galaxy-formation

First results from the methods: numerical, galaxy formation, large-scale structure of Universe: matter and galaxy clustering

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Abstract

Hydrodynamical simulations of galaxy formation have now reached sufficient volume to make precision predictions for clustering on cosmologically relevant scales. Here, we use our new IllustrisTNG simulations to study the non-linear correlation functions and power spectra of baryons, dark matter, galaxies, and haloes over an exceptionally large range of scales. We find that baryonic effects increase the clustering of dark matter on small scales and damp the total matter power spectrum on scales up to k ∼ 10 h Mpc−1 by 20 per cent. The non-linear two-point correlation function of the stellar mass is close to a power-law over a wide range of scales and approximately invariant in time from very high redshift to the present. The two-point correlation function of the simulated galaxies agrees well with Sloan Digital Sky Survey at its mean redshift z ≃ 0.1, both as a function of stellar mass and when split according to galaxy colour, apart from a mild excess in the clustering of red galaxies in the stellar mass range of109–1010 h−2 M⊙. Given this agreement, the TNG simulations can make valuable theoretical predictions for the clustering bias of different galaxy samples. We find that the clustering length of the galaxy autocorrelation function depends strongly on stellar mass and redshift. Its power-law slope γ is nearly invariant with stellar mass, but declines from γ ∼ 1.8 at redshift z = 0 to γ ∼ 1.6 at redshift z ∼ 1, beyond which the slope steepens again. We detect significant scale dependences in the bias of different observational tracers of large-scale structure, extending well into the range of the baryonic acoustic oscillations and causing nominal (yet fortunately correctable) shifts of the acoustic peaks of around ∼ 5 per cent.

Magnetic field discovery gives clues to galaxy-formation processes

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Astronomers making a detailed, multi-telescope study of a nearby galaxy have discovered a magnetic field coiled around the galaxy’s main spiral arm. The discovery, they said, helps explain how galactic spiral arms are formed. The same study also shows how gas can be funneled inward toward the galaxy’s center, which possibly hosts a black hole.

“This study helps resolve some major questions about how form and evolve,” said Rainer Beck, of the Max-Planck Institute for Radio Astronomy (MPIfR), in Bonn, Germany.

The scientists studied a galaxy called IC 342, some 10 million light-years from Earth, using the National Science Foundation’s Karl G. Jansky Very Large Array (VLA), and the MPIfR’s 100-meter Effelsberg radio telescope in Germany. Data from both radio telescopes were merged to reveal the magnetic structures of the galaxy.

The surprising result showed a huge, helically-twisted loop coiled around the galaxy’s main spiral arm. Such a feature, never before seen in a galaxy, is strong enough to affect the flow of gas around the .

“Spiral arms can hardly be formed by gravitational forces alone,” Beck said. “This new IC 342 image indicates that magnetic fields also play an important role in forming spiral arms.”

The new observations provided clues to another aspect of the galaxy, a bright central region that may host a black hole and also is prolifically producing new stars. To maintain the high rate of star production requires a steady inflow of gas from the galaxy’s outer regions into its center.

“The magnetic field lines at the inner part of the galaxy point toward the galaxy’s center, and would support an inward flow of gas,” Beck said.

Large-scale Effelsberg radio image of IC 342. Lines indicate orientation of magnetic fields. Credit: R. Beck, MPIfR.

The scientists mapped the galaxy’s magnetic-field structures by measuring the orientation, or polarization, of the radio waves emitted by the galaxy. The orientation of the radio waves is perpendicular to that of the magnetic field. Observations at several wavelengths made it possible to correct for rotation of the waves’ polarization plane caused by their passage through interstellar magnetic fields along the line of sight to Earth.

The Effelsberg telescope, with its wide field of view, showed the full extent of IC 342, which, if not partially obscured to visible-light observing by dust clouds within our own Milky Way Galaxy, would appear as large as the full moon in the sky. The high resolution of the VLA, on the other hand, revealed the finer details of the galaxy. The final image, showing the , was produced by combining five VLA images made with 24 hours of observing time, along with 30 hours of data from Effelsberg.

Scientists from MPIfR, including Beck. were the first to detect polarized radio emission in galaxies, starting with Effelsberg observations of the Andromeda Galaxy in 1978. Another MPIfR scientist, Marita Krause, made the first such detection with the VLA in 1989, with observations that included IC 342, which is the third-closest spiral galaxy to Earth, after the Andromeda Galaxy (M31) and the Triangulum Galaxy (M33).