Transcript 슬라이드 1
KASI Galaxy Evolution Journal Club A Massive Protocluster of Galaxies at a Redshift of z ~ 5.3 - P. L. Capak et al. 2011, Nature, in press (arXive:1101.3586) - January 27, 2011 Joon Hyeop Lee Cosmological simulation progenitors of present-day galaxy clusters: - Mhalo>2x1011M⊙, Mstars>4x109M⊙ at z ~ 6 - Local overdensities of massive galaxies on comoving scale of 2-8 Mpc (coherently extend over tens of Mpc) - Owing to high mass densities and high merger rate, extreme phenomena (starbursts, quasars) should preferentially exist. Springel et al. (2005) Nature, 435, 629 Previous observations - Overdensities around radio galaxies on ~ 10-20 Mpc scales at z>5 - Large gas masses around quasars at z>5 - The available data are not comprehensive enough to constrain the mass of those protoclusters (and thus cosmological models) Overzier et al. (2009) ApJ, 704, 548 Walter et al. (2003) Nature, 424, 406 COSMOS + Keck II spectroscopy COSMOS (Cosmological Evolution Survey): Search for starbursts, quasars and massive galaxies as signposts of potential overdensities at high-z - Optical + NIR: optically bright objects at z>4 - mm + radio: extreme star formation activity - X-ray: potential luminous quasars Keck II + DEIMOS spectroscopy: measuring redshifts. A Grouping of 4 major objects at z = 5.30 A Grouping of 4 major objects at z = 5.30 COSMOS AzTEC-3: extreme starburst galaxy (Riechers et al. 2010, ApJ, 720, L131) - molecular gas >5.3x1010M⊙, dynamical mass (+ DM) >1.4x1011M⊙ Overdensity near AzTEC-3: - L60-120um ~ (1.7±0.8)x1013L⊙ → SFR > 1500M⊙ /yr : 100 times the rate of an average L* galaxy at z = 5.3 ... from empirical estimates based on: sub-mm flux, radio flux, CO luminosity + model fitting - 2.2x1013L⊙ < L8-1000um < 11x1013L⊙ ... Large uncertainty: many assumptions in the models, lack of data constraining 140um emission ... Sub-mm spectral slope favors models with colder dust (lower L) Significance of the overdensity In the 1’x1’ (2.3x2.3 Mpc2 at z = 5.3) area around AzTEC-3: - Expect to find 0.75±0.04 bright (z850<26) LBGs (z=5.3) - Actually find 8 LBGs (11-fold overdensity) Bouwens et al. (2007) ApJ, 670, 928 Overdensity Within 2 Mpc radius of AzTEC-3, 11 objects > L* → 11-fold overdensity → >9-σ significance, even if only the spectroscopically confirmed systems are considered. 3 of 11 objects are within properdistance of 12.2 kpc (2”) of AzTEC-3 +2 within 95 kpc (15.5”) X-ray selected Quasar - X-ray selected z>5 quasars are extremely rare (detection limit) → One is found within 13 Mpc of AzTEC-3. → co-moving distance scale expected for protoclusters at z~5 - Optical spectrum: deep, blueshifted gas absorption features → strong winds driven by the energy dissipated from the rapid black-hole growth 11L , L 11 - - L ~ 1.9x10 X ⊙ bolometric > 8.3x10 L⊙ (from SED) → LBH > 3x107L⊙ (supposing accretion at the Eddington rate) LBH > 3x108L⊙ (typical accretion of 1/10 Eddington rate) - Final BH – stellar mass relation: M BH ~ 0.002Mstar → This object will eventually have a stellar mass > 1010 - 1011M⊙ ~ to be placed the most luminous and massive objects at this redshift Stellar mass - Stellar population model (single recent burst of SF) fitting to the rest-frame UVoptical SED - [OII] and Ha emission lines were added to the templates - Insufficient to fully break the age-dust degeneracy - However, the age 10 Myr is consistent with the Keck spectra features. → the resulting stellar mass is probably accurate to a factor of ~ 2 (Conservative) Stellar mass estimates AzTEC-3 10 - M star ~ (1-2)x10 M⊙ → The baryonic matter is >70% gas. → gas nearly twice that found in typical starburst systems 11 objects in the protocluster core 10 9 9 - M star, total > 2x10 M⊙ , individually 0.06x10 M⊙ < Mstar < 10x10 M⊙ - Lower limit on the total mass of this system (assuming a global DM/baryon ratio of 5.9; Larson et al. 2010, ApJS, in press): Mhalo, total > 4x1011M⊙ , with AzTEC-3 in a halo of mass > 1011M⊙ → comparable to the halo masses predicted for galaxies that will eventually merge into present-day galaxy clusters (Walter et al. 2010, Nature, 424, 406) - Actuall mass is probably much higher, because much of the baryonic mass is probably in ubobserved hydrogen gas, and the starburst object alone accounts for >37% of the total mass. - The contribution of significantly more numerous, fainter (<L ) galaxies * are not counted in this mass estimate. 3 objects around AzTEC-3 Probably represent the progenitor of a massive central cluster galaxy (type cD) at lower redshift. - Already within the radius of a typical local cD galaxy - Dynamical timescale ~ 60 Myr (assuming σ ~ 200km/s) - Even for the objects at 95kpc, the dynamical timescale < 0.5 Gyr However, the observed stellar mass in these galaxies is significantly less than ~ 1011M⊙ - 1012M⊙ in a typical local cD galaxy. → The majority of the stars have yet to form. Comparison with simulations Protocluster - in qualitative and quantitative agreement : Spatial extent, specific SFR, gas properties, and so on. Quasar - In agreement with with the models of the later phases of massivegalaxy formation when the quasar becomes visible. Unlike for previously described overdensities at z>5: - Strong spectroscopic and photometric evidence for a range of objects including massive, heavily star forming and active galaxies. - The effect of environment on galaxy formation as early as z~5 can be studied. Conclusion: This region contains a large-scale baryonic overdensity in the very early Universe that will evolve into a high-mass cluster like those observed at lower redshifts.