[Paper Review] Bars in Starbursts and AGNs -- A Quantitative Reexamination

Lei Hao, Shardha Jogee+3 more|ArXiv.org|Oct 20, 2009

Astronomy and Astrophysical Research1 references3 citations

TL;DR

This study re-evaluates the link between galactic bars and nuclear activity using a large, consistent sample of 2,000 SDSS disk galaxies. It finds that both active galactic nuclei (AGNs) and star-forming galaxies have similar optical bar fractions (47% and 50%, respectively), significantly higher than inactive galaxies (29%), suggesting bars drive gas inflow to fuel central activity but do not directly correlate with AGN triggering beyond this mechanism.

ABSTRACT

Galactic bars are the most important driver of secular evolution in galaxies. They can efficiently drive gas into the central kiloparsec of galaxies, thus feed circumnuclear starbursts, and possibly help to fuel AGN. The connection between bars and AGN activities has been actively debated in the past two decades. Previous work used fairly small samples and often lacked a proper control sample. They reported conflicting results on the correlation between bars and AGN activity. Here we revisit the bar-AGN and bar-starburst connections using the analysis of bars in a large sample of about 2000 SDSS disk galaxies (Barazza, Jogee, & Marinova 2008). We find that AGN and star-forming galaxies have similar optical bar fractions, 47% and 50%, respectively. Both bar fractions are higher than that in inactive galaxies (29%). We discuss the implications of the study on the relationship between host galaxies and their central activities.

Motivation & Objective

• To resolve conflicting prior results on the correlation between galactic bars and AGN activity by using a larger, more consistent sample.
• To address limitations in previous studies, including small sample sizes, inconsistent bar identification methods, and mismatched control samples.
• To investigate whether bars are overrepresented in AGNs compared to inactive or star-forming galaxies using consistent spectral and morphological classifications.
• To examine whether AGN activity weakens or distorts bars, testing theoretical predictions on bar stability.
• To quantify the role of bars in driving gas to the inner kiloparsec, fueling circumnuclear starbursts and potentially AGNs.

Proposed method

• Utilized a sample of 1,144 moderately inclined disk galaxies from the SDSS (0.01 < z < 0.03), selected via blue colors and structural analysis.
• Applied ellipse fitting to identify and characterize bars in optical images, ensuring consistent morphological classification across all galaxies.
• Used SDSS spectra (3'' aperture, 606–1,780 pc) to classify galaxies into spectral types: inactive (EW(Hα) < 3 Å), star-forming, and AGN (spectral classes 4–6).
• Ensured spectral classification consistency by applying uniform line measurement and stellar absorption subtraction techniques from Hao et al. (2005).
• Compared bar fractions across spectral classes, controlling for redshift, inclination, and host galaxy properties.
• Analyzed bar ellipticity as a proxy for bar strength, testing for weakening in AGN hosts.

Experimental results

Research questions

• RQ1 Is there a statistically significant excess of bars in AGN hosts compared to inactive galaxies when using a large, consistent sample?
• RQ2 How does the bar fraction in star-forming galaxies compare to that in AGNs and inactive galaxies?
• RQ3 Does the presence of an AGN lead to observable weakening or morphological distortion of the primary bar?
• RQ4 To what extent do bars drive gas inflow to fuel circumnuclear starbursts and AGN activity?
• RQ5 How do inconsistent spectral classifications and small sample sizes in prior studies affect the reliability of bar-AGN correlation claims?

Key findings

• The optical bar fraction in AGNs is 47%, statistically indistinguishable from the 50% bar fraction in star-forming galaxies.
• The bar fraction in inactive galaxies is significantly lower at 29%, indicating a clear excess in both active and star-forming galaxies.
• There is no evidence that AGN activity weakens the bar, as mean bar ellipticity remains constant across spectral classes.
• The lack of bar weakening supports theoretical models showing that supermassive black holes do not significantly disrupt bars.
• The results suggest that bars efficiently drive gas to the inner kiloparsec, fueling starbursts and potentially enabling AGN fueling, but secondary mechanisms are likely needed for further inward transport.
• The study's consistent methodology and large sample size help resolve prior contradictions in the literature, which were likely due to inconsistent classifications and small samples.