Dynamical Friction in Nonlocal Gravity

We study dynamical friction in the Newtonian regime of nonlocal gravity (NLG), which is a classical nonlocal generalization of Einstein’s theory of gravitation. The nonlocal aspect of NLG simulates dark matter. The attributes of the resulting effective dark matter are described and the main physical predictions of NLG, which has a characteristic length scale of order 1 kpc, for galactic dynamics are presented. Within the framework of NLG, we derive the analog of Chandrasekhar’s formula for dynamical friction. The astrophysical implications of the results for the apparent rotation of a central bar subject to dynamical friction in a barred spiral galaxy are briefly discussed.

Mapping the Cosmos

There is no way to transcribe the features of the Earth’s spherical surface onto a flat map without some distortion. All maps distort the geography of the sphere. The familiar Mercator maps inflate regions close to the poles compared to regions in the tropics. In 1973, Arno Peters promoted the Gall–Peters projection that compensates for the expansion of polar regions compared to the tropics. Buckminster Fuller invented a map called the Dymaxion in which the globe is projected onto an icosahedron, which is then unfolded into an icosahedral net. Another interesting projection is the Pierce Quincuncial projection invented by Charles Sanders Pierce. The Milky Way galaxy was recently mapped using data from NASA’s Wide-field Infra-red Survey Explorer (WISE) and shown to be a barred spiral galaxy. Pablo Carlos Budassi has created a map of the entire visible universe using NASA images by representing radial distances on a logarithmic scale.

Comparison of galaxy spiral arm pitch angle measurements using manual and automated techniques

ABSTRACT The pitch angle (PA) of arms in spiral galaxies has been found to correlate with a number of important parameters that are normally time intensive and difficult to measure. Accurate PA measurements are therefore important in understanding the underlying physics of disc galaxies. We introduce a semi-automated method that improves upon a parallelized two-dimensional fast Fourier transform algorithm (p2dfft) to estimate PA. Rather than directly inputting deprojected, star subtracted, and galaxy centred images into p2dfft, our method (p2dfft:traced) takes visually traced spiral arms from deprojected galaxy images as input. The tracings do not require extensive expertise to complete. This procedure ignores foreground stars, bulge and/or bar structures, and allows for better discrimination between arm and interarm regions, all of which reduce noise in the results. We compare p2dfft:traced to other manual and automated methods of measuring PA using both simple barred and non-barred spiral galaxy models and a small sample of observed spiral galaxies with different representative morphologies. We find that p2dfft:traced produces results that, in general, are more accurate and precise than the other tested methods and it strikes a balance between total automation and time-consuming manual input to give reliable PA measurements.

A three-dimensional map of the Milky Way using classical Cepheid variable stars

The Milky Way is a barred spiral galaxy, with physical properties inferred from various tracers informed by the extrapolation of structures seen in other galaxies. However, the distances of these tracers are measured indirectly and are model-dependent. We constructed a map of the Milky Way in three dimensions, based on the positions and distances of thousands of classical Cepheid variable stars. This map shows the structure of our Galaxy’s young stellar population and allows us to constrain the warped shape of the Milky Way’s disk. A simple model of star formation in the spiral arms reproduces the observed distribution of Cepheids.

WALLABY Early Science – IV. ASKAP H i imaging of the nearby galaxy IC 5201

ABSTRACT We present a Wide-field ASKAP L-Band Legacy All-sky Blind surveY (WALLABY) study of the nearby (vsys = 915 km s−1) spiral galaxy IC 5201 using the Australian Square Kilometre Array Pathfinder (ASKAP). IC 5201 is a blue, barred spiral galaxy that follows the known scaling relations between stellar mass, SFR, H i mass, and diameter. We create a four-beam mosaicked H i image cube from 175 h of observations made with a 12-antenna sub-array. The root mean square noise level of the cube is 1.7 mJy beam−1 per channel, equivalent to a column density of $N_{\rm H\, \small {I}}$ = 1.4 × 1020 cm−2 over 25 km s−1. We report nine extragalactic H i detections – five new H i detections including the first velocity measurements for two galaxies. These sources are IC 5201, three dwarf satellite galaxies, two galaxies, and a tidal feature belonging to the NGC 7232/3 triplet and two potential infalling galaxies to the triplet. There is evidence of a previous tidal interaction between IC 5201 and the irregular satellite AM 2220−460. A close fly-by is likely responsible for the asymmetric optical morphology of IC 5201 and warping its disc, resulting in the irregular morphology of AM 2220−460. We quantify the H i kinematics of IC 5201, presenting its rotation curve as well as showing that the warp starts at 14 kpc along the major axis, increasing as a function of radius with a maximum difference in position angle of 20°. There is no evidence of stripped H i, triggered or quenched star formation in the system as measured using DECam optical and GALEX UV photometry.

Zoom-in cosmological hydrodynamical simulation of a star-forming barred, spiral galaxy at redshift z = 2

ABSTRACTWe present gas and stellar kinematics of a high-resolution zoom-in cosmological chemodynamical simulation, which fortuitously captures the formation and evolution of a star-forming barred spiral galaxy, from redshift z ∼ 3 to z ∼ 2 at the peak of the cosmic star formation rate. The galaxy disc grows by accreting gas and substructures from the environment. The spiral pattern becomes fully organized when the gas settles from a thick (with vertical dispersion σv > 50 km s−1) to a thin (σv ∼ 25 km s−1) disc component in less than 1 Gyr. Our simulated disc galaxy also has a central X-shaped bar, the seed of which formed by the assembly of dense gas-rich clumps by z ∼ 3. The star formation activity in the galaxy mainly happens in the bulge and in several clumps along the spiral arms at all redshifts, with the clumps increasing in number and size as the simulation approaches z = 2. We find that stellar populations with decreasing age are concentrated towards lower galactic latitudes, being more supported by rotation, and having also lower velocity dispersion; furthermore, the stellar populations on the thin disc are the youngest and have the highest average metallicities. The pattern of the spiral arms rotates like a solid body with a constant angular velocity as a function of radius, which is much lower than the angular velocity of the stars and gas on the thin disc; moreover, the angular velocity of the spiral arms steadily increases as a function of time, always keeping its radial profile constant. The origin of our spiral arms is also discussed.

CO Multi-line Imaging of Nearby Galaxies (COMING). III. Dynamical effect on molecular gas density and star formation in the barred spiral galaxy NGC 4303

We present the results of $^{12}\textrm{C}$$\textrm{O}$(J = 1–0) and $^{13}\textrm{C}$$\textrm{O}$(J = 1–0) simultaneous mappings toward the nearby barred spiral galaxy NGC 4303 as part of the CO Multi-line Imaging of Nearby Galaxies (COMING) project. Barred spiral galaxies often show lower star-formation efficiency (SFE) in their bar region compared to the spiral arms. In this paper, we examine the relation between the SFEs and the volume densities of molecular gas n(H2) in the eight different regions within the galactic disk with $\textrm{C}$$\textrm{O}$ data combined with archival far-ultraviolet and 24 μm data. We confirmed that SFE in the bar region is lower by 39% than that in the spiral arms. Moreover, velocity-alignment stacking analysis was performed for the spectra in the individual regions. Integrated intensity ratios of $^{12}\textrm{C}$$\textrm{O}$ to $^{13}\textrm{C}$$\textrm{O}$ (R12/13) ranging from 10 to 17 were the results of this stacking. Fixing a kinetic temperature of molecular gas, $n(\rm {H_2})$ was derived from R12/13 via non-local thermodynamic equilibrium (non-LTE) analysis. The density n(H2) in the bar is lower by 31%–37% than that in the arms and there is a rather tight positive correlation between SFEs and n(H2), with a correlation coefficient of ∼0.8. Furthermore, we found a dependence of $n(\rm {H}_2)$ on the velocity dispersion of inter-molecular clouds (ΔV/sin i). Specifically, n(H2) increases as ΔV/sin i increases when ΔV/sin i < 100 km s−1. On the other hand, n(H2) decreases as ΔV/sin i increases when ΔV/sin i > 100 km s−1. These relations indicate that the variations of SFE could be caused by the volume densities of molecular gas, and the volume densities could be governed by the dynamical influence such as cloud–cloud collisions, shear, and enhanced inner-cloud turbulence.