[Paper Review] Evidence for the connection between star formation rate and evolutionary phases of quasars

Zhifu Chen, Zhicheng He+6 more|arXiv (Cornell University)|Nov 18, 2021

Galaxies: Formation, Evolution, Phenomena50 references44 citations

TL;DR

This study uses the [O II]/[Ne V] emission line flux ratio as a proxy for star formation rate (SFR) in quasar host galaxies across different broad absorption line (BAL) types—LoBAL, HiBAL, and non-BAL—using SDSS DR16 data. It finds that SFR decreases from LoBAL to HiBAL quasars and then rebounds in non-BAL quasars, supporting an evolutionary sequence where outflows quench star formation temporarily before it recovers, indicating negative global feedback from quasar-driven outflows.

ABSTRACT

Both theory and observations suggest that outflows driven by an active central supermassive black hole (SMBH) has a feedback effect on shaping the global properties of the host galaxy. However, whether feedback from the outflow is effective, and if so, whether it is positive or negative, has long been controversial. Here, using the latest catalog from the Sloan Digital Sky Survey (SDSS), we use the flux ratio of the [O II] to [Ne V] emission lines as a proxy to compare the star formation rate (SFR) in the hosts of quasars with different types of broad absorption lines (BALs): low-ionization (Lo)BAL, high-ionization (Hi)BAL, and non-BAL. We find that SFR decreases from LoBAL to HiBAL quasars, and then increases from HiBAL to non-BAL quasars. Assuming that the sequence of LoBAL to HiBAL to non-BAL represents evolution, our results are consistent with a quenching and subsequent rebound of star formation in quasar host galaxies. This phenomenon can be explained that the SFR is suppressed by the outflow, which then rebounds once the outflow disappears as the quasars evolve from HiBALs to non-BALs. Our result suggests that the quasar outflow has a negative global feedback on galaxy evolution.

Motivation & Objective

• To investigate the connection between star formation rate (SFR) and evolutionary phases of quasars.
• To test whether quasar outflows have a negative feedback effect on host galaxy star formation.
• To determine if the observed BAL types (LoBAL, HiBAL, non-BAL) represent distinct evolutionary stages.
• To use the [O II]/[Ne V] flux ratio as a proxy for SFR in quasars with different BAL classifications.

Proposed method

• Collected 407 LoBAL, 714 HiBAL, and 14,144 non-BAL quasars from the SDSS DR16 quasar catalog.
• Constructed median composite spectra for each BAL type to analyze emission line features.
• Used the flux ratio R = EW[O II]/EW[Ne V] as a proxy for SFR, where higher R indicates higher SFR.
• Applied the Small Magellanic Cloud extinction law to correct for dust reddening in LoBAL and HiBAL quasars.
• Binned HiBAL quasars by C IV BAL equivalent width (EWBAL_C IV) to trace SFR evolution within the HiBAL phase.
• Performed statistical significance tests (Nσ) to assess differences in R across BAL types and bins.

Experimental results

Research questions

• RQ1 Does the star formation rate (SFR) vary systematically across different BAL quasar types (LoBAL, HiBAL, non-BAL)?
• RQ2 Is the observed SFR variation consistent with an evolutionary sequence from LoBAL to non-BAL quasars?
• RQ3 Does the strength of the C IV broad absorption line correlate with SFR evolution within the HiBAL phase?
• RQ4 Can the [O II]/[Ne V] flux ratio reliably trace SFR in quasar hosts despite potential contamination from shocks or narrow-line region diversity?
• RQ5 Is the observed SFR evolution best explained by an evolutionary scenario rather than an orientation-dependent model?

Key findings

• The [O II]/[Ne V] flux ratio R decreases significantly from LoBALs (R = 2.228 ± 0.158) to HiBALs (R = 0.668 ± 0.039), indicating a strong drop in SFR (9.6σ significance).
• R then increases from HiBALs to non-BALs (R = 1.002 ± 0.013), showing a significant rebound in SFR (8.1σ significance), suggesting recovery after quenching.
• Within the HiBAL sample, R decreases with increasing C IV BAL equivalent width (from 1.779 ± 0.282 at EWBAL_C IV > 25 Å to 0.597 ± 0.066 at 10–15 Å), indicating SFR suppression during strong outflows.
• The SFR rebound begins when EWBAL_C IV drops below 10 Å, suggesting the outflow's quenching effect diminishes at this stage.
• The R ratio remains significantly higher in LoBALs than in HiBALs (10σ significance) even after correcting for [Ne V] weakness, confirming enhanced SFR in LoBAL hosts.
• The SFR trends are independent of black hole mass, as shown across three MBH bins (≤10^8.5, 10^8.5–10^9.0, ≥10^9.0 M⊙), supporting robustness of the results.