Enhanced Star Formation Efficiency in the Central Regions of Nearby Quasar Hosts

Abstract

We combine Atacama Large Millimeter/submillimeter Array and Multi Unit Spectroscopic Explorer observations tracing the molecular gas, millimeter continuum, and ionized gas emission in six low-redshift (z ≲ 0.06) Palomar–Green (PG) quasar host galaxies to investigate their ongoing star formation at roughly kiloparsec-scale resolution. The AGN contribution to the cold dust emission and the optical emission-line flux is carefully removed to derive spatial distributions of the star formation rate (SFR), which, complemented with the molecular gas data, enables the mapping of the depletion time (t dep). We report ubiquitous star formation activity within the quasar host galaxies, with the majority of the ongoing star formation occurring in the galactic center. The rise of the SFR surface density (ΣSFR) toward the nucleus is steeper than that observed for the cold molecular gas surface density, reaching values up to ΣSFR ≈ 0.15–0.80 M ⊙ yr−1 kpc−2. The gas in the nuclear regions is converted into stars at a shortened depletion time (t dep ≈ 0.2–2.0 Gyr), suggesting that those zones can be deemed as starbursts. At large galactocentric radius, we find that the ongoing star formation takes place within spiral arms or H ii region complexes, with an efficiency comparable to that reported for nearby inactive spirals (t dep ≈ 1.8 Gyr). We find no evidence of star formation activity shutoff in the PG quasar host galaxies. On the contrary, these observations shed light on how the central environments of galaxies hosting actively accreting supermassive black holes build up stellar mass.

Dynamics of Molecular Gas in the Central Region of the Quasar I Zwicky 1

“…The star formation activity distribution was confirmed with newly developed integrated field unit (IFU) observations (Perna et al 2021;Lamperti et al 2022;Molina et al 2022). The significant star formation activity has also been confirmed by a combination of optical and submillimeter observations (Molina et al 2023). Previous IRAM and ALMA observations have already suggested that I Zw 1 has a rich molecular gas reservoir, mainly concentrated in its circumnuclear zone Eckart et al 1994;Schinnerer et al 1998;Tan et al 2019).…”

“…Assuming a MW-like α CO increases the molecular gas surface density significantly, while the data points are still close to a ULIRG-like KS relation, and are still well above the KS relation for local star-forming galaxies (Figure 7). Nuclear starbursts have also been found in other low-z quasars (Cresci et al 2004;Schweitzer et al 2006;Romeo & Fathi 2016;Molina et al 2023).…”

“…Molina et al (2021) found that the molecular gas in this galaxy is centrally concentrated, and rotating in a disk with negligible noncircular motions. Moreover, the continuum map reveals a centrally enhanced star formation (see also Molina et al 2023), which also argues against the suppression of star formation from AGN feedback. From our kinematic and dynamical analysis, we find no evidence of AGN-driven outflow or an external gas energy budget.…”

“…The ALMA continuum image reveals a 1.3 mm continuum flux density of ∼1.1 mJy from the central 3σ contour region. Molina et al (2023) decomposed the SED of I ZW 1. They estimated that the AGN contribution, including the nonthermal synchrotron emission extrapolated from the radio bands and the thermal free-free emission, contribute about 28% (∼0.30 mJy) of the ALMA continuum, and the remaining 72% is likely to be from the thermal dust heated by nuclear star formation.…”

“…This value is consistent with that derived from the synchrotron and free-free components in the SED decomposition, considering the large uncertainty of 0.45 dex of the νL ν,mm -L 2−10 keV relation. Therefore, we adopt the nuclear SFR of 5.43 M e yr −1 from Molina et al (2023) in the analysis here.…”