracers in the outer regions of early-type galaxies (ellipticals and lenticulars), we re-examine the role of angular momentum in galaxies of all types. We ...present new methods for quantifying the specific angular momentum j, focusing mainly on the more challenging case of early-type galaxies, in order to derive firm empirical relations between stellar j and mass M (thus extending earlier work by Fall).We carry out detailed analysis of eight galaxies with kinematical data extending as far out as 10 effective radii, and find that data at two effective radii are generally sufficient to estimate total j reliably. We suggest that a combination of early collapse and multiple mergers (major or minor) may account naturally for the trend for ellipticals. More generally, the observed variations in angular momentum represent simple but fundamental constraints for any model of galaxy formation.
ABSTRACT We carry out a joint spatial-kinematical-metallicity analysis of globular clusters (GCs) around the Andromeda Galaxy (M31), using a homogeneous, high-quality spectroscopic data set. In ...particular, we remove the contaminating young clusters that have plagued many previous analyses. We find that the clusters can be divided into three major metallicity groups based on their radial distributions: (1) an inner metal-rich group (Fe/H > ); (2) a group with intermediate metallicity (with median Fe/H = −1); and (3) a metal-poor group, with Fe/H < . The metal-rich group has kinematics and spatial properties like those of the disk of M31, while the two more metal-poor groups show mild prograde rotation overall, with larger dispersions-in contrast to previous claims of stronger rotation. The metal-poor GCs are the least concentrated group; such clusters occur five times less frequently in the central bulge than do clusters of higher metallicity. Despite some well-known differences between the M31 and Milky Way GC systems, our revised analysis points to remarkable similarities in their chemodynamical properties, which could help elucidate the different formation stages of galaxies and their GCs. In particular, the M31 results motivate further exploration of a metal-rich GC formation mode in situ, within high-redshift, clumpy galactic disks.
We present Hubble Space Telescope/Advanced Camera for Surveys (ACS) imaging of 23 very low surface brightness ( e,V ∼ 25-27.5) galaxies detected in the fields of four nearby galaxy groups. These ...objects were selected from deep optical imaging obtained with the Dragonfly Telephoto Array. Seven are newly identified, while most of the others had been seen previously in visual surveys of deep photographic plates and more recent surveys. Few have previously been studied in detail. From the ACS images, we measure distances to the galaxies using both the tip of the red giant branch method and the surface brightness fluctuations method. We demonstrate that the two methods are consistent with each other in the regime where both can be applied. The distances to 15 out of 20 galaxies with stable measurements are consistent with that of the targeted group within errors. This suggests that assuming group membership based solely on projected proximity is ∼75% successful in this regime. The galaxies are nearly round, with a median axis ratio of 0.85, and visually resemble dwarf spheroidal galaxies. The objects have a range of sizes, from Re = 0.4 kpc to Re = 1.8 kpc, with a median . They range in luminosity from MV = −11.4 to MV = −15.6, with a median . Galaxies with Re ∼ 1 kpc and MV ∼ −12 are fairly rare in the Local Group, but we find many of them in this relatively small sample. Four of the objects fall in the class of ultra-diffuse galaxies, with Re > 1.5 kpc and 0,V > 24 mag arcsec−2, including the recently identified dark matter deficient galaxy NGC 1052-DF2.
A galaxy lacking dark matter van Dokkum, Pieter; Danieli, Shany; Cohen, Yotam ...
Nature,
03/2018, Volume:
555, Issue:
7698
Journal Article
Peer reviewed
Open access
Studies of galaxy surveys in the context of the cold dark matter paradigm have shown that the mass of the dark matter halo and the total stellar mass are coupled through a function that varies ...smoothly with mass. Their average ratio M
/M
has a minimum of about 30 for galaxies with stellar masses near that of the Milky Way (approximately 5 × 10
solar masses) and increases both towards lower masses and towards higher masses. The scatter in this relation is not well known; it is generally thought to be less than a factor of two for massive galaxies but much larger for dwarf galaxies. Here we report the radial velocities of ten luminous globular-cluster-like objects in the ultra-diffuse galaxy NGC1052-DF2, which has a stellar mass of approximately 2 × 10
solar masses. We infer that its velocity dispersion is less than 10.5 kilometres per second with 90 per cent confidence, and we determine from this that its total mass within a radius of 7.6 kiloparsecs is less than 3.4 × 10
solar masses. This implies that the ratio M
/M
is of order unity (and consistent with zero), a factor of at least 400 lower than expected. NGC1052-DF2 demonstrates that dark matter is not always coupled with baryonic matter on galactic scales.
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