Exploring the Roles of Galaxy Star Formation and Environment in the Tidal Triggering of Bars

James Garland (he/him)

Advised by Professor Karen Masters (she/her)

Haverford College

Bars in galaxies

• Formed by kinematic instabilities
• Drive gas inflow and quick star formation burst
• Exhaust star forming material, quenching galaxy

NASA/Hubble

(Secular) bar behavior

BUT!

There are few

universal rules

Tidal interactions & bars

• Motivated by Peschken & Łokas (2019) on behavior of bars in Illustris connected to tidal interactions

• Some observations suggest bar suppression in close pairs

Casteels et al. (2013)

Peschken & Łokas (2019)

• Tidal interactions form or strengthen bars on order proportional to scale of interaction

​

Current research

• Bimodal star formation population (secular)
• Quiescent galaxies prefer bars
• pbar is inversely related to gas content
• Overall, star formation is enhanced by tidal interaction (Scudder et al. 2012) and bars tend to quench

How can we combine SF, bar formation, and tidal interaction into one picture?

Cheung et al. (2013)

Data

• GZ2 face-on disk galaxies (p_{features, disk} ≥ 0.430, p_{not edge-on} ≥ 0.715, N_{not edge-on} ≥ 20)
• 0.01 < z < 0.06
• MPA-JHU masses + total and fiber specific SFR (SDSS DR7 spectra)
• At least 1 neighbor within ΔV < 500 km s^-1

16,607 galaxies

Quantifying tidal interaction

• Logarithmic tidal strength parameter, Q
• Quantifies influence of nearby neighbors
• Proportional to separation, galaxy diameter, relative mass

Alonso et al. (2018)

Casteels et al. (2013)

Q

Q

Verley et al. (2007)

Methods

• For each GZ2-classified galaxy:
• Select nearest neighbor on sky
• Check if within ΔV < 500 km s^-1
• Calculate tidal force contribution
• Continue until N_neighbors = 5 or checked out to 10th nearest angular neighbor
• Caveat:
• Galaxies that are entirely secular (i.e. no near-velocity neighbor out to 10th nearest neighbor on sky) are not included in final sample, due to log(0)
• This brings valid GZ2 sample from 19,191 galaxies to 16,607

Population distribution by sSFR

Population distribution by p_bar

A combined perspective

Quenching

Specific Star Formation

Tidal Strength

Bar Likelihood

Bar population at high Q

Bar Likelihood

Tidal Strength

Most SFR

Least SFR

Overall

Star formation behavior

Tidal Strength

Specific Star Formation

Entire Galaxy

Galaxy Center

Strongly barred

Strongly barred

Unbarred/

Weakly Barred

Unbarred/

Weakly Barred

Disk SFR enhancement?

Post-Q=0 sSFR enhancement above unbarred: ~0.5 dex

(~0.4 dex above weak bars)

Post-Q=0 sSFR enhancement above unbarred: ~0.2 dex

(~0.15 dex above weak bars)

What have we learned?

• Passive galaxies disfavor bars in dense regions.
• Environmental density has no global rules for bar formation and destruction, but may change how bars behave.
• Barred galaxies may not quench as effectively at high Q.

Potential avenues

• Measure local gas content; test for the abundance of extragalactic inflow material?
• Bulge morphology (via Sérsic index) as a tracer of tidally-induced vs secular bars?
• Other signatures of tidal interaction
• Q is not a measure of where galaxies may be in an interaction
• Kinematic asymmetry from MaNGA may trace whether a galaxy is pre- or post-interaction (Feng et al. 2020)

We are open to ideas!

Questions?

Feel free to contact me:

jgarland@haverford.edu

Caveats with Q

• Expected SFR enhancement and, in Illustris, bar formation/strengthening is delayed after closest pass
• There may be an intermediate Q range where these effects arise, but it is difficult to distinguish stage of interaction
• Highest Q bins may reflect disruptive mergers where bars are no longer possible

Scudder et al. (2012)