Hydrodynamic simulations of the triaxial bulge of M 31
TL;DR: In this article, the potential of the stellar bulge is derived from its surface brightness prole, which is assumed to be triaxial and rotating in the same plane as the disk in order to explain the twisted nature of M 31's central isophotes and the non circular gas velocities in the inner disk.
Abstract: The interstellar gas flow in the inner disk of M 31 is modelled using a new, two dimensional, grid based, hydrodynamics code. The potential of the stellar bulge is derived from its surface brightness prole. The bulge is assumed to be triaxial and rotating in the same plane as the disk in order to explain the twisted nature of M 31's central isophotes and the non circular gas velocities in the inner disk. Results are compared with CO observations and the bulge is found to be a fast rotator with a B-band mass-to-light ratio, B =6 .5 0.8, and a ratio of co-rotation radius to bulge semi-major axis,R =1 .2 0.1, implying that any dark halo must have a low density core in contradiction to the predictions of CDM. These conclusions would be strengthened by further observations conrming the model's o axis CO velocity predictions.
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TL;DR: In this article, the authors apply standard disk formation theory with adiabatic contraction within cuspy halo models predicted by the standard cold dark matter (?CDM) cosmology.
Abstract: We apply standard disk formation theory with adiabatic contraction within cuspy halo models predicted by the standard cold dark matter (?CDM) cosmology. The resulting models are confronted with the broad range of observational data available for the Milky Way and M31 galaxies. We find that there is a narrow range of parameters that can satisfy the observational constraints, but within this range, the models score remarkably well. Our favored models have virial masses of 1012 and 1.6 ? 1012 M? for the Galaxy and for M31, respectively, average spin parameters ? ? 0.03-0.05, and concentrations Cvir = 10-17, typical for halos of this mass in the standard ?CDM cosmology. The models require neither dark matter modifications nor flat cores to fit the observational data. We explore two types of models, with and without the exchange of angular momentum between the dark matter and the baryons. The models without exchange give reasonable rotation curves, fulfill constraints in the solar neighborhood, and satisfy constraints at larger radii, but they may be problematic for fast rotating central bars. We explore models in which the baryons experience additional contraction due to loss of angular momentum to the surrounding dark matter. These models produce similar global properties, but the dark matter is only a 25% of the total mass in the central 3 kpc region, allowing a fast rotating bar to persist. According to preliminary calculations, our model galaxies probably have sufficient baryonic mass in the central ~3.5 kpc to reproduce recent observational values of the optical depth to microlensing events toward the Galactic center. Our dynamical models unequivocally require that about 50% of all the gas inside the virial radius must not be in the disk or in the bulge, a result that is obtained naturally in standard semianalytic models. Assuming that the Milky Way is typical, we investigate whether the range of virial masses allowed by our dynamical models is compatible with constraints from the galaxy luminosity function. We find that if the Milky Way has a luminosity MK = -24.0, then these constraints are satisfied, but if it is more luminous (as expected if it lies on the Tully-Fisher relation), then the predicted space density is larger than the observed space density of galaxies of the corresponding luminosity by a factor of 1.5-2. We conclude that observed rotation curves and dynamical properties of normal spiral galaxies appear to be consistent with standard ?CDM.
737 citations
Cites background from "Hydrodynamic simulations of the tri..."
...Those anomalies are attributed to the presence of a triaxial bar/bulge in M31 (e.g. Stark 1977; Stark & Binney 1994; Berman 2001)....
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TL;DR: In this article, the two spiral arms appear to start at the ends of a bar in the nuclear region and extend beyond the star-forming ring, and the star forming ring is very circular except for a region near M32 where it splits.
Abstract: New images of M31 at 24, 70, and 160 μm taken with the Multiband Imaging Photometer for Spitzer (MIPS) reveal the morphology of the dust in this galaxy. This morphology is well represented by a composite of two logarithmic spiral arms and a circular ring (radius ~10 kpc) of star formation offset from the nucleus. The two spiral arms appear to start at the ends of a bar in the nuclear region and extend beyond the star-forming ring. As has been found in previous work, the spiral arms are not continuous, but composed of spiral segments. The star-forming ring is very circular except for a region near M32 where it splits. The lack of well-defined spiral arms and the prominence of the nearly circular ring suggest that M31 has been distorted by interactions with its satellite galaxies. Using new dynamical simulations of M31 interacting with M32 and NGC 205, we find that, qualitatively, such interactions can produce an offset, split ring like that seen in the MIPS images.
198 citations
Cites background from "Hydrodynamic simulations of the tri..."
...The location of these spots and appearance of the nuclear region (especially at 70 µm) is similar to the triaxial bulge simulations of Berman (2001)....
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TL;DR: M 31, the closest large spiral galaxy to our own, is the best object for studying molecular clouds and their relation to the spiral structure as discussed by the authors, and it is also one of the best places where to estimate molecular clouds masses through the Virial Theorem.
Abstract: M 31, the closest large spiral galaxy to our own, is the best object for studying molecular clouds and their relation to the spiral structure. As one of the astronomical objects with the best known distance (0.78 ± 0.02 Mpc), it is also one of the best places where to estimate molecular clouds masses through the Virial Theorem.
193 citations
Cites background from "Hydrodynamic simulations of the tri..."
...Its non-circular orbit may be caused by the central bar (Berman 2001; Berman & Loinard 2002)....
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TL;DR: In this article, a tilted ring model was used to fit the rotation curve of the nearest spiral galaxy, M 31, using a deep, full-disk 21-cm imaging survey smoothed to 466-pc resolution.
Abstract: Aims. We test cosmological models of structure formation using the rotation curve of the nearest spiral galaxy, M 31, determined using a recent deep, full-disk 21-cm imaging survey smoothed to 466 pc resolution.Methods. We fit a tilted ring model to the HI data from 8 to 37 kpc and establish conclusively the presence of a dark halo and its density distribution via dynamical analysis of the rotation curve.Results. The disk of M 31 warps from 25 kpc outwards and becomes more inclined with respect to our line of sight. Newtonian dynamics without a dark matter halo provide a very poor fit to the rotation curve. In the framework of modified Newtonian dynamic (MOND) however the 21-cm rotation curve is well fitted by the gravitational potential traced by the baryonic matter density alone. The inclusion of a dark matter halo with a density profile as predicted by hierarchical clustering and structure formation in a Λ CDM cosmology makes the mass model in newtonian dynamic compatible with the rotation curve data. The dark halo concentration parameter for the best fit is C = 12 and its total mass is 1.2 1012 . If a dark halo model with a constant-density core is considered, the core radius has to be larger than 20 kpc in order for the model to provide a good fit to the data. We extrapolate the best-fit Λ CDM and constant-density core mass models to very large galactocentric radii, comparable to the size of the dark matter halo. A comparison of the predicted mass with the M 31 mass determined at such large radii using other dynamical tracers, confirms the validity of our results. In particular the Λ CDM dark halo model which best fits the 21-cm data well reproduces the mass of M 31 traced out to 560 kpc. Our best estimate for the total mass of M 31 is 1.3 1012 , with 12% baryonic fraction and only 6% of the baryons in the neutral gas phase.
175 citations
Cites background from "Hydrodynamic simulations of the tri..."
...However, notice that this is shown not just to point out the consistency of the molecular and atomic gas velocities, but to emphasize one has to consider non-circular motion to properly trace the rotation curve in the inner region (Berman 2001; Berman & Loinard 2002)....
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...In particular Berman (2001); Berman & Loinard (2002) have shown that the anomalous velocities observed in the inner region of M31 can be explained as the response of the gas to the potential of a triaxial rotating bulge....
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...Detailed modelling of the surface brightness shows that at very least the bulge is an obl te spheroid with axis ratio of 0.8 (Kent 1983) but most likely it is a triaxial bulge (e.g. Berman 2001, and references therein)....
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...Hydrodynamic simulations of the triaxial bulge of M31 by Berman (2001) found a B-band mass-to-light ratio of 6.5 for the bulge i.e. a stellar mass of 1010 M⊙....
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References
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TL;DR: A review of the observational evidence for dark matter in spiral galaxies can be found in this paper, where a comparison of the distribution of light and the rotation curve is made, and it is shown that dark matter is needed to produce the flat observed rotation curves in the outer region.
Abstract: Mass models of spiral galaxies based on the observed light distribution, assuming constant M/L for bulge and disc, are able to reproduce the observed rotation curves in the inner regions, but fail to do so increasingly towards and beyond the edge of the visible material. The discrepancy in the outer region can be accounted for by invoking dark matter; some galaxies require at least four times as much dark matter as luminous matter. There is no evidence for a dependence on galaxy luminosity or morphological type. Various arguments support the idea that a distribution of visible matter with constant M/L is responsible for the circular velocity in the inner region, i.e. inside approximately 2.5 disc scalelengths. Luminous matter and dark matter seem to 'conspire' to produce the flat observed rotation curves in the outer region. It seems unlikely that this coupling between disc and halo results from the large-scale gravitational interaction between the two components. Attempts to determine the shape of dark halos have not yet produced convincing results. The aim of this paper is to review the observational evidence for dark matter in spiral galaxies, and to discuss some of its consequences. That dark matter is needed can be inferred from a comparison of the distribution of light and the rotation curve. Considerable progress has been made in this area in recent years. The amount of dark matter detected so far in spiral galaxies is rather modest from the cosmological point of view. Total mass: light ratios of individ
285 citations
244 citations
"Hydrodynamic simulations of the tri..." refers background in this paper
...TheB-band surface brightness profile of the bulge of M31 follows ther 1 4 law of de Vaucouleurs (1958): I(s) = Ie exp(−7.67[(s/re) 14 − 1]) , wherere is the effective radius,Ie is the surface brightness at re ands is the semi-major axis length of the isophote....
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215 citations
"Hydrodynamic simulations of the tri..." refers background in this paper
...Using stellar kinematic data, Kormendy and Illingworth (1982) provide evidence that many SA bulges (in unbarred galaxies) are rotationally flattened, and consistent with being oblate spheroids....
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01 Jan 1993
TL;DR: Mass models of spiral galaxies based on the observed light distribution, assuming constant M /L for bulge and disc, are able to reproduce the observed rotation curves in the inner regions, but fail to do so increasingly towards and beyond the edge of the visible material.
205 citations
"Hydrodynamic simulations of the tri..." refers background in this paper
...Many rotation curves can be fitted by models ranging from zero-mass disks to ‘maximum disks’ (van Albada and Sancisi 1986), that is disks that are as massive as possible whilst ensuring that the halo is not hollow....
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