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: The noncircular streaming motions in a barred galaxy are sensitive to the mass of the bar and can be used to lift the degeneracy between disk and dark matter halo encountered when fitting axisymmetric rotation curves of disk galaxies as mentioned in this paper.
Abstract: The noncircular streaming motions in barred galaxies are sensitive to the mass of the bar and can be used to lift the degeneracy between disk and dark matter halo encountered when fitting axisymmetric rotation curves of disk galaxies. In this paper we present photometric and kinematic observations of NGC 4123, a barred galaxy of modest size (Vrot = 130 km s-1, L = 0.7L*), that reveal strong noncircular motions. The bar has straight dust lanes and an inner Lindblad resonance. The disk of NGC 4123 has no sign of truncation out to 10 scale lengths, and star-forming regions are found well outside R25. A Fabry-Perot Hα velocity field shows velocity jumps of greater than 100 km s-1 at the location of the dust lanes within the bar, indicating shocks in the gas flow. VLA observations yield the velocity field of the H I disk. Axisymmetric mass models yield good fits to the rotation curve outside the bar region for disk I-band M/L of 2.25 or less, and dark halos with either isothermal or power-law profiles can fit the data well. In a companion paper we model the full two-dimensional velocity field, including noncircular motions, to determine the stellar M/L and the mass of the dark halo.
73 citations
TL;DR: In this article, the authors present a morphologie of the bulbe de la galaxie, deduite d'observations HI and CO, with a size r=500 kpc.
Abstract: La courbe de rotation apparente de la galaxie, deduite d'observations HI et CO, presente un pic etroit (ν max ≃250-260 kms −1 ) a r=500 pc, puis un lent declin pour 600 pc≤r≤1,5 kpc. On montre que cette morphologie ne peut etre reconciliee avec les observations que si le bulbe de la galaxie n'est pas a symetrie axiale et le potentiel resultant triaxial
66 citations
"Hydrodynamic simulations of the tri..." refers background in this paper
...Hydrodynamical and computational parameters Number of grid cells I × J 80× 160 Gas recycling parameter† α 0.3 pc2M−1 yr −1 Initial gas density Σ0 1M pc−2 Constant mass radius rM 10 kpc Sound speed‡ cg 10 km s−1 Galaxy radius rG 10 kpc Courant number C 0.5 † Athanassoula (1992),‡ Cowie (1980) Simulations start with the gas at a uniform surface density Σ0 = 1 M pc−2 and on circular orbits at a speed given by βvA(r) with β chosen such thatβ2v2A(r) = GM(rM )/rM , whererM = 10 kpc andM(rM ) is the mass of the model at rM ....
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...However, to recover the true bulge surface brightness profile, this must be corrected for absorption both in M31 and in our own Galaxy....
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...Using the values ofAB = 0.32 mag for our Galaxy in the direction of M31 (Burstein and Heiles 1982),AB = 0.98 mag for the bulge region of M31 (van Genderen 1973) andM B = 5.48 for the absoluteB-band solar magnitude (Allen 1973), the value of Ie = 22.2 mag arcsec−2 in Walterbos and Kennicutt (1988) is transformed intoIe = 289L pc−2....
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...Triaxiality has been demonstrated for both NGC 4845 (Gerhard et al. 1989) and for our Galaxy (Gerhard and Vietri 1986)....
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...As the closest large spiral to our own Galaxy, Andromeda provides us with an ideal laboratory for probing the structure and dynamics of spiral galaxies....
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TL;DR: In this paper, the effects of a reduced viscosity of a barred spiral galaxy were investigated on the basis of high-resolution time-dependent gas-dynamical equations.
Abstract: The considered investigation is concerned with conditions in that type of spiral galaxy which is known as a 'barred spiral'. The lack of star formation and the smoothness of the dark dust lanes may indicate that the bar region is pervaded by a more homogeneous interstellar medium with a lower viscosity than that characteristic of the outer regions. The effects of such a reduced viscosity are investigated on the basis of high-resolution time-dependent gasdynamical equations. Calculations modeling the gas flow in a given bar potential are presented. The calculations were performed on grids with various mesh sizes and consequently had different resolutions and numerical viscosities. A reduction in the numerical diffusion and viscosity was accompanied by changes in the flow pattern, including the appearance of a strong shock in the bar region and the occurrence of postshock outflow.
62 citations
"Hydrodynamic simulations of the tri..." refers methods in this paper
...The response of the gas is determined using a parallel, hydrodynamics code, GALAHAD, based on the shock capturing FS2 algorithm (van Albada and Roberts 1981, van Albada et al. 1982)....
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55 citations
"Hydrodynamic simulations of the tri..." refers background in this paper
...These effects have been seen in NGC 2784 (Bertola et al. 1988) and in a sample of 10 unbarred galaxies (Zaritsky and Lo 1986)....
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TL;DR: In this article, it was shown that for dynamical time scales longer than 10 million years, the ensemble of interstellar cold diffuse clouds may be described by equations of mass and momentum conservation where the pressure has the form p/v-squared/ where p is the mass averaged square velocity dispersion.
Abstract: It is shown that for dynamical time scales longer than 10 million years the ensemble of interstellar cold diffuse clouds may be described by equations of mass and momentum conservation where the pressure has the form p/v-squared/ where /v-squared/ is the mass averaged square velocity dispersion. Equilibrium cloud spectra are derived and an equation of state obtained. The equation of state undergoes a phase transition at a critical density close to the local interstellar gas density. At higher densities, more massive clouds are formed and the cloud fluid softens. It is suggested that this mechanism may naturally regulate the interstellar gas density if star formation is very efficient in the cooler high-density systems.
55 citations