X-rays from disk galaxy halos- testing galaxy formation

Jesper Rasmussen (Univ.

of Copenhagen, Denmark)+ Jesper Sommer-Larsen, Kristian Pedersen (Copenhagen), Sune

Toft (Yale), Richard Bower (Durham, UK), Andy Benson (Oxford, UK)

Theory + sim’s: Disk galaxies should possess hot, gaseous halos.

Cooling halo gas emits X-rays signature of active (gas-)disk assembly.

Cosmological simulations of galaxy formation (Toft, Rasmussen, Sommer-Larsen, Pedersen 2002):

LX Vc5

> 95% of LX inside ~20 kpc

Background

1. Direct searches:

Benson et al. (2000): ROSAT analysis of NGC2841 (D = 13.9 Mpc, Vc = 317 km/s) LX < 3.7 x 1040 erg/s (3σ).

NGC2841 (ROSAT) :

Previous work

2. Other works:

NGC4631 (Wang et al. 2001) – hot halo likely fueled by disk outflows.

NGC891 (Bregman & Pildis 1994) – probably something similar (Strickland et al. 2004)

Milky Way (e.g. Wolfire et al. 1995, Pietz et al. 1998, Kuntz & Snowden 2000) – some evidence for a hot halo, with LX ~ few times 1039 erg/s.

Theory + sim’s: Disk galaxies should possess hot, gaseous halos.

Cooling halo gas emits X-rays signature of active (gas-)disk assembly.

Hot halos not yet observed testing predictions using

(1) More sophisticated simulations + higher resolution

(2) Sensitive X-ray data of carefully selected candidates

Cosmological simulations of galaxy formation (Toft, Rasmussen, Sommer-Larsen, Pedersen 2002):

LX Vc5

> 95% of LX inside ~20 kpc

Background

Sim’s include: star formation, stellar feedback, radiative cooling, UV radiation field, self-consistent chemical evolution.

1: Novel simulations(J. Sommer-Larsen + collaborators)

Temperatures: 104 105 106 K

Densities, log N: -3.5 -3 -2 cm-3

Vc ~ 250 km/s

Temperatures: 104 105 106 K

2: X-ray/optical observationsNGC5170 NGC5746

Vc = 252 km/s, D = 24 Mpc Vc = 307 km/s, D = 29 Mpc

Chandra: 2 x 37 ks

XMM: 33 ks (NGC5170 only) Danish 1.54m: 160/120 min Hα (+UBVRI)

—— 16 arcmin ≈ 140 kpc —— —— 16 arcmin ≈ 100 kpc ——

An example: NGC1800

Extent ≈ 2 kpc,

Lx = 1.3 +/- 0.3 x 1038 erg/s

0.3-5 keV, adaptively smoothed X-ray/optical overlay

Most distant dwarf starburst (D = 7.4 Mpc) with clear detection of diffuse X-rays.

D25 (25

mag/arcsec2)

~ 2kpc

45 ks of Chandra/ACIS exposure (Rasmussen, Stevens, Ponman 2004):

A hot halo around NGC1800?

LX Vc5

No detection in annulus

Lx < 1038 erg/s (3σ) (assuming

T = 0.1 keV, Z = 0.2 Zsun )