Galaxies and Galaxy Formation

ALMA and the VLA are primary tools for studying the molecular gas out of which stars form throughout the Universe, while the ultra-high resolution of the VLBA can probe radio structures down to sub-parsec scales in the most distant galaxies.

The VLA and VLBA have observed HI 21cm absorption toward the radio galaxy 3C84 (Figure 8; Morganti et al. 2023), at the center of the rich Perseus cluster of galaxies. Broad HI 21cm absorption is detected (500 km/s) with the VLA toward the nucleus at arcsec resolution. The line width suggests that the detected gas is close to the SMBH, but no absorption is detected with the VLBA at 7 mas resolution. Comparison with the circum-nuclear disk (CND) seen in CO emission with ALMA, shows similar velocity and spatial structure, suggesting the HI is associated with the fast rotating CND down to 20 parsec radius. The radio continuum providing the background for absorption arises from non-thermal synchrotron emission from the star formation activity in the CND, whose presence has been reported by earlier VLBA studies.

Figure 8. Left to right: VLBA image of 3C84 at 1.4GHz, 7 mas resolution. VLA image at 1.5” resolution. HI 21cm absorption spectra from the VLA in the inner 1” region. ALMA CO velocity field of the circumnuclear disk plus the VLBA 330 MHz image (Morganti et al. arXiv:2309:10358).

Multifrequency VLA observations of the prototype Fanaroff-Riley (FRI) radio galaxy, Hydra A (Figure 9; Baidoo et al. 2023), reveal extreme Faraday rotation measures (RM), with magnitudes as large as -12300 rad m⁻², the majority of which arises in the magnetized thermal cluster gas. The radio emission also depolarizes systematically with decreasing frequency and decreasing resolution. These results can be modeled by a Faraday screen of magnetized cluster gas, external to the radio lobes, with field strengths of a few mG, and with both large scale ordered fields on tens of kiloparsec scales, plus turbulent field structures down to scales of 1 kpc, or less. Such fields have important implications on cluster thermal conductivity and turbulence.

Figure 9. VLA polarimetric imaging of Hydra A, from left to right: total intensity at 2GHz, 1” resolution; magnetic field vectors; rotation measures; depolarization at 6 GHz from 3” to 0.5” resolution (Baidoo et al. 2023, ApJ 955, 16).

One of the most impressive results from JWST has been the discovery of ultra-high-z galaxies, out to z ~ 16, or a look-back time of 13.5 Gyr (the Big Bang occurred 13.7 Gyr ago). The mere existence of these ultra-redshift galaxies presents an interesting challenge to models of galaxy formation and cosmology. ALMA provides the unique ability to image the gas and dust in these first galaxies. ALMA is now regularly detecting fine structure line emission from z ~ 8 to 10 galaxies, revealing gas physical conditions, distribution, and dynamics, complimenting JWST studies of the stars and gas. For the current JWST redshift record holder, a candidate galaxy at z_phot ~ 16, a tantalizing spectral feature seen with ALMA may be [OIII] 52um emission at this redshift, but requires confirmation, since it could be [CII] 157 um emission at z = 4.6. Study of the gas and dust in the ultra-high redshift universe promises to become a major growth area for ALMA in the coming years (Figure 10; Fujimoto et al. 2023).

Figure 10. Top: images of dust continuum (non-detection), possible [OIII] 88um emission, and the JWST near-IR image of a z ~ 16 JWST galaxy candidate (Fujimoto ea. 2023, ApJ, 955, 130). Bottom: ALMA spectrum.

The VLBI has observed M81*, one of the closest low-luminosity active galactic nuclei, at 8.8, 22, and 44 GHz. A bright knot in the core-jet is seen at 1.2mas distance (0.023 pc) from the core. Multi-epoch observations show that the knot has a low apparent speed of 0.1c. X-ray observations show a moderate X-ray flare occurred when the knot launched from the core region. Three possible origins are currently possible: an episodic jet ejection, a low-speed shock wave, or a possible secondary black hole in a binary system (Figure 11; Wang et al. 2023).

Figure 11. Left: An image of the core and bright knot in the core-jet in M81* at 22 GHz with a resolution of 0.4 mas, plus the spectral index between 22 GHz and 44 GHz. Center: fractional polarization of the system at 8.8 GHz plus polarization vectors. Right: measure proper motion of the knot (Wang et al. 2023, arXiv:2310.08902).

The first results from the Physics at High Angular Resolution in Nearby GalaxieS (PHANGS) large program at ALMA have appeared, which, in concert with the JWST, has produced spectacular images of the relationship between the molecular gas, dust, stars, and star formation down to tens of parsec scales in a large sample of nearby galaxies. At higher redshift, the ALMA Large Proposal of gAlactic Cold gAs (ALPACA) program is probing the molecular gas in a large sample of galaxies at the peak epoch of galaxy formation, z ~ 1.