Galaxies and Galaxy Formation

The VLA and ALMA provide the unique capability to study the cool gas and dust in galaxies. These are key constituents in fueling star formation, and act as star formation tracers. The VLBA provides the complimentary ability to study the bright radio continuum structures on tens of pc-scales, associated with the most distant Active Galactic Nuclei (AGN).

The order of magnitude decline in the cosmic star formation rate density over the last 10 Gyr is a well constrained, but not well understood, phenomenon. It has long been assumed that the cause for cessation of star formation in galaxies is depletion of the molecular gas via strong winds and shocks driven by the star formation itself.  SDSS J1448+1010, at z = 0.65, is a post-merger galaxy which had a sharp drop-off in star formation some 70Myr ago.  ALMA observations of CO 2-1 emission show that this system still has plenty of molecular gas, but that the gas has been torn from the main galaxy into a 64 kpc tidal stream due to the recent merger. These observations suggest an alternative mechanism for quenching star formation in galaxies via major mergers, as opposed to just starburst driven dynamics (Figure 9; Spilker et al. 2022, ApJ 936, L11).

Figure 9. ALMA observations of CO2-1 emission from a post-merger, post-starburst galaxy at z = 0.64. Most of the molecular gas has been stripped into tidal tails extending 64 kpc (Spilker et al. 2022, ApJ 936, L11)

The VLBA remains the only instrument that can probe AGN radio structure down to tens of pc-scales in the most distant galaxies. A recent exciting result came from 1.4 GHz VLBA observations of the low luminosity radio AGN in the z ~ 6 quasar, J2242+0334. These observations imply a highly resolved radio source at 10 mas resolution (60 pc), implying a wind-like radio emitting region, and not a typical high brightness relativistic core-jet. These observations provide clues as to the nature of the radio emission from the lower luminosity radio AGN in the early Universe.

ALMA, working in concert with the JWST, has identified a dust and molecule rich grand design spiral galaxy at z = 3.06. The galaxy has a radius of 7 kpc, and a star formation rate of 85 M_⊙/yr, with the dust and star formation revealed by ALMA concentrated in the galaxy center. The CO lines provided the spectroscopic redshift for this galaxy. Existence of an apparently mature, metal enriched, large spiral galaxy just 2 Gyr after the Big Bang challenges mechanisms for galaxy formation in the early Universe (Figure 10; Wu et al. 2022).

Figure 10. JWST image of a grand design spiral at z = 3.0. Center: stellar mass surface density (color) and ALMA 150 GHz dust emission (green contours). Right: ALMA detection of molecular gas (Wu et al. arXiv:2208:08473).

The ALMA Large Programs: Reionization Era Bright Emission Line Survey (REBELS), ALMA Large Program to Investigate C+ at Early Times (ALPINE), and ALMA Lensing Cluster Survey (ALCS), have targeted identifying and performing a first characterization of many of the most luminous star-forming galaxies known in the z > 6.5 universe. REBELS systematically scans 40 bright UV-selected galaxies in a 7 degree square area for bright [C II]158 μm and [O III]88 μm lines and dust-continuum emission. Initial results (Bouwens et al. 2022, ApJ 931, 160) identified 18 objects with [C II] emission (see Figure), 13 of which also show dust continuum emission. An analysis of these search results vs. Star Formation Rate (SFR) suggests an ~79% efficiency in scanning for [CII] when the SFR(UV+IR) is in excess of 28 M_o/yr.

The first ultra-deep fields with the JWST have identified an unexpectedly large number of extreme redshift galaxy candidates at z > 10,  or within 500 Myr of the Big Bang. However, these first galaxy candidates are based on photometric redshifts only—spectroscopic verification of the redshifts remains critical. ALMA provides the unique potential to obtain spectroscopic redshifts using fine structure lines. ALMA has observed the z ~ 12.0 JWST candidate, GLASS-z13, at 250 GHz, searching for both the [OIII] 83um line and thermal continuum emission. Non-detection of the dust continuum implies a dust-poor galaxy, consistent with the blue optical color. A marginal (4𝝈) detection is made of the [OIII] line at z = 12.1, offset by 0.5” from the optical galaxy. While the probability of this line being noise remains significant (Kaasinen et al. 2022), the general point remains that ALMA can be an important tool in the spectroscopic verification, and characterization, of the first galaxies candidates from JWST (Figure 11; Bakx et al. 2023).

Figure 11. Tentative [OIII] 88um emission seen by ALMA at 256 GHz, from the z = 12.1 JWST galaxy candidate GLASS-z13 (Bakx et al. 2022, arXiv:2208.13642).