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Register a new userGas Dynamics in the Galaxy: Total Mass Distribution and the Bar Pattern Speed
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Zhi Li
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Gas morphology and kinematics in the Milky Way contain key information for understanding the formation and evolution of our Galaxy. We present a high resolution hydrodynamical simulation based on a realistic barred Milky Way potential constrained by recent observations. Our model can reproduce most features in the observed longitude-velocity diagram, including the Central Molecular Zone, the Near and Far 3-kpc arms, the Molecular Ring, and the spiral arm tangents. It can also explain the non-circular motions of masers obtained by the recent BeSSeL2 survey. The central gas kinematics are consistent with a mass of $6.9times10^8; {rm M}_{odot}$ in the Nuclear Stellar Disk. Our model predicts the formation of an elliptical gaseous ring surrounding the bar, which is composed of the 3-kpc arms, Norma arm, and the bar-spiral interfaces. This ring is similar to those inner rings in some Milky Way analogs with a boxy/peanut-shaped bulge. The kinematics of gas near the solar neighbourhood are governed by the Local arm, which is induced by the four major stellar spiral arms. The bar pattern speed constrained by our gas model is $37.5-40; {rm km};{rm s}^{-1};{rm kpc}^{-1}$, corresponding to a corotation radius of $R_{rm CR}=6.0-6.4;{rm kpc}$. The rotation curve of our model rises gently within the central $sim5;{rm kpc}$, which is significantly less steep than those predicted by modern zoom-in cosmological simulations such as Auriga.
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We construct a large set of dynamical models of the galactic bulge, bar and inner disk using the Made-to-Measure method. Our models are constrained to match the red clump giant density from a combination of the VVV, UKIDSS and 2MASS infrared surveys together with stellar kinematics in the bulge from the BRAVA and OGLE surveys, and in the entire bar region from the ARGOS survey. We are able to recover the bar pattern speed and the stellar and dark matter mass distributions in the bar region, thus recovering the entire galactic effective potential. We find a bar pattern speed of $39.0 pm 3.5 ,rm{km,s^{-1},kpc^{-1}}$, placing the bar corotation radius at $6.1 pm 0.5 rm{kpc}$ and making the Milky Way bar a typical fast rotator. We evaluate the stellar mass of the long bar and bulge structure to be $M_{rm{bar/bulge}} = 1.88 pm 0.12 times 10^{10} , rm{M}_{odot}$, larger than the mass of disk in the bar region, $M_{rm{inner disk}} = 1.29pm0.12 times 10^{10} , rm{M}_{odot}$. The total dynamical mass in the bulge volume is $1.85pm0.05times 10^{10} , rm{M}_{odot}$. Thanks to more extended kinematic data sets and recent measurement of the bulge IMF our models have a low dark matter fraction in the bulge of $17%pm2%$. We find a dark matter density profile which flattens to a shallow cusp or core in the bulge region. Finally, we find dynamical evidence for an extra central mass of $sim0.2times10^{10} ,rm{M}_{odot}$, probably in a nuclear disk or disky pseudobulge.
An important dynamic parameter of barred galaxies is the bar pattern speed. Among several methods that are used for the determination of the pattern speed the Tremaine-Weinberg method has the advantage of model independency and accuracy. In this work we apply the method to a simulated bar including gas dynamics and study the effect of 2D spectroscopy data quality on robustness of the method. We added a white noise and a Gaussian random field to the data and measured the corresponding errors in the pattern speed. We found that a signal to noise ratio in surface density ~5 introduces errors of ~20% for the Gaussian noise, while for the white noise the corresponding errors reach ~50%. At the same time the velocity field is less sensitive to contamination. On the basis of the performed study we applied the method to the NGC 3367 spiral galaxy using H{alpha} Fabry-Perot interferometry data. We found for the pattern speed 43 pm 6 km/s/kpc for this galaxy.
We present the first galactic-scale model of the gas dynamics of the prototype barred Seyfert 1 galaxy NGC1097. We use large scale FaNTOmM Fabry-Perot interferometric data covering the entire galactic disc and combine the distribution and kinematics maps with high resolution two-dimensional spectroscopy from the Gemini telescope. We build a dynamical model for the gravitational potential by applying the analytic solution to the equations of motion, within the epicyclic approximation. Our model reproduces all the significant kinematic and structural signatures of this galaxy. We find that the primary bar is 7.9+/-0.6 kpc long and has a pattern speed of 36 +/- 2 km s^-1 kpc^-1. This places the corotation radius at 8.6 +/-0.5 kpc, the outer Lindblad resonance at 14.9+/-0.9 kpc and two inner Lindblad resonances at 60+/-5 pc and 2.9+/-0.1 kpc. These derivations lead to a ratio of the corotation radius over bar length of 1.0--1.2, which is in agreement with the predictions of simulations for fast galaxy bars. Our model presents evidence that the circumnuclear ring in this galaxy is not located near any of the resonance radii in this galaxy. The ring might have once formed at the outer inner Lindblad resonance radius, and it has been migrating inward, toward the centre of the galactic gravitational potential.
We compare distance resolved, absolute proper motions in the Milky Way bar/bulge region to a grid of made-to-measure dynamical models with well defined pattern speeds. The data are obtained by combining the relative VVV Infrared Astrometric Catalog v1 proper motions with the Gaia DR2 absolute reference frame. We undertake a comprehensive analysis of the various errors in our comparison, from both the data and the models, and allow for additional, unknown, contributions by using an outlier-tolerant likelihood function to evaluate the best fitting model. We quantify systematic effects such as the region of data included in the comparison, with or without possible overlap from spiral arms, and the choice of synthetic luminosity function and bar angle used to predict the data from the models. Resulting variations in the best-fit parameters are included in the final error budget. We measure the bar pattern speed to be Omega_b=35.4+-0.9 km/s/kpc and the azimuthal solar velocity to be V_phi_sun= 251.4+-1.7 km/s. These values, when combined with recent measurements of the Galactic rotation curve, yield the distance of corotation, 6.3 < R_(CR) [kpc] < 6.8, the outer Lindblad resonance (OLR), 10.5 < R_(OLR) [kpc] < 11.5, and the higher order, m=4, OLR, 8.5 < R_(OLR_4) [kpc] < 9.0. The measured low pattern speed provides strong evidence for the long-slow bar scenario.
Using photometric galaxies from the HSC survey, we measure the stellar mass density profiles for satellite galaxies as a function of the projected distance, $r_p$, to isolated central galaxies (ICGs) selected from SDSS/DR7 spectroscopic galaxies at $zsim0.1$. By stacking HSC images, we also measure the projected stellar mass density profiles for ICGs and their stellar halos. The total mass distributions are further measured from HSC weak lensing signals. ICGs dominate within $sim$0.15 times the halo virial radius ($0.15R_{200}$). The stellar mass versus total mass fractions drop with the increase in $r_p$ up to $sim0.15R_{200}$, beyond which they are less than 1% while stay almost constant, indicating the radial distribution of satellites trace dark matter. The total stellar mass in satellites is proportional to the virial mass of the host halo, $M_{200}$, for ICGs more massive than $10^{10.5}M_odot$, i.e., $M_{ast,mathrm{sat}} propto M_{200}$, whereas the relation between the stellar mass of ICGs $+$ stellar halos and $M_{200}$ is close to $M_{ast,mathrm{ICG+diffuse}}propto M_{200}^{1/2}$. Below $10^{10.5}M_odot$, the change in $M_{200}$ is much slower with the decrease in $M_{ast,mathrm{ICG+diffuse}}$. At fixed stellar mass, red ICGs are hosted by more massive dark matter halos and have more satellites. At $M_{200}sim10^{12.7}M_odot$, both $M_{ast,mathrm{sat}}$ and the fraction of stellar mass in satellites versus total stellar mass, $f_mathrm{sat}$, tend to be slightly higher around blue ICGs, perhaps implying the late formation of blue galaxies. $f_mathrm{sat}$ increases with the increase in both $M_{ast,mathrm{ICG+diffuse}}$ and $M_{200}$, and scales more linearly with $M_{200}$. We provide best-fitting formulas for these scaling relations and for red and blue ICGs separately.
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