Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.atnf.csiro.au/research/cena/documents/presentations/rejkuba.pdf Äàòà èçìåíåíèÿ: Thu Jul 16 04:37:07 2009 Äàòà èíäåêñèðîâàíèÿ: Fri Nov 27 08:35:45 2009 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: m 43

The stellar halo of NGC 5128
Marina Rejkuba
European Southern Observatory

ESO/MPIA 2.2m+WFI Made by: B. Vandame


Galaxy halos
· Historical record containing the oldest and the most metalpoor stars ­ how galaxies assembled their mass ­ early phases of galaxy formation Do all galaxies have extended halos? What is the metallicity distribution of halo stars? How old are halo stars?

Ibata et al. 2007


Galaxy formation scenarios
· Early monolithic collapse scenario: ­ assembly in a dissipative gaseous collapse, either from a unique cloud or many gaseous clumps, but not out of preexisting stars (e.g. Eggen, Lynden-Bell & Sandage 1962) Expect red HB · Hierarchical merging scenario: ­ successive non-dissipative mergers of smaller systems over an extended time likely forming from pre-existing disk galaxies' stars (Toomre 1977, Kauffmann+93) Expect blue (extended) HB

Ikuta 2007


N GC 5 1 2 8
· Nearest easily observable gE/S0
­ 3.8+/-0.4 Mpc (Rejkuba 2004) ­ 3.4+/-0.2+/-0.3 Mpc (Ferrarese+07)

· Recent merger · Radio galaxy - but not unusual (Struve) · AGN with central supermassive BH - not unusual (Neumayer) · "a normal early type galaxy" (Harris, Oosterloo)


Simple stellar population
Asymptotic giant branch (AGB) Horizontal He-shell burning branch (HB) He-core burning Red giant branch H-shell burning Turn-off H-core exhaustion White Dwarf (WD) cooling sequence
(RGB)

Tool: Stellar populations in CMDs

Composite stellar population

Main sequence (MS) H-core burning

MAGNITUDE Luminosity

Template globular cluster (Harris 2000)

m s as

COLORT

eff

Template galaxy (Aparicio 1998)


Tool: simulated CMDs
Composite stellar population Random extraction of: ­ mass(IMF) ­ age(SFR) ­ metallicity(Z(t)) · Interpolation of the extracted star (m,t,Z) on the HRD (log L, log Teff) · Determination and application of bolometric corrections: BC(T, g, Z) · Application of observational uncertainties: ­ photometric error ­ incompleteness
Aparicio & Gallart 2004


Resolving stars in NGC 5128
· Soria et al. 1996 (WFPC2) - RGB tip and distance · Harris, Harris & Poole 1999; Harris & Harris 2000, 2002 (WFPC2) - metallicity distribution in the halo · Mould et al. 2000 (WFPC2) - recent star formation in the halo; Fassett & Graham 2000, Rejkuba et al. 2001, 2002 (VLT, Magellan), Graham & Fasset 2002 (du Pont 2.5m) · Marleau et al. 2000 (NICMOS) - the first view of the stellar populations of Cen A in near-IR · Rejkuba et al. 2003 (VLT, ISAAC) - RGB and AGB stars in nearIR and the first variable star study in Cen A · Rejkuba et al. 2005 (ACS) - Red Clump, AGB bump and the first handle on the age of the old stellar population · Ferrarese et al. 2007 (WFPC2) - Cepheids



NGC 5128 = Centaurus A

Field 0 (center) VLT+FORS2 BVI

Field 1 (shell) VLT+FORS1 U+V Box: VLT+ISAAC

DSS 25'x25'

Field 3 (jet) MagI+CCD U+V+I

Field 2 (halo) VLT+FORS1 U+V Box: VLT+ISAAC


Recent star formation in the halo

Rejkuba et al. 2001, 2002, Fassett & Graham 2000, Graham & Fassett 2002


Recent star formation in the halo
Mould+00

Rejkuba, Greggio & Zoccali 2004
· Star formation still on-going or at most stopped ~2Myr ago
· · · ·

Oldest stars observed ~100 Myr, but there could be some older stars! IMF slope: 2< <2.6 (Salpeter =2.35) low SFR ~0.007MH /yr, low metallicity Z=0.004-0.008 Total mass formed in stars over the last 100 Myr ~1x106 M


Recent star formation in the halo
Schiminovich+94
QuickTime and a BMP decompressor are needed to see this picture.

285
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285

QuickTime and a BMP decompressor are needed to see this picture.

=285± 54 km/s

Brogaard, Nikolov, Hunter, Miralles, Rejkuba


Could jet trigger this star formation?!

Kraft+09

Oosterlo+Morganti 05


"Normal" halo CMDs
Soria et al. 1996 38 kpc Field Rejkuba et al. 2005

Harris & Harris 2002 21 kpc field Harris et al. 1999

Harris & Harris 2000


Intermediate-age component?
Marleau+00 Rejkuba+01 Rejkuba+03

­Presence of extended giant branch:
· · ·

intermediate-age AGB metal-rich & old blends?


K

0.0

0.5

1.0 phase ( )

1.5

0.0

0.5

1.0 phase ( )

1.5

2.0

Rejkuba+03

Mira long period variable stars period distribution - comparison of the old and intermediate-age populations


RGB color Metallicity distribution

= [m/H] HST ACS Cycle 11, 24 orbits total: 12 V (F606W) +12 I (F814W) 8.6 h per filter
Rejkuba et al. 2005


Getting a handle on age

AGB bump (AGBb) Red clump (RC)


= n(Z) * Mag(Z,age) * dZ

Average age of halo stars in NGC 5128 ~8±3 Gyr
(Rejkuba et al. 2005)


Finding the best fit model CMD
Random extraction of a mass, age, metallicity Interpolation of the extracted star (m,t,Z) on the theoretical HR diagram (log L, log Teff) using -enhanced and solar scaled stellar evolutionary models (Pietrinferni+04) application of bolometric corrections: BC(T, g, Z) Application of observational uncertainties: · photometric error · incompleteness



2

Comparison of V, and I luminosity functions, and full CMD


A sanity check
· Comparison model vs. model: input 10 Gyr old single age ("single starburst") simulation vs. · all single burst simulations have input observed MDF ­ Most sensitive diagnostic is the full CMD fit

=
2


i

(N

obs,i

-N N obs,i

s im , i

)

2


The best fitting single age model
· Best fitting age: 10-12 Gyr · Solar scaled models produce worse 2, larger discrepancy between diagnostics and lower sensitivity to age


Two bursts
Combining single age simulations in different proportions: 80% 12 Gyr + 20% 3 Gyr Much improved LF(I) fit - see the width of the red clump

· Best fits for: 70-80% old 12-12.5 Gyr + 20-30% intermediate-age 3-4 Gyr · No improvement by combining old -enhanced with younger solar scaled simulations


Complex star formation history


S um ma r y
· MDF is broad and has very minor metal-poor component · He burning stars are concentrated in a red clump (red HB) · Metal-poor halo, similar to the MW halo, is not detected up to ~7Reff, and halo metallicity gradient is shallow · Bulk of the stars in NGC 5128 are old: ~70-80% of stars 11-12.5 Gyr · Single age old population models do not fit well the observations · 20-30% intermediate-age population 3-4 Gyr old stars in the halo · Young stars in the halo confined to star forming regions associated with optical ionized gas filaments along the NE jet


Ks IMAGES
VLT+ISAAC, Ks 2400-3600 s/epoch One epoch shown

VLT+FORS1 U+V, 1800s

HST/NIC3 F222M, 256s


Luminosity function AGB bump
NGC 5128 luminosity function

I

Gallart `98

AGB bump ­ as the central He burning finishes, the core contracts rapidly, until He burning shell is ignited. The ensuing violent expansion pushes the star temporarily out of thermal equilibrium and to brighter luminosities. When the He shell is fully ignited, thermal equilibrium is restored and the evolution proceeds on a nuclear timescale bump in the LF