Radio continuum observations have proven to be a workhorse in our understanding of the star formation process (i.e., stellar birth and death) from galaxies both in the nearby universe and out to the highest redshifts. In this article we focus on how
the ngVLA will transform our understanding of star formation by enabling one to map and decompose the radio continuum emission from large, heterogeneous samples of nearby galaxies on $gtrsim 10$,pc scales to conduct a proper accounting of the energetic processes powering it. At the discussed sensitivity and angular resolution, the ngVLA will simultaneously be able to create maps of current star formation activity at $sim$100,pc scales, as well as detect and characterize (e.g., size, spectral shape, density, etc.) discrete H{sc ii} regions and supernova remnants on 10,pc scales in galaxies out to the distance of the Virgo cluster. Their properties can then be used to see how they relate to the local and global ISM and star formation conditions. Such investigations are essential for understanding the astrophysics of high-$z$ measurements of galaxies, allowing for proper modeling of galaxy formation and evolution.
H$_2$O megamasers residing in the accretion disks of active galactic nuclei (AGN) exhibit Keplerian rotation about the central supermassive black hole (SMBH). Such disk maser systems are excellent tools for diagnosing the kinematic status of the SMBH
, and they currently provide the only direct and unambiguous measure of SMBH velocities outside of the Milky Way. We have measured the galaxy recession velocities for a sample of 10 maser disk systems using a combination of spatially resolved HI disk modeling, spatially integrated HI profile fitting, and optical spectral line and continuum fitting. In comparing the SMBH velocities to those of their host galaxies, we find two (out of 10) systems -- J0437+2456 and NGC 6264 -- for which the SMBH and galaxy velocities show a statistically significant ($>$3$sigma$) difference. For NGC 6264 the apparent velocity offset can likely be explained by ionized gas motion within the host galaxy (e.g., from AGN-driven shocks). The velocity measurements for J0437+2456, however, imply a SMBH peculiar velocity of $69.6 pm 12.7$ km s$^{-1}$ (5.5$sigma$). We thus consider J0437+2456 to be a promising candidate for hosting either a recoiling or binary SMBH, though additional observations are necessary to exclude the possibility of a systematic offset between the galactic recession velocity and that measured using the optical spectrum.
(Abridged) Our sensitive ($sigma_{rm n} = 572,{rm nJy,beam}^{-1}$), high-resolution (FWHM $theta_{1/2} = 220,{rm mas} approx2mathrm{,kpc~at~}zgtrsim1$) 10$,$GHz image covering a single Karl G.~Jansky Very Large Array (VLA) primary beam (FWHM $Theta_{
1/2} = 4.25$) in the GOODS-N field contains 32 sources with $S_{rm p}gtrsim2,mu{rm Jy~beam}^{-1}$ and optical and/or near-infrared (OIR) counterparts. Most are about as large as the star-forming regions that power them. Their median FWHM major axis is $langletheta_{rm M} rangle=167pm32,{rm mas} approx 1.2pm0.28,{rm kpc}$ with rms scatter 91 was $approx$ 0.79 kpc. In units of the effective radius $r_{rm e}$ that encloses half their flux, these radio sizes are $langle r_{rm e}rangle = 69pm13{rm mas} approx pm114mathrm{,pc}$ and have rms scatter $38mathrm{,mas}approx324mathrm{,pc}$. These sizes are smaller than those measured at lower radio frequencies, but agree with dust emission sizes measured at mm/sub-mm wavelengths and extinction-corrected H$alpha$ sizes. We made a low-resolution ($theta_{1/2}=1.0$) image with $approx10times$ better brightness sensitivity to detect extended sources and measure matched-resolution spectral indices $alpha_{1.4}^{10}$. It contains 6 new sources with $S_{rm p}gtrsim3.9,mu{rm Jy~beam}^{-1}$ and OIR counterparts. The median redshift of all 38 sources is $1.24pm0.15$. The 19 sources with 1.4$,$GHz counterparts have median spectral index $-0.74pm0.10$ with rms scatter $0.35$. Including upper limits on $alpha$ for sources not detected at 1.4$,$GHz flattens the median to $gtrsim-0.61$, suggesting that the $mu$Jy radio sources at higher redshifts, and hence selected at higher rest-frame frequencies, may have flatter spectra. If the non-thermal spectral index is -0.85, the median thermal fraction at rest-frame frequency 20$,$GHz is $gtrsim$48%.
We use single-dish radio spectra of known 22 GHz H$_2$O megamasers, primarily gathered from the large dataset observed by the Megamaser Cosmology Project, to identify Keplerian accretion disks and to investigate several aspects of the disk physics. W
e test a mechanism for maser excitation proposed by Maoz & McKee (1998), whereby population inversion arises in gas behind spiral shocks traveling through the disk. Though the flux of redshifted features is larger on average than that of blueshifted features, in support of the model, the high-velocity features show none of the predicted systematic velocity drifts. We find rapid intra-day variability in the maser spectrum of ESO 558-G009 that is likely the result of interstellar scintillation, for which we favor a nearby ($D approx 70$ pc) scattering screen. In a search for reverberation in six well-sampled sources, we find that any radially-propagating signal must be contributing $lesssim$10% of the total variability. We also set limits on the magnetic field strengths in seven sources, using strong flaring events to check for the presence of Zeeman splitting. These limits are typically 200--300 mG ($1sigma$), but our most stringent limits reach down to 73 mG for the galaxy NGC 1194.
The shape of the curves defined by the counts of radio sources per unit area as a function of their flux density was one of the earliest cosmological probes. Radio source counts continue to be an area of interest, used to study the relative populatio
ns of galaxy types in the Universe (as well as investigate any cosmological evolution in luminosity functions). They are a vital consideration for determining how source confusion may limit the depth of a radio interferometer observation, and are essential for characterising extragalactic foregrounds in CMB experiments. There is currently no consensus as to the relative populations of the faintest (sub-mJy) source types, where the counts turn-up. Most of the source counts in this regime are gathered from multiple observations that each use a deep, single pointing with a radio interferometer. These independent measurements show large amounts of scatter (factors ~ a few) that significantly exceeds their stated uncertainties. In this article we use a simulation of the extragalactic radio continuum emission to assess the level at which sample variance may be the cause of the scatter. We find that the scatter induced by sample variance in the simulated counts decreases towards lower flux density bins as the raw source counts increase. The field-to-field variations are significant, and could even be the sole cause at >100 {mu}Jy. We present a method for evaluating the flux density limit that a survey must reach in order to reduce the count uncertainty induced by sample variance to a specific value. We also derive a method for correcting Poisson errors on counts in order to include the uncertainties due to the cosmological clustering of sources. An empirical constraint on the effect of sample variance at these low luminosities is unlikely to arise until the completion of new large-scale surveys with next-generation radio telescopes.
Despite decades of study, it remains unclear whether there are distinct radio-loud and radio-quiet populations of quasi-stellar objects (QSOs). Early studies were limited by inhomogeneous QSO samples, inadequate sensitivity to probe the radio-quiet p
opulation, and degeneracy between redshift and luminosity for flux-density-limited samples. Our new 6 GHz EVLA observations allow us for the first time to obtain nearly complete (97%) radio detections in a volume-limited color-selected sample of 179 QSOs more luminous than M_i = -23 from the Sloan Digital Sky Survey (SDSS) Data Release Seven in the narrow redshift range 0.2 < z < 0.3. The dramatic improvement in radio continuum sensitivity made possible with the new EVLA allows us, in 35 minutes of integration, to detect sources as faint as 20 microJy, or log[L_6 (W/Hz)] ~ 21.5 at z = 0.25, well below the radio luminosity, log[L_6 (W/Hz)] ~ 22.5, that separates star-forming galaxies from radio-loud active galactic nuclei (AGNs) driven by accretion onto a super-massive black hole. We calculate the radio luminosity function (RLF) for these QSOs using three constraints: (a) EVLA 6 GHz observations for log[L_6 (W/Hz)] < 23.5, (b) NRAO-VLA Sky Survey (NVSS) observations for log[L_6 (W/Hz)] > 23.5, and (c) the total number of SDSS QSOs in our volume-limited sample. We show that the RLF can be explained as a superposition of two populations, dominated by AGNs at the bright end and star formation in the QSO host galaxies at the faint end.
