Ch.5

absorption lines

*QSO

zemzabs<zem

they allow to use quasars as cosmological probes to study the Universe at large distances and large look-back times

absorption line system:system of absorption lines at the same zabs, presumably associated with the same absorber

absorption lines

study of great potential interest to investigate the gas distribution in the Universe

difficulties:* weak and unresolved lines need high spectral resolution, and high S/N ratio for weak sources need big telescopes* study must be done as a function of redshift need wide spectral rangeevery given QSO can include between 0 and hundreds of absorption lines in his spectrum, depending on(1) redshift zem (2) observed spectral region(3) limit EW (function of spectral resolution and S/N ratio)

most common lines: Lyalpha 1216, CIV 1548,1551, MgII 2795,2802other common lines: CII 1335, Si IV1394,1403, MgI 2852

absorption lines

“metal” line system

“damped” Lyα

large column density, probably due to the cross of a galactic disk

absorption lines

Lyα systemsLy limit system

Broad Absorption Lines and BAL QSOs

BAL QSOs are:•X-ray-weak, lower than for non-BAL•usually radio quiet

broad (104 km/s)

profiles PCygni-like, absorptions shifted by ~ 30000 km/s

probably associated with outflows from nuclear region

Gibson et al 2008

intrinsic to the quasar, not

intergalactic !

BAL QSOs

model by Elvis (2000)

~15-20% of radio-quiet AGNs

outflows are evolutionary phenomenon, independent on orientationoutflows are present in every quasar, but cover only ~20% of solid angle

two classes of models

statistic of absorption lines

number of absorbers crossed per unit path length

proper length

number of absorption lines per unit redshift

cross section of the absorbers

density of the absorbers:constant comoving densityn(z)=n0(1+z)3it is assumed that

comoving density andcross section areboth constant

more generally, dependence on z is assumed parametrically

q0=0

q0=1/2implies evolution

for the non evolutionary case, no, σo const, we have:for Λ≠0 change to:

risultati statistici

clear evidence of evolution

possibly, lower metallicity at high z

effects near the QSO

number of absorbers can increase for zabs~zem because some absorbers could be physically related to the QSO

viceversa in many cases (e.g. Lyα) number of absorption lines decreases due to the higher ionization level (proximity effect or inverse effect)to remove the effect, absorption systems within an appropriate velocity interval from QSO are excludedin the QSO rest-frame, a cloud moving toward the observer produces an absorption line at

in the observer frame:the corresponding velocity can be found:

typically, bias is removed excluding absorptions with zabs less than the value corresponding to β~0.1

high redshift galaxies

they appear different from nearby galaxies, both for observational effects and for intrinsic differences

main effects:* redshift-dependence of surface brightness* K-correction* passive and active evolution

light of distant galaxies comes mainly from massive, young stars: observing at high redshift, we see cosmic epochs of vigorous star formation

portion of the Hubble Deep Field

galaxies appear more irregular thanpresent day galaxies

we see them through the light emittedin UV by the young stars

but in UV also nearby galaxiesappear less regular

cosmic distances and surface brightnesslike for quasars, also for galaxies we must use luminosity distance

moreover, because galaxies have extended images, it is also important theangular diameter distance

the different dipendence on redshift has important consequenceson surface brightness, and is due to the fact that in one casephotons are dispersed on detector’s area at z=0,while in the other we observe photons emitted by anarea of the source at zem

surface brightness falls rapidly, making photometry difficultthe apparent sizes of corresponding isophotes shrink

fall of the isophotal diameter R25

exponential disk

spheroid R1/4

at low z there is a larger effect for the spheroid, at high z for the exponential disk

z

A(z)

luminosity distance

in units of c/H0

0

z

Aa(z)

angular diameter distance

in units of c/H0

K-correction(evolutionary correction)

we cannot measure the spectrum of a distant galaxy like it is now, but we can compute how a galaxy identical to one present day galaxy would appear if placed at a redshift zelliptical, falls rapidly in the rest-

frame UV corresponding to observed Bstarburst, small or no decrease, because of young stars emitting in this band

effect in the I band is lower for both spectra

same spectrum shifted in lambda

effect for z=0.5

K-correction

K-correction for ellipticals, Sb spirals, blue irregolar/starburst galaxies, in the bands BJ, I, K

evolutionary correction

different spectrum at t0 and te

3 possibilities:

•burst of star formation, and then rapid death of massive stars and progressive dimming of the other stars (passive evolution)

•further episodes of star formation

•addition of stars and/or gas in merging episodes

approximate expression in terms of the evolutionary luminosity change dL/dt, for small z and Λ=0:

Δt being the look-back time

Hubble diagram

in the K band for some samples of radiogalaxies

curves show the effect of two models of passive evolution with star formation burst at z=20

passive and active evolution

passive evolution is called the change of galactic properties due to the aging of stellar population born initially in the original star formation burst. active evolution indicates instead the effect on galactic properties due to secondary events of star formation, e.g. produced by merging

population synthesis: a galactic spectrum can be written simply as sum of the spectra of constituent stars (ignoring complications such as internal absorption by dust or co-evolving binary systems):

theoretical stellar spectra can be used, or even empirical stellar spectra, if they can be observed for a grid of values of temperature, luminosity, and chemical composition

need to specify the initial mass function (IMF) with which stars are born. at high redshift IMF can be much different than present IMF, probably peaked toward very massive stars

most common models use star formation with a single burst, or exponentially decreasing, or constant. results show that much of the initial emission is in the UV. later, a strong characteristic spectral feature is produced, called HK break or 4000Å break, a blend of absorption lines near the HK CaII doublet. the amplitude of the break increases with age and is little dependent on other factors

evolution of a galactic spectrum

color bimodality

luminosity, mass, color, morphology, stellar population of galaxies are strongly related. analysis of such properties in the cosmic time started first with the study of the luminosity function but later included galaxy counts as function of the various parameters

however, almost all these properties are unimodal, and galaxies tend to occupy a big cloud in the parameter space, and it is often difficult to distinguish if a change in a particular cell of the parameter space is due to a global number change or to a shift towards/from nearby cells

in this sea of unimodal functions, one function appears different for his bimodal character, the color function. bimodality is evident, e.g. in the color-magnitude diagram (CMD), where two populations are clearly distingushed, the BLUE CLOUD and the RED SEQUENCE

Hogg et al 2003

Baldry et al 2004

color bimodality

Baldry et al 2004otherwise, this can be viewed with color distributions in bins of absolute magnitude, approximated by double Gaussians

bimodality is present also for other parameters, morphology, metallicity, SFR, but color bimodality is much more clear, and is observed up to z~1, and partially for z>1.

Lilly et al 1995

blue and red luminosity function

bimodal behavior is also clear from the luminosity function, where a steepening is observed in the low luminosity part of the LF of blue galaxies for z > ~0.5, and instead a substantial lack of evolution for red galaxies (Lilly et al 1995)

these observations were interpreted with the conclusion that red galaxies formed first, in accordance with the so-called “monolithic collapse” scenario (Eggen Lynden-Bell Sandage 1962), and that blue galaxies are still evolving

Faber et al 2007

more recent studies based on 39,000 galaxies from surveys DEEP2 and COMBO-17 (Faber et al 2007) have provided evidence also for evolution of the LF of red galaxies, with a decrease of MB* and an increase of φ* (parameters of the Schechter LF)

it is found also a substantial constancy of the luminosity density for z<1

as stellar evolution models for red galaxies predict an increase of the ratio M/LB of 1-2 mag, constancy of jB implies that stellar mass of red galaxies is at least doubled from z=1

blue and red luminosity function

color-stellar mass diagram

besides the color-magnitude diagram, bimodality is represented also with the color-stellar mass diagram

e.g. Taylor et al 2009

Bundy et al 2005color-stellar mass diagram

estimate of stellar masses uses multiband photometry and redshift to compare the observed SED with a grid of synthetic SEDs depending on star formation history, age, metallicity, dust content.for each grid model the computed quantities are M*/LK, M*, chi2, and the probability that the model represents the data

probabilities are then summed on the grid and probability histograms by stellar mass are produced. so for each galaxy a probability distribution of M* is found, and the median value is adopted as measure of M*

evolution in the color-stellar mass diagram

Track C is intermediate, with contributions by both mechanisms. this scenario is in better agreement with the properties of elliptical galaxies, both distant and local

Track B is the opposite extreme, with a late star formation quenching. in this case, galaxies collect most of their mass in the blue phase, and then are subject to merging and become red, without further “dry merging”

Track A represents an early quenching of star formation, when galaxy fragments are still small. in this case, most of the galaxy growth occurs in “dry mergers”

Faber et al 2007 assume that galaxies can transit from BLUE CLOUD to RED SEQUENCE when star formation stops during a “major merger” (merging between galaxies with nearly equal masses). the stop of star formation(quenching) is represented by nearly vertical lines. mergers are gas-reach (wet mergers) because progenitor galaxies are blue galaxies with star formation. once on the red sequence, galaxies can be subject to gas-poor mergers (dry mergers), described by the white arrows. three cases are proposed:

Dekel et al 2006variants of the color-stellar mass diagram

Cattaneo et al 2006

variants of the color-stellar mass diagram

Cattaneo et al 2009

Hasinger 2008

Smolcic 2009

green valley