Pharos:distant beacons as cosmological probes

Fabrizio Fiore and V. D’Elia, S. Piranomonte, R. Perna, D. Lazzati, D. Guetta,L. Stella, A. Antonelli, P. Ward and many others

The “Pharos” of Alexandria, oneof the Seven Wonders of the ancient world, was the tallestbuilding on Earth (120m). Its mysterious mirror, which reflection could be seen more than 55 km off-shore fascinated scientists forcenturies.

Gamma ray bursts: one of the great wonders of the Universe• GRBs combine 4 themes of early 21st Century astrophysics:

– Among the most energetic events in the Universe 1997 11997 1stst GRB redshift (thank to BeppoSAX) GRB redshift (thank to BeppoSAX)

– Galaxies in the age of star formation metal abundances, dynamics, gas

ionization, dust– The recombination epoch 2000-200? Gunn-Peterson trough at z~6-?2000-200? Gunn-Peterson trough at z~6-?– The fate of the baryons & large scale structure (See Nicastro et al. poster) 1999 Warm IGM simulations1999 Warm IGM simulations 2001 12001 1stst Warm IGM detection (thank to Chandra) Warm IGM detection (thank to Chandra)

Minutes after the GRB event their afterglows are the brightest sources in the sky at cosmological redshift.

Afterglows can be used to probe the high redshift Universe :

Can we use GRB to track star-formation at high-z? through: • High resolution spectroscopy of UV lines

• Statistical studies

GRB01022210 Crab!

Crab

1mCrab i.e. a bright AGN

Galaxies in the Age of Star Formation

Mann et al. 2002 MNRAS, 332, 549

• Star formation in the Universe peaked at z~2

• Studies of z=>1-2 galaxies are biased against dusty environments.

• GRB hosts are normal galaxies

• GRB afterglows will reveal host

Galaxy dynamics, abundances, & dust content at z>1

redshift, (1+z)

sta

r fo

rmat

ion

rate

GRB Hosts

peak of star formation

GRBs also probe normal high z galaxies

The history of the metal enrichment in the Universe

Savaglio 2003

Prochaska et al. 2003

Spectroscopy of UV lines

Star-formation regions (and the interstellar matter in general) are complex. Can we truly learn anything about star-forming regions from afterglow’s spectroscopy? Or, are we just doing meteorology?Just like to try to understand the physics of the atmosphere from the observation of one (a few) lightning …

Statistical studies

Porciani & Madau 2001, Schmidt 2001,Guetta et al. 2004, Jakobsson et al. 2005and many others.

But…. Too few z, 30-50% only of well selected GRB samples. Selection effects complex anddifficult to account for.

Redshift distributions

30 Swift GRB with spectroscopic redshift 27 BeppoSAX and HETE2 GRB with spectroscopic redshift

Peak flux distributions

NH distributions

118 Swift and 44 BeppoSAX+HETE2 GRBs localized in regions with NH(Gal)<41021

Due to the different Eband of localization? (BAT 15-150keV, WFC and WXC 2-20 keV)

Selection effectfor different localization bands

Z=9 5.3 3 1 .5 0 NH=1024cm-2

€

NH (z = 0) ≈ NH (z)(1+ z)−2.5

AV ≈NH

a few ×1021cm−2X-ray localization: biased against large z=0 obscuration==> BSAX and HETE2 samples miss highly obscured, low-z GRB.

==> Swift localizes highly obscured, low-z GRB, but dust extinction makes their optical afterglow faint, and so more difficult z determinations

Selection effects

Redshift

1) Sensitivity: BSAX and HETE2 samples biased against X-ray faint GRB2) Localization Eband: BSAX and HETE2 samples biased against highly

obscured GRB (preferably at low z)3) Spectroscopic identification: dust in the host galaxy plays a major role

Spectroscopy of UV lines

1- The GRB surrounding medium: its physical and dynamical state can be easily modified by the GRB and its afterglow. Geometry, density and physical state of the gas can be constrained by line variability and comparison of line ratios to expectation of time dependent photoionization codes.

2- The ISM of the host galaxy. Metal column densities, gas ionization and kinematics.These studies have so far relied upon either Lyman Break Galaxies or Damped Lyman Alpha systems, which may not truly be representative of the whole high-z galaxy population.

GRB afterglows can provide new, independent tools to study high z galaxies.

DATA

UVES spectra 3200-9400 A, slit 1”, resolution=42,000FORS spectra 3800-9500 A, slit 1”, resolution=1000

GRB020813: z=1.245 - 24 hours after the GRB; R=20.4

GRB021004: z=2.328 - 12 hours after the GRB; R=18.6

GRB050730: z=3.968 - 4hr after the GRB; R18

GRB050922C: z=2.199 – 3.5hr after the GRB; R18

The ISM probed by GRB afterglow

Complex, with many components spanning a velocity ranges from a few hundred to a few thousands km/s

GRB0500922C UVES spectrum

Separating different components Piranomonte et al. 2006

GRB050730 UVES spectrum

Strong fine structure transition: CII*, SiII*, OI*, OI**, FeII*, FeII**, FeII***. Prochaska et al. 2006: Observed abundances of excited ions are well explained by UV pumping with the gas at r~a few hundred pc from the GRB

GRB050730: a slightly different approach:Separating different components

D’Elia et al. 2006

GRB050730

Strong CII* for both components 2 and 3

3 2 1

GRB050730

Strong SiII*,OI* and OI** only for component 2

3 2

4 3 2

GRB050730

4 3 2

Strong FeII and FeII* transitions only for component 2

Fine structure lines

Two main processes: 1) Radiative excitation; 2) Collisional exitation

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GRB050730 Fine structure linescomponent 2

T=2650+3000-1000 KNe>a few 100Assuming collisional excitation

NCII/NCII*~0.5

GRB050730 fine structure linesComponent 3

NCII/NCII*~1.8, higher than that of Component 2!

Ne = a few 10Assuming collisional excitation

GRB050730 gas ionization and distance of the clouds from the GRBHuge (and variable) ionizing flux! CI<12.3

[CIV/CII] and [SiIV/SiII] of components 2 and 3 similar. If density is different by a factor 10 (100), the distance from the GRB of component 3 is 3 (10) times higher than that of component 2.Similar conclusion reached assuming UV pumping as dominant mechanism for excited transitions

Need detailed time-dependent photoionization codes!

Time dependent photoionization Nicastro et al.1999 Perna, Lazzati et al.

• Strong variation with time of ion abundances

• A tool to constrain gas geometry and density through ion ratio variations

Lazzati et al. 2006

Metallicities

In case of multiple components it is truly difficult to estimate H columns for each, even with high quality spectroscopy. The spatial distribution of heavy elements can be very different from that of H.

The star-forming regions hosting the GRB are likely to be much more enriched than the outer galaxy regions

GRB host galaxy metallicities

050730

030323

000926

050820

050401

060206

050904

GRB host galaxy metallicities

However… metallicity depends on: 1)Impact factor2)Galaxy mass3)Star-formation rate4)Etc….

1) Metallicity depend on impact factor

galaxy

DLA

GRB021004

2) Metallicity depends on galaxy mass:

Savaglio et al. 2005,2006 Berger et al. 2006

050904, abs.

Host em. lines

Statistical population studies

… should consider

• Complex selection effects, different localization bands and NH distributions should be taken into account

• SFR / metallicities / masses appropriate for the typical GRB hosts