the role of galaxy interactions and stellar bars · (jogee+ 2009, robaina + 2009) ... • visual...

The role of Galaxy Interactions

and Stellar Bars

Preethi Nair STScI

ICTP: Interacting GalaxiesDate: April 3, 2012

Galaxy Interactions and Stellar Bars

Collaborators

• Sara Ellison, Dave Patton, Luc Simard

• Bob Abraham, Sidney van den Bergh

April 3rd, 2012 ICTP: Interacting Galaxies & Binary quasars Preethi Nair

Sept 15th, 2011 Preethi Nair Bologna Seminar

Cosmological or



2 main mechanisms for gas inflow to build bulge.

Galaxy-galaxy mergers(e.g. the mice)

Galaxy bars(e.g. NGC 1300)

Theory vs. Observations for pairs

• Close pair interactions should show enhanced star formation rates.

• depends on pair separation?

• depends on mass ratio? (Cox et al. 2008, Ellison et al. 2008)

• 30% - 200%

• Level of SFR enhancement is the same out to z ~1. (Jogee+ 2009, Robaina + 2009)

• Shallower metallicity gradients. (Rupke et al. 2010, Kewley et al. 2010)

• Central metallicities are lower in close pair galaxies - (0.05 dex to 0.1 dex decrement).

• AGN triggers? (talks by Sara Ellison, John Silverman)

• Bar triggers? (this talk, if I have the time)



Why are stellar bars important?

1. Redistribute angular momentum between baryonic and dark matter components- cusp/core problem

2. Gas inflows - trigger central starbursts (Norman et al. 1987,1988, Ho et al 1997)

NGC 1300

4. Trigger AGN (?) (Norman et al. 1988, Shlosman et al. 1989)- black hole mass vs. bulge mass correlation

5. Metallicity gradients6. Drive spiral arms and rings7. Up to 70% (?) of disk galaxies are barred. Bars are ‘normal’.

PROBLEMS- fractions (?)- timescales (?)- destruction mechanisms? (?)- Found in mature disks (?)- Triggered in unstable disks?- contribution to star formation rate density, AGN fraction or bulge growth(?)

3. Build bulges - (pseudo-bulges, Norman et al. 1996, Athanssoula +, Kormendy & Kennicutt 2004)



Role of bars

• Why is it hard to determine bar fractions?

• Are barred galaxies different from unbarred galaxies?

• Can they be triggered by close pair interactions?



The Role of Secular Features

Barred vs. Unbarred

SDSS DR4: g<16Sloan Digital Sky Survey-Nair & Abraham 2010 sample

- Sample drawn from SDSS Data Release 4 to match high-z galaxies- g < 16 and 0.01 < z < 0.1- ~15,000 galaxies - visually (T-types) and quantitatively (A,C,G,M20,E, etc) classified

Catalog is public.



• Visual catalog of Nair & Abraham 2010.

• 0.02 < z < 0.1

• S/N>5 in [OII], H_beta, [OIII], H_alpha, [NII]

• AGN are excluded.

• 311 barred galaxies and 806 unbarred galaxies.

Ellison et al. 2011



Properties of Barred Galaxies- Star Formation Rates

Ellison et al. 2011

~ No broad star formation rate enhancement.



Properties of Barred Galaxies- Star Formation Rates

Ellison et al. 2011

~ No broad star formation rate enhancement.

Central star formation rate does show enhancement



Properties of Barred Galaxies- Metallicities

Metallicity vs. total stellar mass Metallicity vs. fiber stellar mass

0.06 dex enhancement

- Barred galaxies more metal-rich at all masses.

- Bars sufficiently long lived to show enrichment, even after burst of SF.



Properties of Barred Galaxies- Star Formation Rates & Metallicity

Offset from Metallicity vs. fiber stellar mass

0.06 dex enhancement

pairs

bars


So while there is metallicity enhancement below Log M~10, there is no enhancement in SFR.

So while there is metallicity enhancement below Log M~10, there is no enhancement in SFR.



Quick Detour:Bar Fractions

(SDSS)

Bar Fraction vs. Stellar Mass

9 10 11 120.0

0.1

0.2

0.3

0.4

0.5

Log M

f BAR

AllC!0.29

0.29!C!0.38C"0.38


Nair & Abraham 2010

• Visual catalog of Nair & Abraham 2010.

• Bar Fractions strongly depend on mass

• Bar Fractions are bimodal

• The also depend on the central concentration of a galaxy


- So, on average strong bar fraction in disk galaxies is ~30%.- For all galaxies including ellipticals and no i cut, average bar fraction ~ 15%

Properties of Barred Galaxies- Contributions of bars and CP to SF

>3, I.e. at least 3.5 times more central star formation comes from bars than pairs.

Ratio of bar and pair fraction in galaxy population.

Ratio of bar and pair fraction in star-forming sample.Ratio of SFR enhancements bar and pair star-forming sample (this ratio ~1).

Ratio of enhanced star formation coming from bars and pairs.

Ellison et al. 2011



Properties of Barred Galaxies- Contributions of bars and CP to SF

>3, I.e. at least 3.5 times more central star formation comes from bars than pairs.

Ratio of bar and pair fraction in galaxy population.

Ratio of bar and pair fraction in star-forming sample.Ratio of SFR enhancements bar and pair star-forming sample (this ratio ~1).

Ratio of enhanced star formation coming from bars and pairs.

Ellison et al. 2011

So from SDSS, bars cause the same level of SFR

enhancement as close pairs.Maybe because close pairs

trigger bars which cause the gas inflow.



Back to Bar Fractions

9 10 11 120.0

0.1

0.2

0.3

0.4

0.5

Log M

f BAR

AllC!0.29

0.29!C!0.38C"0.38



-From SDSS, bar fractions are bimodal.

-For the low mass peak, bar fractions at a given stellar mass decreases as concentration increases.

-Bar fractions decrease with increasing stellar mass.

- central concentrations are expected to destroy bars, but only for more massive galaxies

Back to Bar Fractions

9 10 11 120.0

0.1

0.2

0.3

0.4

0.5

Log M

f BAR

AllC!0.29

0.29!C!0.38C"0.38



-For the high mass peak, bar fractions at a given stellar mass increase as concentration increases.

-Bar fractions shows no dependence on stellar mass.

- Bulges may be stabilizing bars.

Bar Fraction vs. Concentration

0.2 0.4 0.6 0.80.0

0.1

0.2

0.3

0.4

Concentration

f BAR

AllLog M!10.3Log M"10.3

Keyed in Mass bins

0.2 0.4 0.6 0.80.0

0.1

0.2

0.3

0.4

Concentration

f BAR

AllSbc ! Sd

SbS0 ! Sab

Keyed in Hubble bins

So central concentration is important to bar fractions. But it depends on the stellar mass and morphology of the galaxy!




Do Close Pairs Trigger Bars?

Talk to me offline.

• Fiber metallicities of barred galaxies are uniformly higher by 0.06 dex.

• Fiber star-formation rates of barred galaxies are higher by 60%.

• Bars account for 3.5 times more triggered central star formation.

• Bar Fraction strongly depend on the mass and morphology of a galaxy

Conclusions from SDSSGalaxy Interactions and Stellar Bars


the role of galaxy interactions and stellar bars · (jogee+ 2009, robaina + 2009) ... • visual...

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