Intergalactic Medium and Lyman-Alpha / Metal Absorbers

Transcription

1 Intergalactic Medium and Lyman-Alpha / Metal Absorbers Image credit: Tom Abel & Ralf Kaehler (Stanford) Ji-hoon Kim (TAPIR)! Slides provided by: Phil Hopkins and Ji-hoon Kim

2 Today s Agenda What are there between the galaxies? How can we know this? How can we utilize QSO spectra to probe IGM? What absorbers? What do they say about Universe s history? Image credit: Abel & Kaehler

3 Quasi-Stellar Object (QSO) 3C 273: QSOs with strange emission lines later identified as extragalactic source (1963), with hydrogen lines receding at 0.158c Image credit: CSIRO

4 Quasi-Stellar Object (QSO) 3C 273: QSOs with strange emission lines later identified as extragalactic source (1963), with hydrogen lines receding at 0.158c Most likely an active galactic nucleus (AGN) at high-z outshining its host galaxy (LQSO > Lsun) class next week Spectra of QSOs look similar at all z (power from radio to γ-rays)

5 Quasi-Stellar Object Spectrum What is going on?

6 Intergalactic Medium Ji-hoon Kim /

7 Intergalactic Medium (IGM)! Essentially, baryons between galaxies! Its density evolution follows the LSS formation, and the potential wells defined by the DM, forming a web of filaments, the cocalled Cosmic Web! An important distinction is that this gas unaffiliated with galaxies samples the low-density regions, which are still in a linear regime! Gas falls into galaxies, where it serves as a replenishment fuel for star formation! Likewise, enriched gas is driven from galaxies through the radiatively and SN powered galactic winds, which chemically enriches the IGM! Chemical evolution of galaxies and IGM thus track each other! Star formation and AGN provide ionizing flux for the IGM! Image credit: Abel & Kaehler

8 Intergalactic Medium (IGM)! Essentially, baryons between galaxies! Its density evolution follows the LSS formation, and the potential wells defined by the DM, forming a web of filaments, the cocalled Cosmic Web! An important distinction is that this gas unaffiliated with galaxies samples the low-density regions, which are still in a linear regime! Gas falls into galaxies, where it serves as a replenishment fuel for star formation! Likewise, enriched gas is driven from galaxies through the radiatively and SN powered galactic winds, which chemically enriches the IGM! Chemical evolution of galaxies and IGM thus track each other! Star formation and AGN provide ionizing flux for the IGM!

9 IGM in Cosmic Energy Budget Fukugita & Peebles (2004)

10 Intergalactic Medium (IGM)! Essentially, baryons between galaxies! Its density evolution follows the LSS formation, and the potential wells defined by the DM, forming a web of filaments, the cocalled Cosmic Web! How can we probe these diffuse regions? An important distinction is that this gas unaffiliated with galaxies samples the low-density regions, which are still in a linear regime! Gas falls into galaxies, where it serves as a replenishment fuel for star formation! Likewise, enriched gas is driven from galaxies through the radiatively and SN powered galactic winds, which chemically enriches the IGM! Chemical evolution of galaxies and IGM thus track each other! Star formation and AGN provide ionizing flux for the IGM!

11 Cosmic Web: Numerical Simulations! Our lines of sight towards some luminous background sources intersect a range of gas densities, condensed clouds, galaxies! Cen & Simcoe (1997) (from R. Cen)!

12 QSO Absorption Line Systems! An alternative to searching for galaxies by their emission properties is to search for them by their absorption! Quasars are very luminous objects and have very blue colours which make them relatively easy to detect at high redshifts! Nowadays, GRB afterglows provide a useful alternative! Note that this has different selection effects than the traditional imaging surveys: not by luminosity or surface brightness, but by the cross section (size) and column density!

13 QSO Absorption Line Systems! An alternative to searching for galaxies by their emission properties is to search for them by their absorption! Quasars are very luminous objects and have very blue colours which make them relatively easy to detect at high redshifts! Nowadays, GRB afterglows provide a useful alternative! Note that this has different selection effects than the traditional imaging surveys: not by luminosity or surface brightness, but by the cross section (size) and column density! (1+zabs)λ0 (1+zQSO)λ0 Schematic of Ly-α absorbers

14 Lyman Series Emission & Absorption Transition between different ionization states of Hydrogen Ly-α Emission Ly-L Absorption

15 Animation: Lyman-Alpha Absorbers Ji-hoon Kim /

16 QSO Spectrum QSO at z=3.12 DLA LAF LLS QSO at z=3.17 Absorption lines (doublet) by C IV, rest wavelength = Å and Å

17 Types of QSO Absorption Lines! Lyman alpha forest:! Numerous, weak lines from low-density hydrogen clouds! Lyman alpha clouds are proto-galactic clouds, with low density, they are not galaxies (but some may be proto-dwarfs)! Lyman Limit Systems (LLS) and Damped Lyman alpha (DLA) absorption lines:! Rare, strong hydrogen absorption, high column densities! Coming from intervening galaxies! An intervening galaxies often produce both metal and damped Lyman alpha absorptions! Helium equivalents are seen in the far UV part of the spectrum! Metal absorption lines! Absorption lines from heavy elements, e.g., C, Si, Mg, Al, Fe! Most are from intervening galaxies! elements heavier than He produced by nuclear burning in stellar cores

18 How to Understand Absorption Profiles Ji-hoon Kim /

19 Fitting the Forest:! Absorption lines contain information about the intervening gas clumps They are not δ-functions! Kirkman & Tytler (1997)

20 Absorption Profile Broadening Broadening function of an absorption line contains two processes Natural Broadening (Lorentzian) by Uncertainty Principle and Collisions Doppler Broadening (Gaussian) by Maxwellian motions in absorbers asymptotes to δ(λ - λ0) when δk 0 and v = 0 Doppler Core Damping Wings Voigt Profile (dashed line)

21 Fitting the Forest:! Voigt profile best fit parameters = (lognhi, b) 1σ error Kirkman & Tytler (1997)

22 Equivalent Width When spectral resolution is too coarse, resort to equivalent width Unresolved Profile Equivalent Width Again, essentially a function of N and b (degenerate) where

23 Curve of Growth Different regimes - linear: - logarithmic: - square-root: logarithmic (flat) regime square-root (damping) regime Curve of growth between W and N (for a fixed b) linear regime Image credit: Churchill

24 Animation: Absorption Profile Ji-hoon Kim /

25 Measuring the Absorbers! We measure equivalent widths of the lines, and in some cases shapes of the line profiles! They are connected to the column densities via curves of growth! The shape of the line profile is also a function of the pressure, which causes a Doppler broadening, and also the global kinematics of the absorbing cloud!

26 Measuring the Absorbers! We measure equivalent widths of the lines, and in some cases shapes of the line profiles! irrespective of b They are connected to the column densities e.g. for Ly-α via curves line (f = 0.416) of growth! Square-root regime - linear regime: The shape of the line profile is also a function of the - square-root pressure, which regime: causes a Doppler broadening, and also the global kinematics of the absorbing cloud! Linear regime LAF DLA

27 Metallic Absorption Lines Metal doublets can be used to locate an absorber on COG e.g. see Galaxies at High Redshift, 2003, ISBN: Transition lines (doublet) by C IV,! λ0 = rest wavelength f = oscillator strength

28 Lyman-Alpha Absorbers: Types Ji-hoon Kim /

29 3 Types of Lyman-Alpha Absorbers QSO at z=3.12 DLA LAF LLS

30 Ly α Absorbers! Ly α Forest: N HI cm -2! Lines are unsaturated! Primordial metalicity < solar! Sizes are > galaxies! Ly Limit Systems (LLS): N HI cm -2! Ly α Lines are saturated! Z ~ 0.01 Zsun LAF DLA N HI is ufficient to absorb all ionising photons shortward of the Ly limit at 912Å in the restframe (i.e., like the UV-drop out or Lyman-break galaxies)! Damped Ly α (DLA) Systems: N HI cm -2! Line heavily saturated! Profile dominated by damped Lorentzian wings! Almost surely proto-disks or their building blocks!

31 Ly α Absorbers! Ly α Forest: N HI cm -2! Lines are unsaturated! Primordial metalicity < solar! Sizes are > galaxies! Ly Limit Systems (LLS): N HI cm -2! Ly α Lines are saturated! N HI is ufficient to absorb all ionising photons shortward of the Ly limit at 912Å in the restframe (i.e., like the UV-drop out or Lyman-break galaxies)! Damped Ly α (DLA) Systems: N HI cm -2! Line heavily saturated! LLS Optically thick to photons having energy above 13.6eV Profile dominated by damped Lorentzian wings! Almost surely proto-disks or their building blocks! U-drop out: visible in B-band but not in U-band LBGs: break at 912 Å, typical of gas-rich galaxies

32 Ly α Absorbers! Ly α Forest: N HI cm -2! Lines are unsaturated! Primordial metalicity < solar! Sizes are > galaxies! DLA Ly Limit Systems (LLS): N HI cm -2! Ly α Lines are saturated! N HI is ufficient to absorb all ionising photons shortward of the Ly limit at 912Å in the restframe (i.e., like the UV-drop out or Lyman-break galaxies)! Damped Ly α (DLA) Systems: N HI cm -2! Line heavily saturated! Comparable to present-day galactic ISM; Z ~ 0.1 Zsun Profile dominated by damped Lorentzian wings! Almost surely proto-disks or their building blocks!

33 Absorber Cross Sections! Steidel (1993) LLS: Likely an extended gas halo of a bright galaxy DLA: Likely a thick (proto-)disk Column density of neutral H is higher at smaller radii, so LLS and DLA absorbers are rare!! Metals are ejected out to galactic coronae, and their column densities and ionization states depend on the radius! LAF: Not necessarily associated with a galaxy

34 Distribution of Column Densities! f (N HI ) ~ N HI -1.7! Ly α Forest! LLS! DLA! Hu et al. (1995), Storrie-Lombardi & Wolfe (2000)

35 Lyman-Alpha Absorbers: Number Evolution in Time Ji-hoon Kim /

36 Evolution of Ly α Absorbers! (from Rauch 1998, ARAA, 36, 267)! Essentially a function of z and σ(z) deceleration parameter q0 = (1/2)Ω0 (NB: this is for Λ = 0 cosmology!)! Typical γ ~ 1.8 (at high z s)!

37 Evolution of Ly α Absorbers! The numbers are higher at higher z s, but it is not yet clear how much of the effect is due to the number density evolution, and how much to a possible cross section evoluton - nor why is there a break at z ~ 1.5! z = 1.5 γ = 1.85 γ = 0.5 Lu et al. (1991), Bechtold (1994), Weymann et al. (1998)

38 More Recent Plot γ = 1.85 Janknecht (2006) γ = 0.5 z = 1.5 Lu et al. (1991), Bechtold (1994), Weymann et al. (1998) γ = 2.47 γ = 0.13

39 Gunn-Peterson Test Ji-hoon Kim /

40 The Gunn-Peterson Effect! Even a slight amount of neutral hydrogen in the early IGM can completely absorb the flux blueward of Lyα! Fan et al. (2006) Thus, if there is a significant amount of hydrogen in IGM, it must be very highly ionized! (from Fan et al. 2006, ARAA, 44, 415)!

41 Clumpy Absorbers at low-z QSO at z=3.12 DLA LAF LLS QSO at z=3.17 Other than the clumps, the rest of the IGM is pre-ionized before the light arrives But is this true at all redshift?

42 QSO Spectra at z > 6 Spectra of the first z > 6 quasars discovered in 2001 (Becker et al.) GP trough Strong Ly-α absorption right blueward of Ly-α emission peak for a z = 6.28 quasar (Gunn-Peterson trough)! suggests that the IGM was not ionized until z ~ 6 Becker et al. (2001)

43 Gunn-Peterson Troughs at z > 6 Universe approaches the end of reionization epoch at z ~ 6 Fan et al. (2003) Fan et al. (2006) GP trough

44 Reionization Completed By z ~ 6 Universe initially ionized after Big Bang became neutral after recombination at z ~ 1100, and the cosmic dark age followed First stars and first galaxies illuminate and reionize" the IGM began at z ~ 15 and completed at z ~ 6 (epoch of reionization) H II regions (bubbles/islands) growing in size Image credit: NASA/ESA

45 Reionization Completed By z ~ 6 Alvarez et al. (2009)

46 Transmitted Lyα Flux vs. Redshift! (from Fan et al. 2006, ARAA, 44, 415)!

47 Metallic Line Absorbers Ji-hoon Kim /

48 The Absorber - Galaxy Connection! Metallic line absorbers are generally believed to be associated with galaxies (after all, stars must have made the metals)! Origin of metals in galaxies (and IGM) ejecta from supernovae explosion An example with multiple metallic line systems:! Steidel et al. (1997) Steidel et al. (1997)

49 Galaxy Counterpart of Metal Absorbers A plausible galaxy counterpart near the QSO s line of sight is identified for nearly every metallic absorption lines Shen et al. (2013), simulation of the growth of metal-enriched bubbles

50 Clustering of Metallic Absorbers! Metallic absorbers! Metallic absorbers are found to cluster in redshift space, even at high z s, while Ly α clouds do not. This further strengthens their association with galaxies! Another evidence that metallic line absorbers are linked to galaxies Ly α clouds! Fernandez-Soto et al. (1996), two-point correlation function of C IV and Ly- α absorbers

51 Metal Abundances of DLAs DLAs always cause metallic absorption, but the reverse is not true DLAs are galactic (proto-)disks surrounded by media of metal absorbers (metal-enriched halo)! DLAs are less metal-rich than MW disk stars, but more so than halo stars Pettini et al. (1997), DLAs + Milky Way stars

52 Metallic Line Absorbers: Number Evolution in Time Ji-hoon Kim /

53 Number Density Evolution of Absorbers! While the H I seems to decline in time (being burned out in stars?), the density of metals seems to be increasing, as one may expect! Storrie-Lombardi & Wolfe (2001), Stengler-Larrea et al. (1995), York et al. (1991)

54 Solar!! Chemical Enrichment Evolution of DLA Systems! (Wolfe et al.)!

55 Numerical Simulations of IGM! DLA systems as the densest knots in the cosmic web!! However, the simulations cannot resolve whether these are rotating (proto)disks!! still largely true after ~20 years (from Katz et al. 1996)!

56 Metal Enrichment of IGM Starbursts can drive metal-enriched galactic winds out to IGM ejected gas may again accrete onto the galaxy Movie credit: Abel, Wise & Kaehler M82 Image credit: Smith, Gallagher & Westmoquette, BVI continuum (HST) + Hα (magenta, WYIN)

57 Metal Enrichment of IGM Starbursts can drive metal-enriched galactic winds out to IGM ejected gas may again accrete onto the galaxy Shen et al. (2013), simulation of the growth of metal-enriched bubbles M82 Image credit: Smith, Gallagher & Westmoquette, BVI continuum (HST) + Hα (magenta, WYIN)

58 IGM Summary! Intergalactic medium (IGM) is the gas associated with the large scale structure, rather than galaxies themselves; e.g., along the still collapsing filaments, thus the cosmic web! However, large column density hydrogen systems, and strong metallic absorbers are always associated with galaxies! It is condensed into clouds, the smallest of which form the Ly α forest! It is ionized by the UV radiation from star forming galaxies and quasars! It is metal-enriched by the galactic winds, which expel the gas already processed through stars; thus, it tracks the chemical evolution of galaxies! Studied through absorption spectra against background continuum sources, e.g., quasars or GRB afterglows! Thank You!

59 [Supplemental Slides] Ji-hoon Kim /

60 The Forest Thickens!

61 Estimating the Evolution of Gas Density! (from Wolfe et al. 2005, ARAA, 43, 861)!

62 Evolution of Neutral Gas!

63 Galaxy Counterparts of DLA Systems! Several examples are known with Lyα line emission! Properties (size, luminosity, SFR) are typical of field galaxies at such redshifts, and consistent with being progenitors of z ~ 0 disks!

64 But different types of systems may be evolving in different ways! Pettini (1999) (from M. Pettini)!