Massive and Dark System: Mass Distributions in NGC 1407 and Its Associated Group

1. A Massive and Dark System: Mass Distributions in NGC 1407 and Its Associated Group Zhongli Zhang (张仲莉)1, Haiguang Xu (徐海光)1,2, Yu Wang (王宇)1, Tao An (安涛)3, Yueheng Xu (许玥恒)4 & Xiang-Ping Wu (武向平)2 • Department of Astrophysics, Shanghai Jiaotong University • National Astronomical Observatories, Chinese Academy of Sciences • Shanghai Astronomical Observatory, Chinese Academy of Sciences • Department of Physics and Astronomy, University of Leicester

2. Introduction • X-ray observations of NGC 1407 • Total gravitating mass • Stellar mass analyses • Dark matter distribution • Discussion

3. X-ray observations Z = 0.0059 (26.8 Mpc) (a) Chandra ~50 ks observation ROSAT ~21 ks observation • Resolving 41 X-ray point sources with ~2Re. Most of them are LMXBs. • 18% resolved, 21% unresolved point sources of the total emission (Zhang et al. 2004). • X-ray gas halo extending to ~12'. • Diffuse X-ray structure aligned in 1400-1407 direction (possible dwarf galaxy; Trentham et al, 2006).

4. Temperature and surface brightness profiles • Combining Chandra and ROSAT sbp: • Two-beta model give acceptable fit: • 1= 0.70 ± 0.01 rc1= 0.14 ± 0.01 • 2= 0.45 ± 0.01 rc2= 0.97± 0.01 • The central excess of the galaxy, • corresponding to ne = 0.1 cm-3

5. Total gravitating mass • In galactic scale: The total mass: • 2.75±0.51×1011atRe • The mass-to-light radio • ~ 5-10 within 1' • (NGC 1600; NGC 720) • In group scale: • The total mass: • 6.64±1.20×1012 at 10Re • The mass-to-light radio • 90 at 12'(gas halo boundary) • (2-6 times of other galaxies, • Humphrey et al. 2006) • Modeling: c > 68% scatter for the halo at z = 0 for the given Mvir(Jing 2000) Slop change Rs = 4.0'±0.5' c=18.6±1.5

6. Stellar mass •Optical surface brightness (B-band; de Carvalho et al. 1991) •B-band stellar mass-to-light ratio Line indices (Rampazzo et al. 2005) PEGASA-HR code (Fioc Rocca-Volmerange 1997) Assuming: 1. Line-strength indices and color gradients are due to the metallicity gradients; 2. The star formation rate is decreasing exponentially; 3. Adopting Kroupa IMF. Fitting parameters: b-r, b-k(2mass), Hβ, MgFe (Thomas et al. 2003 or Cappellari et al. 2006) Thomas and Maraston models (Thomas et al. 2003; Maraston 2005)Steps: Thomas Hβ, Mgb, Fe5270, Fe5335, Fe5015 Maraston Age, [α/Fe], metallicity MStellar/LB

7. Average Mstellar/LB for Kroupa IMF( PEGASE code, [MgFe]T and [MgFe]C ): 0-1/16 Re: 5.4±0.7 1/16-1/8 Re: 5.3±0.7 1/8-1/4 Re: 5.2±0.9 1/4-1/2 Re: 4.5±0.8 > 1/2 Re: 4.5±0.8 (2MASS H-K and J-K colors keep nearly unchanged in 0.5-2 Re) Stellar mass profile <20% errors Errors: Model uncertainties on the IMF profile ~10% Systematic and stellar spectrum library of Pegase code ~10% Star formation history modeling ~5% Photometric data and line indices <10%

8. Dark matter distribution (c) Slop change • Central dark matter excess • Dark matter begins to dominate the • galaxy outside 1'. • Cuspy central total mass and dark • matter distribution. • Power-law profile with indices of • 2~3, while the dark matter is steeper.

9. Discussion The slope change on the total mass distribution Other examples Clusters: Fornax, Centaurus, A1795 (Xu et al. 1998; Ettori et al. 2002; Makishima et al. 2001) Galaxies with the evidence: NGC 4472 NGC 4649 NGC 507 NGC 1600 (Brighenti & Mathews 1997) (Paolillo et al. 2003) (Sivakoff et al. 2004) IC 1459 NGC 1399 NGC 4125 NGC 4261 (Fukazawa et al. 2006) (Humphrey et al. 2006)

10. Galaxies without the evidence: • Explanation • For galaxies with the evidence, half of them the temperature is • increasing outwards (NGC 1600, IC 1459, NGC1399 & NGC 4261). • For the other half, obvious two-beta gas densities were found • (NGC 4472, NGC 4649 & NGC 507). • The slope change is always seen in the cluster-group dominating • galaxies, or galaxies dominating a sub-cluster, which indicates that • the halo collapse has not enter the final stage. NGC 1404 NGC 4555 NGC 4636 NGC 6482 (Paolillo et al. 2002) (O’Sullivan et al. 2004) (Brighenti et al. 1997) (Humphrey et al. 2006)

11. Summary We study the mass distributions in NGC 1407 and its associated group fromChandraandROSATX-ray observation,opticalline strength indices andmulti-colorphotometric data. NGC 1407 group isextremelydark systemeven comparable to many clusters of galaxies. Total mass and dark matter in the outer regions can be modeled by the NFW model. While the central distributions are cuspy. The total mass and dark matter profiles both show aslope changeat ~1', which may be a mark of the boundary between galaxy and group halos.