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Journal ArticleDOI

Hydrodynamic simulations of the triaxial bulge of M 31

S. Berman1
01 May 2001-Astronomy and Astrophysics (EDP Sciences)-Vol. 371, Iss: 2, pp 476-486

AbstractThe 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.

Topics: Bulge (55%), Radius (51%), Dark matter halo (50%)

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Citations
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Journal ArticleDOI
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.

718 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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Journal ArticleDOI
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.

192 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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Journal ArticleDOI
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.

181 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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Journal ArticleDOI
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.

162 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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Journal ArticleDOI
Abstract: We use high-resolution N-body simulations to study the equilibrium density profiles of dark matter halos in hierarchically clustering universes. We find that all such profiles have the same shape, independent of the halo mass, the initial density fluctuation spectrum, and the values of the cosmological parameters. Spherically averaged equilibrium profiles are well fitted over two decades in radius by a simple formula originally proposed to describe the structure of galaxy clusters in a cold dark matter universe. In any particular cosmology, the two scale parameters of the fit, the halo mass and its characteristic density, are strongly correlated. Low-mass halos are significantly denser than more massive systems, a correlation that reflects the higher collapse redshift of small halos. The characteristic density of an equilibrium halo is proportional to the density of the universe at the time it was assembled. A suitable definition of this assembly time allows the same proportionality constant to be used for all the cosmologies that we have tested. We compare our results with previous work on halo density profiles and show that there is good agreement. We also provide a step-by-step analytic procedure, based on the Press-Schechter formalism, that allows accurate equilibrium profiles to be calculated as a function of mass in any hierarchical model.

9,149 citations


"Hydrodynamic simulations of the tri..." refers background or methods in this paper

  • ...Hence, for a given value of Hubble’s constant, the halo circular speed curve is fully specified byv200 and c. Navarro et al. (1997) find a relationship between the characteristic density and radius (or, equivalently, betweenc andv200) of a halo in a given cosmology, thereby reducing the NFW…...

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  • ...If v200 = v0 = 257 km s−1, then the method of Navarro et al. (1997) gives a halo mass fraction at 3.5 kpc of 58% and a disk mass of only 5.8× 109M ....

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  • ...From Navarro et al. (1997), at a redshiftz = 0, r200 = ( h v200 km s−1 ) kpc....

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Journal ArticleDOI
Abstract: High resolution N-body simulations show that the density profiles of dark matter halos formed in the standard CDM cosmogony can be fit accurately by scaling a simple “universal” profile. Regardless of their mass, halos are nearly isothermal over a large range in radius, but significantly shallower than r -2 near the center and steeper than r -2 in the outer regions. The characteristic overdensity of a halo correlates strongly with halo mass in a manner consistent with the mass dependence of the epoch of halo formation. Matching the shape of the rotation curves of disk galaxies with this halo structure requires (i) disk mass-to-light ratios to increase systematically with luminosity, (ii) halo circular velocities to be systematically lower than the disk rotation speed, and (iii) that the masses of halos surrounding bright galaxies depend only weakly on galaxy luminosity. This offers an attractive explanation for the puzzling lack of correlation between luminosity and dynamics in observed samples of binary galaxies and of satellite companions of bright spiral galaxies, suggesting that the structure of dark matter halos surrounding bright spirals is similar to that of cold dark matter halos.

7,147 citations


"Hydrodynamic simulations of the tri..." refers background or methods or result in this paper

  • ...One method of breaking this degeneracy is to represent the halo by an NFW profile (Navarro et al. 1996) derived from cosmologicalN -body simulations, ρ(r) ρs = 1 (r/rs)(1 + r/rs)2 , where ρs is a characteristic density,rs = r200/c is a characteristic radius andc is a dimensionless concentration…...

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  • ...This is in stark contrast to the predictions of Navarro et al. (1996), which imply that the halo mass fraction within 3.5 kpc is between 44% and 58%....

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  • ...However, Navarro et al. (1996) state that the maximum rotation velocity can be as high as1....

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Journal ArticleDOI

1,021 citations


"Hydrodynamic simulations of the tri..." refers methods in this paper

  • ...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)…...

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  • ...…of inner isophotes∗ β 1.54 Bulge to disk isophote angle∗∗ ψ 10◦ Max. bulge isophote semi-major axis§ rmax 11′.4 At the distance of M31,1′ = 200 pc on the major axis † Hodge (1992),∗ Walterbos and Kennicutt (1988) ‡ Burstein and Heiles (1982) and van Genderen (1973) ∗∗ Lindblad (1956),§ Stark (1977)...

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Journal ArticleDOI
Abstract: I study the flow of gas in and around the bars or ovals of model barred galaxies and show that it is intimately linked to the properties of the periodic orbits. Simulations show that the density of the gas in and around the bar region is low, except for the centre and two narrow lanes which are the loci of shocks. Such shocks form if the x 1 periodic orbits have either loops or large curvature values at their apocentres. The form of the shock loci depends on a number of parameters characterizing the bar and disc potentials. In order for shocks to be offset from the bar major axis towards its leading side, the x 2 and x 3 families must not only exist but also cover a sufficient extent along the bar major axis

892 citations


"Hydrodynamic simulations of the tri..." refers background or methods in this paper

  • ...Athanassoula (1992) shows that these shocks are intimately linked to the dust lanes which appear in many barred galaxies....

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  • ...However, unlike the galaxy models of Athanassoula (1992), M31 is a real galaxy, modelled with a somewhat arbitrary bulge edge....

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  • ...…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…...

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  • ...As in Athanassoula (1992), GALAHAD includes a routine to compensate for this by crudely simulating the interaction of stars with the ISM: star formation occurs and gas is removed in areas of high density, whereas stellar mass loss is assumed to take place at a steady rate across the galaxy....

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  • ...Following Athanassoula (1992), we set rco = R a, wherea is the length of the bulge semi-major axis....

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Journal ArticleDOI
Abstract: This paper commences a series of investigations into the stellar populations of local elliptical galaxies as determined from their integrated spectra. The goal of the series is to determine the star formation and chemical evolution histories of present-day elliptical galaxies. The primary galaxy sample analyzed is that of Gonzalez, which consists of 39 elliptical galaxies drawn primarily from the local field and nearby groups, plus the bulge of Messier 31. Single-burst stellar population (SSP)–equivalent ages, metallicities, and abundance ratios are derived from Hβ, Mg b, and Fe line strengths using an extension of the Worthey models that incorporates nonsolar line-strength "response functions" by Tripicco & Bell. These functions account for changes in the Lick/IDS indices caused by nonsolar abundance ratios, allowing us to correct the Worthey models for the enhancements of Mg and other α-like elements relative to the Fe-peak elements. SSP-equivalent ages of the Gonzalez elliptical galaxies are found to vary widely, 1.5 Gyr t 18 Gyr, while metallicities [Z/H] and enhancement ratios [E/Fe] are strongly peaked around [Z/H] = +0.26 and [E/Fe] = +0.20 (in an aperture of radius re/8). The enhancement ratios [E/Fe] are milder than previous estimates because of the application of nonsolar abundance corrections to both Mg b and Fe for the first time. While [E/Fe] is usually greater than zero, it is not the "E" elements that are actually enhanced but rather the Fe-peak elements that are depressed; this serves not only to weaken Fe but also to strengthen Mg b, accounting for the overall generally mild enhancements. Based on index strengths from the Lick/IDS galaxy library (Trager et al.), C is not depressed with Fe but rather seems to be on a par with other elements such as Mg in the E group. Gradients in stellar populations within galaxies are found to be mild, with SSP-equivalent age increasing by 25%, metallicity decreasing by [Z/H] = 0.20 dex, and [E/Fe] remaining nearly constant out to an aperture of radius re/2 for nearly all systems. Our ages have an overall zero-point uncertainty of at least ~25% because of uncertainties in the stellar evolution prescription, the oxygen abundance, the effect of [E/Fe] ≠ 0 on the isochrones, and other unknowns. However, the relative age rankings of stellar populations should be largely unaffected by these errors. In particular, the large spread in ages appears to be real and cannot be explained by contamination of Hβ by blue stragglers or hot horizontal-branch stars, or by fill-in of Hβ by emission. Correlations between these derived SSP-equivalent parameters and other galaxy observables will be discussed in future papers.

653 citations