Abstract
The inner kiloparsec regions surrounding sub-Eddington (luminosity less than 10
−3
in Eddington units,
L
Edd
) supermassive black holes (BHs) often show a characteristic network of dust ...filaments that terminate in a nuclear spiral in the central parsecs. Here we study the role and fate of these filaments in one of the least accreting BHs known, M31 (10
−7
L
Edd
) using hydrodynamical simulations. The evolution of a streamer of gas particles moving under the barred potential of M31 is followed from kiloparsec distance to the central parsecs. After an exploratory study of initial conditions, a compelling fit to the observed dust/ionized gas morphologies and line-of-sight velocities in the inner hundreds of parsecs is produced. After several million years of streamer evolution, during which friction, thermal dissipation, and self-collisions have taken place, the gas settles into a disk tens of parsecs wide. This is fed by numerous filaments that arise from an outer circumnuclear ring and spiral toward the center. The final configuration is tightly constrained by a critical input mass in the streamer of several 10
3
M
☉
(at an injection rate of 10
−4
M
⊙
yr
−
1
); values above or below this lead to filament fragmentation or dispersion respectively, which are not observed. The creation of a hot gas atmosphere in the region of ∼10
6
K is key to the development of a nuclear spiral during the simulation. The final inflow rate at 1 pc from the center is ∼1.7 × 10
−7
M
☉
yr
−1
, consistent with the quiescent state of the M31 BH.
Abstract
We investigate the evolution of mass segregation in initially substructured young embedded star clusters with two different background potentials mimicking the gas. Our clusters are ...initially in virial or subvirial global states and have different initial distributions for the most massive stars: randomly placed, initially mass segregated or even inversely segregated. By means of N-body simulation, we follow their evolution for 5 Myr. We measure the mass segregation using the minimum spanning tree method ΛMSR and an equivalent restricted method. Despite this variety of different initial conditions, we find that our stellar distributions almost always settle very fast into a mass segregated and more spherical configuration, suggesting that once we see a spherical or nearly spherical embedded star cluster, we can be sure it is mass segregated no matter what the real initial conditions were. We, furthermore, report under which circumstances this process can be more rapid or delayed, respectively.
Aims. We continue the analysis of the data set of our spectroscopic observation campaign of M 31, whose ultimate goal is to provide an understanding of the three-dimensional structure of the bulge, ...its formation history, and composition in terms of a classical bulge, boxy-peanut bulge, and bar contributions. Methods. We derive simple stellar population (SSP) properties, such as age metallicity and α-element overabundance, from the measurement of Lick/IDS absorption line indices. We describe their two-dimensional maps taking into account the dust distribution in M 31. Results. We found 80% of the values of our age measurements are larger than 10 Gyr. The central 100 arcsec of M 31 are dominated by the stars of the classical bulge of M 31. These stars are old (11−13 Gyr), metal-rich (as high as Z/H ≈ 0.35 dex) at the center with a negative gradient outward and enhanced in α-elements (α/Fe≈ 0.28±0.01 dex). The bar stands out in the metallicity map, where an almost solar value of Z/H (≈0.02 ± 0.01 dex) with no gradient is observed along the bar position angle (55.7 deg) out to 600 arcsec from the center. In contrast, no signature of the bar is seen in the age and α/Fe maps, which are approximately axisymmetric, delivering a mean age and overabundance for the bar and boxy-peanut bulge of 10–13 Gyr and 0.25–0.27 dex, respectively. The boxy-peanut bulge has almost solar metallicity (−0.04 ± 0.01 dex). The mass-to-light ratio of the three components is approximately constant at M/LV ≈ 4.4−4.7 M⊙/L⊙. The disk component at larger distances is made of a mixture of stars, as young as 3–4 Gyr, with solar metallicity and smaller M/LV (≈3 ± 0.1 M⊙/L⊙). Conclusions. We propose a two-phase formation scenario for the inner region of M 31, where most of the stars of the classical bulge come into place together with a proto-disk, where a bar develops and quickly transforms it into a boxy-peanut bulge. Star formation continues in the bulge region, producing stars younger than 10 Gyr, in particular along the bar, thereby enhancing its metallicity. The disk component appears to build up on longer timescales.
Aim.
As the nearest large spiral galaxy, M 31 provides a unique opportunity to study the structure and evolutionary history of this galaxy type in great detail. Among the many observing programs ...aimed at M 31 are microlensing studies, which require good three-dimensional models of the stellar mass distribution. Possible non-axisymmetric structures like a bar need to be taken into account. Due to M 31’s high inclination, the bar is difficult to detect in photometry alone. Therefore, detailed kinematic measurements are needed to constrain the possible existence and position of a bar in M 31.
Methods.
We obtained ≈220 separate fields with the optical integral-field unit spectrograph VIRUS-W, covering the whole bulge region of M 31 and parts of the disk. We derived stellar line-of-sight velocity distributions from the stellar absorption lines, as well as velocity distributions and line fluxes of the emission lines H
β
, O
III
and N
I
. Our data supersede any previous study in terms of spatial coverage and spectral resolution.
Results.
We find several features that are indicative of a bar in the kinematics of the stars, we see intermediate plateaus in the velocity and the velocity dispersion, and correlation between the higher moment
h
3 and the velocity. The gas kinematics is highly irregular, but is consistent with non-triaxial streaming motions caused by a bar. The morphology of the gas shows a spiral pattern, with seemingly lower inclination than the stellar disk. We also look at the ionization mechanisms of the gas, which happens mostly through shocks and not through starbursts.
Dwarf spheroidal (dSph) galaxies are considered the basic building blocks of the galaxy formation process in the Lambda cold dark matter hierarchical cosmological model. These galaxies are believed ...to be the most dark matter (DM) dominated systems known, have the lowest stellar content and are poor in gas. Many theories attempt to explain the formation of dSph galaxies resorting to the fact that these galaxies are mainly found orbiting large galaxies or invoking other mechanisms of interactions. Here, we show the full set of simulation as an extension of our fiducial model, where we study the formation of classical dSph galaxies in isolation by dissolving star clusters within the DM halo of the dwarf galaxy. In our parameter survey, we adopt cored and cusped DM halo profiles and consider different numbers of dissolving star clusters. We investigate the dependence of observable quantities with different masses and scalelengths of the DM halo and different star formation efficiencies. We find that our proposed scenario explains many features of the classical dSph galaxies of the Milky Way, like their morphology and their dynamics. We see trends how the surface brightness and the scalelength of the luminous component vary with the parameters of our simulations. We also identify how irregularities in their shape, i.e. clumpiness and ellipticity vary in our simulations. In velocity space, we identify the parameters leading to flat velocity dispersions curves. We recognize kinematically cold substructures in velocity space, named fossil remnants and stemming from our unique initial conditions, which alter the expected results. These streaming motions are considered as a key feature for future observation with high resolution to validate our scenario.
We examine the effects of gas expulsion on initially substructured distributions of stars. We perform N-body simulations of the evolution of these distributions in a static background potential to ...mimic the gas. We remove the static potential instantaneously to model gas expulsion. We find that the exact dynamical state of the cluster plays a very strong role in affecting a cluster's survival, especially at early times: they may be entirely destroyed or only weakly affected. We show that knowing both detailed dynamics and relative star–gas distributions can provide a good estimate of the post-gas expulsion state of the cluster, but even knowing these is not an absolute way of determining the survival or otherwise of the cluster.
Recent deep photometry of the dwarf spheroidal Ursa Major II's morphology, and spectroscopy of individual stars, have provided a number of new constraints on its properties. With a velocity ...dispersion ∼6 km s−1, and under the assumption that the galaxy is virialized, the mass-to-light ratio is found to be approaching ∼2000 - apparently heavily dark matter dominated. Using N-body simulations, we demonstrate that the observed luminosity, ellipticity, irregular morphology, velocity gradient and the velocity dispersion can be well reproduced through processes associated with tidal mass-loss, and in the absence of dark matter. These results highlight the considerable uncertainty that exists in measurements of the dark matter content of Ursa Major II (UMaII). The dynamics of the inner tidal tails, and tidal stream, causes the observed velocity dispersion of stars to be boosted to values of >5 km s−1. These dispersion boosts occur at each apocentre, and last throughout the time the galaxy is close to apocentre. The model need not be close to destruction to have a boosted velocity dispersion. We additionally note that the velocity dispersion at apocentre is periodically enhanced substantially (e.g >20 km s−1). This occurs most strongly when the model's trajectory is close to perpendicular with the Galaxy's disc at pericentre. This effect is responsible for raising the velocity dispersion of our model to (and beyond) the observed values in UMaII. We test an iterative rejection technique for removing unbound stars from samples of UMaII stars whose positions on the sky, and line-of-sight velocities, are provided. We find that this technique is very effective at providing an accurate bound mass from this information, and only fails when the galaxy has a bound mass less than 10 per cent of its initial mass. However, when <2 per cent mass remains bound, mass overestimation by >3 orders of magnitude are seen. Additionally we find that the technique's mass measurements are sensitive to measurement uncertainty in line-of-sight velocities. Measurement uncertainties of 1-4 km s−1 result in mass overestimates by a factor of ∼1.3-5.7.
We attempt to find a progenitor for the ultrafaint object Segue 1 under the assumption that it formed as a dark matter free star cluster in the past. We look for orbits, using the elongation of Segue ...1 on the sky as a tracer of its path. Those orbits are followed backwards in time to find the starting points of our N-body simulations. The successful orbit, with which we can reproduce Segue 1 has a proper motion of μα = −0.19 mas yr−1 and μδ = −1.9 mas yr−1, placing Segue 1 near its apogalacticon today. Our best-fitting model has an initial mass of 6224 M⊙ and an initial scalelength of 5.75 pc.
Hercules is a dwarf spheroidal satellite of the Milky Way, found at a distance of ≈138 kpc, and showing evidence of tidal disruption. It is very elongated and exhibits a velocity gradient of ...16 ± 3 km s−1 kpc−1. Using these data a possible orbit of Hercules has previously been deduced in the literature. In this study, we make use of a novel approach to find a best-fitting model that follows the published orbit. Instead of using trial and error, we use a systematic approach in order to find a model that fits multiple observables simultaneously. As such, we investigate a much wider parameter range of initial conditions and ensure we have found the best match possible. Using a dark matter free progenitor that undergoes tidal disruption, our best-fitting model can simultaneously match the observed luminosity, central surface brightness, effective radius, velocity dispersion, and velocity gradient of Hercules. However, we find it is impossible to reproduce the observed elongation and the position angle of Hercules at the same time in our models. This failure persists even when we vary the duration of the simulation significantly, and consider a more cuspy density distribution for the progenitor. We discuss how this suggests that the published orbit of Hercules is very likely to be incorrect.
The last few years have seen the discovery of many faint and ultra-faint dwarf spheroidal galaxies around the Milky Way. Among these is a pair of satellites called Leo IV and Leo V. This pair is ...found at large distances from the Milky Way (154 and 175 kpc, respectively). The fairly small difference in radial distance, and the fact that they also show a close projected distance on the sky, has led to the idea that we might be seeing a new pair of bound galaxies – like the Magellanic Clouds. In this paper we investigate this speculation by means of a simple integration code (confirming the results with full N-body simulations). Because the luminous mass of the two faint dwarfs is far too low to allow them to be bound, we simulate the pair assuming extended dark matter haloes. Our results show that the minimum dark matter mass required for the pair to be bound is quite high – ranging from 1.6 × 1010 M⊙ to 5.4 × 1010 M⊙ (within the virial radii). Computing the mass of dark matter within a commonly adopted radius of 300 pc shows that our models are well within the predicted range of dark matter content for satellites so faint. We therefore conclude that it could be possible that the two galaxies constitute a bound pair.
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