The Bar Fraction in COSMOS by Stellar Mass, Morphology, SSFR, & Environmental Density. Ewan Cameron (ETH-Zurich) & many members of the COSMOS team

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1 The Bar Fraction in COSMOS by Stellar Mass, Morphology, SSFR, & Environmental Density Ewan Cameron (ETH-Zurich) & many members of the COSMOS team

2 Bar formation in a cosmological context Self-gravitating, rotationally-supported stellar disks are inherently prone to growth of various instabilities (Toomre 1964) : Q = κ σr / (3.36 G Σ) < 1 (unstable) One key mode of disk instability : bar formation (Hohl 1971) In a cosmological context : the assembly history of the barred galaxy population should reflect the assembly history of stellar disks prone to bar instabilities

3 Bar formation in a cosmological context However Bar instabilities can be effectively damped by the presence of a massive halo or classical bulge (Athanassoula & Sellwood 1986, Athanassoula 2003) Near fly-bys with neighboring galaxies can serve as external triggers of bar formation (Mihos & Hernquist 1994)

4 Bar formation in observational studies Conflicting measurements of the bar fraction at high redshift : Abraham et al. (1999, HDF), van den Bergh et al. (2000, HDF), and Sheth et al. (2008, COSMOS) all find a decreasing bar fraction to z ~ 1 Jogee et al. (2004, GEMS) & Elmegreen et al. (2004, Tadpole ACS field) find a constant bar fraction to z ~ 1

5 Bar formation in observational studies Conflicting measurements of the dependence of bar fraction on disk type : Marinova et al. (2009, STAGES) and Barazza et al. (2009, EDisCs) find bars more prevalent in bluer, later-type disks Sheth et al. (2008, COSMOS) find bars more prevalent in redder, bulge-dominated disks

6 Bar formation in observational studies Until COSMOS (Sheth et al. 2008), previous studies of barred galaxy evolution focussed on luminosity-limited selections Sheth et al. (2008) : adopt an evolving luminosity limit & reveal a stellar mass dependence of bar fraction within this sample And, of course, we know... galaxy evolutionary histories are tied closely to stellar mass (e.g. Bundy et al. 2005, Peng et al. 2010)

7 A new analysis of bars in COSMOS Our aim : To establish explicitly the dependence of the bar fraction in COSMOS on mass, morphological type, SSFR, and environment for a mass-limited sample Sample selection : 916 galaxies from ACS catalog (Leauthaud et al. 2007) 0.2 < z < 0.6 (ZEBRA photo-zs, Feldmann et al. 2006) log M > 10.5 (ZEBRA+ stellar masses, Oesch et al. ~2010) disk morphological types (ZEST+, Carollo et al. ~2010) e < 0.3 (SExtractor) SSFRs from UV + IR (24 µm) (Le Floc h et al. 2009, Rieke et al. 2009) Environments from large-scale surface density catalog (Scoville et al. 2007)

8 ZEST+ classifications & ellipse-fit bars Early-Type Barred Disks Early-Type Non-Barred Disks Late-Type Barred Disks Late-Type Non-Barred Disks ZEST+ performs SVMbased galaxy classifications using: C, S, M20, Gini, e, n, etc. Features improved algorithms for measuring these indices compared to original ZEST Strong bars identified via popular ellipse-fitting method (e > 0.4) plus visual quality control

9 Test of bar detection limits via sims. Bar detection limits tested via artificial galaxy simulations: Input model B/T Bar/T Re/h n nbar Lbar/H (1) Early-Type (2) Early-Type (3) Early-Type (4) Late-Type GALFIT used to generate models, convolved with COSMOS PSF and inserted into COSMOS background sky image Ellipse-fit automatic detection procedure applied to simulated image

10 Test of bar detection limits via sims. Results for both early-type and late-type models demonstrate that bars are readily detectable in our sample, except at small sizes ~ Re < 0.33 arcsec Therefore, a size cut is also needed to account for evolution in apparent angular size : fix limit to Re > 2.2 kpc (i.e., 0.33 arcsec at z = 0.6)

11 One final ingredient : Low z comparison Local universe comparison sample derived from Nair & Abraham (2010) Equivalent sample selection: 0.02 < z < 0.04 Disk morph. log M > 10.5 e < 0.3 Re-run ellipse-fit selection procedure to ensure sample consistency : some disagreements against vis. catalog Galaxies with conflicting bar classifications

12 Bar Fraction High Mass Int. Mass Results : Cameron et al. (2010) Bars more common in early-type disks than late-type disks at intermediate stellar mass (10.5 < log M/Msun < 11)! Morphology SSFR Environ. Density Redshift

13 Bar Fraction High Mass Int. Mass Results : Cameron et al. (2010) And these is moderate evolution in the bar fraction at intermediate stellar mass! Morphology SSFR Environ. Density Redshift

14 Bar Fraction High Mass Int. Mass Results : Cameron et al. (2010) At high stellar mass (log M/Msun > 11) bars are more common in late-type disks! Morphology SSFR Environ. Density Redshift

15 Bar Fraction High Mass Int. Mass Results : Cameron et al. (2010) And the total bar fraction exhibits null evolution, or perhaps a slight decrease, at high stellar mass! Morphology SSFR Environ. Density Redshift

16 Bar Fraction High Mass Int. Mass Results : Cameron et al. (2010) Finally, there appears to be no dependence of the bar fraction on environmental density (caveat: photo-z dens.) Morphology SSFR Environ. Density Redshift

17 Conclusions : Cameron et al. (2010) The galaxy bar fraction and its evolution is strongly mass dependent! Null evolution of the high mass galaxy bar fraction to z ~ 0.6 is consistent with observed null evolution in size function of large disks (Sargent et al. 2007) and TF of massive disks (Ziegler et al. 2002) to z ~ 1, indicating the Hubble sequence is in place by z ~ 1 for high mass galaxies While the disk population at intermediate masses continues to evolve structurally to the present day

18 Conclusions : Cameron et al. (2010) The galaxy bar fraction and its evolution is strongly mass dependent! Earlier build up in the early-type barred galaxy population relative to the late-type barred population at intermediate mass may reflect earlier formation epoch of early-type discs, i.e., these galaxies have evolved for longer in a dynamical state conducive to bar formation Inversion in relative bar fractions of early-type and latetype disks at high mass may reflect a dependence of earlytype bulge-to-disk ratio, or inner disk velocity dispersion, on mass

19 Future Work : Cameron et al. (2010) In COSMOS : Using Zurich bulge-disk decomposition catalog we could explore the relation between bar fraction and the properties of component bulges and disks : e.g., do massive bulges indeed suppress bar formation? e.g., are bars indeed favoured in higher surface mass disks? In ZENS : Role of group environment in promoting, or delaying, bar formation (coming soon!!) In GAMA : A definitive benchmark of the near-ir barred galaxy population from ~50,000 galaxies at z < 0.1 with VISTA sub-arcsec YJHK imaging

20 The Bar Fraction in COSMOS by Stellar Mass, Morphology, SSFR, & Environmental Density Ewan Cameron (ETH-Zurich) THE END