[Paper Review] ALFALFA HI Data Stacking II. HI content of the host galaxies of AGN

S. Fabello, Guinevere Kauffmann+5 more|arXiv (Cornell University)|Apr 3, 2011

Galaxies: Formation, Evolution, Phenomena1 references3 citations

TL;DR

This study uses stacking analysis of ALFALFA H i data to compare the average H i gas content of 1,871 AGN host galaxies with matched non-AGN controls, finding no significant difference in H i mass fraction across accretion rate bins. The results indicate that AGN activity does not significantly influence large-scale gaseous properties in local massive galaxies, though in red sequence hosts, gas fraction and accretion rate show a weak correlation suggesting a mixed fuel supply of stellar mass loss and disk gas.

ABSTRACT

We use a stacking technique to measure the average HI content of a volume-limited sample of 1871 AGN host galaxies from a parent sample of galaxies selected from the SDSS and GALEX imaging surveys with stellar masses greater than 10^10 M_sun and redshifts in the range 0.025

Motivation & Objective

• To measure the average H i gas content in a volume-limited sample of 1,871 AGN host galaxies with stellar masses >10¹⁰ M☉ and redshifts 0.025 < z < 0.05.
• To test whether AGN activity correlates with reduced H i content, as predicted by feedback models that quench star formation via energy output.
• To compare AGN hosts with non-AGN control galaxies matched in NUV–r colour and stellar surface mass density to isolate the effect of nuclear activity.
• To investigate how H i gas fraction varies with black hole accretion rate, as traced by L[O iii]/M_BH.
• To explore differences in gas–accretion relationships between blue and red sequence galaxy populations.

Proposed method

• Stacking H i line profiles from the ALFALFA survey for 1,871 AGN host galaxies in redshift bins to measure average H i mass.
• Constructing a control sample of 1,871 non-AGN galaxies matched to AGN hosts in NUV–r colour and stellar surface mass density (μ⋆) to control for galaxy evolutionary state.
• Using the L[O iii]/σ⁴ indicator as a proxy for black hole accretion rate, dividing galaxies into bins to study trends in H i mass fraction (M_HI/M⋆).
• Analyzing separate trends in blue and red sequence populations to assess whether gas–accretion relationships differ by galaxy colour and morphology.
• Applying population synthesis models (Bruzual & Charlot, 2003; Maraston, 2005) to estimate stellar mass loss rates from observed colours and compare to observed H i fractions.
• Assessing structural changes (e.g., concentration index, μ⋆) across accretion rate bins to test for systematic variations in host galaxy structure.

Experimental results

Research questions

• RQ1 Is there a significant difference in H i gas fraction between AGN host galaxies and non-AGN control galaxies matched in colour and stellar surface density?
• RQ2 Does the H i mass fraction of AGN hosts vary with black hole accretion rate as traced by L[O iii]/σ⁴?
• RQ3 How do the relationships between H i content and accretion rate differ between blue and red sequence galaxies?
• RQ4 To what extent can the observed H i content in red sequence AGN hosts be explained by stellar mass loss alone?
• RQ5 Are structural properties of AGN hosts (e.g., concentration, μ⋆) correlated with accretion rate, potentially biasing gas content measurements?

Key findings

• No significant difference in H i mass fraction (M_HI/M⋆) is found between AGN hosts and matched non-AGN controls across the full range of black hole accretion rates.
• In the blue galaxy population, H i gas fraction is independent of accretion rate, indicating that accretion is not regulated by large-scale interstellar medium conditions.
• In the red galaxy population, a weak correlation is observed between H i gas fraction and accretion rate, with a 0.5 dex variation in log(M_HI/M⋆) across accretion rate bins.
• The observed 0.5 dex variation in H i fraction in red hosts exceeds the 0.3 dex predicted by stellar mass loss models alone, suggesting a contribution from disk gas.
• Structural properties such as stellar surface mass density and concentration index decrease by a factor of ~3 from high- to low-accretion-rate red AGN hosts, indicating evolving host morphology.
• The lack of gas deficiency in AGN hosts contradicts global feedback models that predict AGN to deplete external gas reservoirs, suggesting such feedback is not dominant in local massive galaxies.