Athena Engholm

Luminous infrared (IR) galaxies are key sites of obscured stellar mass assembly at z > 0.5. Their star formation rates (SFRs) are often estimated using the luminosities of the 6.2 and 11.2 μ m polycyclic aromatic hydrocarbon (PAH) features or those of the [Ne ii ] and [Ne iii ] fine-structure lines, as they are minimally affected by obscuration. It is uncertain whether the calibration of these features as SFR tracers depends on the starburst bolometric luminosity or the level of active galactic nucleus (AGN) activity. We here investigate the relationship between the luminosities of PAH and neon lines with SFR for highly luminous objects using radiative transfer modeling and archival observations of 42 local ultraluminous (≥10 ¹² L _⊙ ) IR galaxies (ULIRGs). We find that PAH and [Ne ii ] features arise mainly in star-forming regions, with small contributions from the AGN or host, but that the [Ne iii ] line has a mixed contribution from both star formation and AGN activity. We present relations between L _PAH and L _{[Ne II ]} , and both starburst luminosity and SFR. We find relations for lower-luminosity ( L _IR ≃ 10 ¹⁰ –10 ¹² L _⊙ ) systems underestimate the SFRs in local ULIRGs by up to ∼1 dex. The 6.2 and 11.2 μ m PAH features, and the [Ne ii ] line, are thus good tracers of SFR in ULIRGs. We do not find that a more luminous AGN affects the relationship between SFR and PAH or neon luminosity but that it can make PAH emission harder to discern. Our results and derived relations are relevant to studies of star-forming and composite galaxies at z < 3 with the James Webb Space Telescope.

Infrared-luminous galaxies are important sites of stellar and black hole mass assembly at most redshifts. Their luminosities are often estimated by fitting spectral energy distribution (SED) models to near- to far-infrared data, but the dependence of these estimates on the data used is not well understood. Here, using observations simulated from a well-studied local sample, we compare the effects of wavelength coverage, signal-to-noise ratio, flux calibration, angular resolution, and redshift on the recovery of starburst, active galactic nucleus (AGN), and host luminosities. We show that the most important factors are wavelength coverage that spans the peak in a SED, and dense wavelength sampling. Such observations recover starburst and AGN infrared luminosities with systematic bias below 20%. Starburst luminosities are best recovered with far-infrared observations, while AGN luminosities are best recovered with near- and mid-infrared observations, though the recovery of both are enhanced with near/mid-infrared and far-infrared observations, respectively. Host luminosities are best recovered with near/far-infrared observations, but are usually biased low, by ≳20%. The recovery of starburst and AGN luminosity is enhanced by observing at high angular resolution. Starburst-dominated systems show more biased recovery of luminosities than do AGN-dominated systems. As redshift increases, far-infrared observations become more capable and mid-infrared observations less capable at recovering luminosities. Our results highlight the transformative power of a far-infrared instrument with dense wavelength coverage, from tens to hundreds of microns, for studying infrared-luminous galaxies. We tabulate estimates of systematic bias and random error for use with JWST and other observatories.

Luminous infrared galaxies are key sites of obscured stellar mass assembly at z > 0.5. Their star formation rates (SFRs) are often estimated using the luminosities of the 6.2 micron and 11.2 micron polycyclic aromatic hydrocarbon (PAH) features, or those of the [Ne II] and [Ne III] fine-structure lines, as they are minimally affected by obscuration. It is uncertain whether the calibration of these features as SFR tracers depends on the starburst bolometric luminosity or the level of Active Galactic Nucleus (AGN) activity. We here investigate the relationship between the luminosities of PAH and Neon lines with star formation rate for highly luminous objects using radiative transfer modeling and archival observations of 42 local Ultraluminous (>= 10^12 L_sun) Infrared Galaxies (ULIRGs). We find that PAH and [Ne II] features arise mainly in star-forming regions, with small contributions from the AGN or host, but that the [Ne III] line has a mixed contribution from both star formation and AGN activity. We present relations between L_PAH and L_NeII, and both starburst luminosity and SFR. We find relations for lower luminosity (L_IR ~= 10^10-10^12 L_sun) systems underestimate the SFRs in local ULIRGs by up to ~1 dex. The 6.2 micron and 11.2 micron PAH features, and the [Ne II] line, are thus good tracers of SFR in ULIRGs. We do not find that a more luminous AGN affects the relationship between SFR and PAH or Neon luminosity, but that it can make PAH emission harder to discern. Our results and derived relations are relevant to studies of star-forming and composite galaxies at z < 3 with the James Webb Space Telescope.

The assembly of supermassive black hole (SMBH) mass (M _•) and stellar mass (M _*) in galaxies can be studied via the redshift evolution of the M _•–M _* relation, but the ways in which selection bias and physical assembly channels affect this evolution are uncertain. To address this, we compare the M _•–M _* relation for local massive (M _* > 10^10.5 M _⊙) quiescent early-type galaxies (ETGs) to that for massive ETGs hosting active galactic nuclei (AGN) at z ∼ 0.8. The restrictions on stellar mass and galaxy type limit the assembly channels that may connect the two relations. For the local sample we find log ( M • ) = 8.80 + 1.10 ( log M * − 11 ) , in line with prior work. For the z ∼ 0.8 sample we find a bias-corrected relation: log ( M • ) = 7.80 + 1.25 ( log M * − 11 ) . We show, however, that this relation depends on the stellar and SMBH mass functions used to compute the selection bias, the virial relation, the virial factor, and the active fraction, which together introduce uncertainty of up to ​​​​​​∼0.6 dex in the z ∼ 0.8 relation. Adopting reasonable choices of these parameters then our z ∼ 0.8 relation lies above that for z ∼ 0 AGN by ∼0.5 dex, but below our z ∼ 0 ETG relation by 0.4–1 dex in SMBH mass. We discuss possible sources of this offset, including further bias corrections, “downsizing” in SMBH mass assembly, and preferential SMBH growth. Our results highlight the need to reduce uncertainties from selection and measurement bias in SMBH and stellar masses at all redshifts.

Coupling of black hole mass to the cosmic expansion has been suggested as a possible path to understanding the dark energy content of the Universe. We test this hypothesis by comparing the supermassive black hole (SMBH) mass density at z = 0 to the total mass accreted in active galactic nuclei (AGN) since z = 6, to constrain how much of the SMBH mass density can arise from cosmologically coupled growth, as opposed to growth by accretion. Using an estimate of the local SMBH mass density of ≈1.0 × 10⁶ M _⊙ Mpc⁻¹, a radiative accretion efficiency, η, in the range 0.05 < η < 0.3, and the observed AGN luminosity density at z ≈ 4, we constrain the value of the coupling constant between the scale size of the Universe and the black hole mass, k, to lie in the range 0 < k ≲ 2, below the value of k = 3 needed for black holes to be the source term for dark energy. Initial estimates of the gravitational-wave background (GWB) using pulsar timing arrays, however, favor a higher SMBH mass density at z = 0. We show that if we adopt such a mass density at z = 0 of ≈7.4 × 10⁶ M _⊙ Mpc⁻¹, this makes k = 3 viable even for low radiative efficiencies, and may exclude nonzero cosmological coupling. We conclude that, although current estimates of the SMBH mass density based on the black hole mass–bulge mass relation probably exclude k = 3, the possibility remains open that, if the GWB is due to SMBH mergers, k > 2 is preferred.