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Дата изменения: Thu Jun 26 05:00:50 2008
Дата индексирования: Sat Sep 6 16:03:42 2008
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Поисковые слова: merging galaxies
Mergers of Luminous Early-type Galaxies in the Local Universe and Implication for Gravitational Wave Background
Zhonglue Wen (NAO Fengshan Liu Jinlin Han
China),

(Shenyang Normal University, China), (NAO China)

"merging of black holes in galaxies " conference, Australia, 17/06/2008


Why merger of galaxies is interesting




Star formation in galaxies (Kennicutt 1998), morphological transformation (Barnes 1990; Schweizer 1982; Kormendy & Sanders 1992), growth of massive early-type galaxies (Toomre & Toomre 1972, Barnes & Hernquist 1996, Khochfar & Burkert 2003; Bell et al. 2006; Li et al. 2008a), AGN activity (Kauffmann & Haehnelt 2000; Cattaneo 2001; Urrutia, Lacy, Becker 2008; Li et al. 2008b) Growth of suppermassive black hole (SMBH), formation of black hole binary and gravitational wave (GW) (Phinney 2001; Jaffe & Backer 2003; Wyithe & Lobe 2003; Enoki et al.2004; Sesana et al. 2004)


Outline




Brief review on merger rate of galaxies Mergers of luminous early-type galaxies in the Sloan Digital Sky Survey Implication for SMBHs binaries and GW background


Review on pair fraction and merger rate of galaxies






Strongly disturbed galaxies (Le Fevre 2000; Conselice et al. 2003; Lavery et al. 2004; Bundy et al. 2004; van Dokkum 2005; Kampczyk et al. 2007; De Propris et al. 2007; Lotz et al. 2008) Close pair statistics (velocity difference , projected separation ) (Patton et al. 2000,2002; Lin et al 2004; De Propris et al. 2005; Bell et al. 2006a; Kartaltepe etal 2007; Lin et al. 2008) Two-point correlation function at small scale (Masjedi et al. 2006; Bell et al.2006b)


Review on pairs and mergers of galaxies


pair fraction 0.01-0.1 (z < 1) Pair fraction increases with redshift for global mergers (Patton et al. 2000,2002; Carlberg et al. 2000; Lin et al. 2004; Kartaltepe et al. 2007) Lin et al. (2008) found that pair fraction increases with redshift for late-type mergers, while decreases with redshift for early-type mergers






Recent work on ongoing mergers of massive early-type galaxies (Bell et al. 2006a found 6 mergers; Rine et al. 2007 found 1 mergers; McIntosh et al. 2007 found 38 mergers) The merger rate of massive galaxies can imply that how fast the more massive galaxies are generated, and how many massive galaxies could host massive SMBH binaries? We aim to construct a large and volume-limited complete massive early-type galaxy mergers!




Mergers of galaxies in the Sloan Digital Sky Survey (SDSS)
SDSS has provided photometric data in five bands (u,g,r,I,z) for more than 9000 square degree, and more than 790,000 redshifts of galaxies (90% complete for galaxies of r < 17.7).


SDSS DR6

It provides an opportunity to extract a complete sample of galaxy mergers.



Mergers of galaxies in SDSS


Sample selection


redshift z < 0.12 5. Projected separation 7 kpc < r_p < 50 kpc


How many are ongoing merging systems?


Merging identification: GALFIT (Peng 2002)

Raw


Model

Residual

We model the targeted paired galaxies by Sersic function (1968)


Merging identification: asymmetry factor
Illustration of asymmetry In residual image, the area of each galaxy is determined with Sextractor (Bertin & Arounts 1996) Asymmetry of each galaxy

m_residual <20.5

Ongoing merging

Non-merging

Distribution of calibration sample


Pair fraction and Merger rate





There are 314 pairs classified as merging systems Fraction of galaxies in ongoing merging systems f=0.01 Merger rate of massive galaxies in local universe (assume T_mg ~ 0.3 Gyr)


What implication for SMBH binaries and GW background


Masses of SMBHs in binaries GW background


From merging of galaxies to merging of SMBHs


Early-type galaxies host SMBHs (Kormendy & Richstone 1995; Magorrian et al. 1998; Merritt & Ferrarese 2001) From luminosity of galaxy to masses of SMBH (Tundo et al. 2007)





Process of SMBHs merging (Begelman et al. 1980; Yu 2002)




SMBH mass function in ongoing merging pairs




mass ratios of SMBHs in ongoing merging pairs


Gravitational wave background


If SMBHs merger follows galaxy merger, there are a lot of SMBH binaries emitting GW Merger rate




Strain of GW background

m=2



m=-1

Constraints on merger rate evolution index, m, m < 5.3 for current limit; m < -3.4 for future limit of PPTA




Compare with upper limit merger rate of SMBH from detection of individual GW source by pulsar timing


Summary








We construct a large and volume ­limited complete luminous (M_r<-21.5) early-type galaxy pairs (1209) in the local universe (z<0.12) from SDSS early-type galaxies We identify ~300 ongoing merging systems by image analysis, and determined the pair fraction in ongoing merging pairs (0.01), merger rate of luminous early-type galaxies in the local universe Implication for SMBH binary: distribution of mass ratios in binaries is a power law Implication for GW background: