Download lower dotted line - Dipartimento di Fisica e Astronomia

La storia di formazione stellare e’ una delle caratteristiche principali
delle galassie.
Ci concentreremo per lo piu’ su galassie con formazione stellare in
atto…..
e analizzeremo le tecniche attuali per determinare il tasso di
formazione stellare di una galassia a partire da spettri o colori
integrati, con lo scopo di capire a fondo ciascuna di queste….
…partendo da un esempio concreto e recente, la determinazione
dell’evoluzione cosmica della formazione stellare.
Star Formation Rate (solar masses per year)
SFR
Star Formation History (solar masses/yr at each t) SFH
Space density of SFR (solar masses per year per Mpc3)
Evoluzione cosmica della formazione stellare
SFR (Msun yr-1 Mpc-3)
1+z
Hopkins 2004
Evolution of SFR density with redshift, using a common obscuration correction where necessary. The
points are color-coded by rest-frame wavelength as follows: Blue: UV; green: [O II]; red: H and
H ; pink: X-ray, FIR, submillimeter, and radio. The solid line shows the evolving 1.4 GHz LF derived
by Haarsma et al. (2000). The dot-dashed line shows the least-squares fit to all the z < 1 data points,
log( *) = 3.10 log(1 + z) - 1.80. The dotted lines show pure luminosity evolution for the Condon
(1989) 1.4 GHz LF, at rates of Q = 2.5 (lower dotted line) and Q = 4.1 (upper dotted line). The dashed
line shows the "fossil" record from Local Group galaxies (Hopkins et al. 2001b).
BEST INDICATOR IN THE OPTICAL:
THE Hα LINE (6563 A) IN EMISSION
Elliptical
Sa
Sc
Sm/Irr
Kennicutt 1992
LUCIDI
Indicators of ongoing star-formation activity - Timescales
Emission lines
< 3 x 107 yrs
FIR emission
< a few 10^7 (but…)
Radio emission
as FIR
UV-continuum emission
it depends…
WHY IT WORKS:
The calibration LHalpha-SFR can be used because we know with sufficient
approximation:
a) The stellar ionizing emission
b) The physics of the recombination lines of hydrogen
c) And because empirically the IMF does not vary dramatically from one
galaxy to another, from an HII region to another
a) and c) carry an uncertainty in the calibration – a) at the level of 30%, c)
a factor of 3 between Scalo and Salpeter IMF
Other members of the Balmer family:
For a gas with T=10000 K
Kα/ Kβ = 2.87
K β / Kβ = 1.00
Kγ/ Kβ = 0.47
Kδ/ Kβ = 0.26
Kε/ Kβ = 0.16
…..and other HI families….
KPaschen/ Kβ ~ 0.35
KBrackett/ Kβ ~ 0.18
Emission lines are
present when there are
young stars that ionize
the gas….
BUT older hot stars
(PNae, hot HB stars…)
and AGNs can
contribute
100 MO
ZAMS
2.520
MM
O O
PAGB
0.6 MO
PN
5 MO
2.5 MO
2.5 MO
RGB
ZAHB
To WD
Padova 94 set
Z=Zo Y=0.28
1 MO
1MO
AGN EMISSION
Different source of photoionization, different ionizing spectrum
(power-law, the spectrum of the ionizing radiation extends to much
higher energies)
OIII(5007)/Hbeta(4861)
Conventional method to distinguish between photoionization from
O,B stars and non-thermal processes are the so-called “diagnostic
diagrams”, using line intensity ratios
NII(6583)/Halpha
SII(6716)/Halpha
Veilleux & Osterbrock, 1987, Baldwin et al. 1981
OTHER LIMITATIONS
1. Need to be corrected for underlying stellar absorption and NII
emission
2. Assumption that all the massive star formation is traced by the
ionized gas
Escape fraction of ionizing radiation from individual HII
regions can be high (15-50% - Oey & Kennicutt 1997,
Ferguson et al. 1996)
Escape fraction from a galaxy as a whole generally lower
(3%? Leitherer et al. 1995)
3. Most importantly, dust extinction
BEST INDICATOR IN THE OPTICAL:
THE Hα LINE (6563 A) IN EMISSION
Elliptical
Sa
Sc
Sm/Irr
NB in absorption in passive galaxy spectra
Kennicutt 1992
OTHER LIMITATIONS
1. Need to be corrected for underlying stellar absorption and NII
emission
2. Assumption that all the massive star formation is traced by the
ionized gas
Escape fraction of ionizing radiation from individual HII
regions can be high (15-50% - Oey & Kennicutt 1997,
Ferguson et al. 1996)
Escape fraction from a galaxy as a whole generally lower
(3%? Leitherer et al. 1995)
3. Most importantly, dust extinction
DUST EXTINCTION (I)
It is the most important source of systematic error in Hα-derived SFRs
Methods to estimate it:
using the observed Balmer ratio Hα/ Hβ (Balmer decrement) versus
the theoretical value
comparing Hα with other SF estimator less affected by dust (IR
recombination lines, FIR, thermal radio continuum)
Mean extinction in nearby normal galaxies: A(Hα) ~1 mag (factor 2.5)
Hopkins 2004
SFR (Msun yr-1 Mpc-3)
1+z
Evolution of SFR density with redshift, using a common obscuration correction where necessary. The
points are color-coded by rest-frame wavelength as follows: Blue: UV; green: [O II]; red: H and
H ; pink: X-ray, FIR, submillimeter, and radio. The solid line shows the evolving 1.4 GHz LF derived
by Haarsma et al. (2000). The dot-dashed line shows the least-squares fit to all the z < 1 data points,
log( *) = 3.10 log(1 + z) - 1.80. The dotted lines show pure luminosity evolution for the Condon
(1989) 1.4 GHz LF, at rates of Q = 2.5 (lower dotted line) and Q = 4.1 (upper dotted line). The dashed
line shows the "fossil" record from Local Group galaxies (Hopkins et al. 2001b).
[OII]3727 FORBIDDEN-LINE DOUBLET
The next best thing: widely used as a substitute for Hα at redshifts z>0.3
λobs = λ0 (1+z) thus for a standard configuration up to 7500A:
z=1.0
z=0.6
z=0.3
[OII]3727 FORBIDDEN-LINE DOUBLET
Advantages
Strongest line in the blue part of the spectrum
Easily observable even in low signal-to-noise spectra
Disadvantages
Theoretically, very complex behaviour:
unlike the hydrogen recombination lines, the [OII] luminosity not directly
coupled to the ionizing luminosity (N of ionizing photons). [OII] emission
depends strongly on metallicity and ionization state (stellar radiation field,
gas chemical composition and the gas density distribution). Complex
photoionization models exist. (Stasinska 2000)
Theoretical calibration between line-luminosity and SFR is much harder
[OII]3727 EMPIRICAL CALIBRATION
Flux ratio [OII]/Hα in nearby normal
galaxies
SFR = 2.0 X 10-41 L(OII) E(Hα)
(e.g. Kennicutt 1992, 1998)
N.B. Dust extinction higher at
[OII](3727) than at Hα(6563) (but
calibration is an empirical one…)
Gallagher et al. 1989, Kennicutt 1992
Hopkins 2004
SFR (Msun yr-1 Mpc-3)
1+z
Evolution of SFR density with redshift, using a common obscuration correction where necessary. The
points are color-coded by rest-frame wavelength as follows: Blue: UV; green: [O II]; red: H and
H ; pink: X-ray, FIR, submillimeter, and radio. The solid line shows the evolving 1.4 GHz LF derived
by Haarsma et al. (2000). The dot-dashed line shows the least-squares fit to all the z < 1 data points,
log( *) = 3.10 log(1 + z) - 1.80. The dotted lines show pure luminosity evolution for the Condon
(1989) 1.4 GHz LF, at rates of Q = 2.5 (lower dotted line) and Q = 4.1 (upper dotted line). The dashed
line shows the "fossil" record from Local Group galaxies (Hopkins et al. 2001b).
THE ULTRAVOLET EMISSION AS
INDICATOR OF ONGOING AND RECENT
STAR FORMATION
Wavelength range 1200-2500 A dominated by young stars (where
there are)
First points placed in Madau plot were based on UV (Lilly et al. 1996,
Madau et al. 1996)
Advantages: it can be used to study high-z galaxies
computing the UV flux does not require spectroscopy
Drawbacks:
relatively few UV facilities to study local UV Universe (now GALEX!)
conversion between UV flux and SFR based on assumptions that may
be unrealistic in some cases, and this introduces an uncertainty
As usual, sensitive to extinction and IMF – not useful for IR-luminous
starbursts
THE ULTRAVOLET EMISSION AS
INDICATOR OF ONGOING AND RECENT
STAR FORMATION
Leitherer and
collaborators –
STARBURST99
SPECTROPHOTOMETRIC MODELS
Simply adding up the light of all stars:
a Single Stellar Population (SSP)
Monochromatic luminosity emitted by a star
with mass m, metallicity Z and age T
stellar IMF
SPECTROPHOTOMETRIC MODELS
Simply adding up the light of all stars:
a galaxy (composite spectrum)
THE ULTRAVOLET EMISSION AS
INDICATOR OF ONGOING AND RECENT
STAR FORMATION
Leitherer and
collaborators –
STARBURST99
Indicators of ongoing star-formation activity - Timescales
Emission lines
< 3 x 107 yrs
UV-continuum emission
it depends…
FIR emission
< a few 10^7 (but…)
Radio emission
as FIR (?)
THE ULTRAVOLET EMISSION AS
INDICATOR OF ONGOING AND RECENT
STAR FORMATION
 Emission et 2000 A is dominated by stars with 2-5 Msun (10^8yrs)
 Calibration using spectrophotometric models
 Assuming continuous and well-behaved SF over timescales of 10^8 yrs or
longer.
Lnu at 1500 A (flat between 1500-2800 A)
For young starbursts, the proportionality
constant can be significantly different
(using this calibration the SFR would be
overestimated)
Madau et al. 1998
Log SFR
SFR = 1.4 X 10-28 Lnu (ergs/s/Hz)
L(1550, 2800 A)
OTHER CALIBRATIONS
Assuming constant SFR on timescale of some 10^8yr:
SFR (solar masses/year) =
0.3 X 10-38 LUV (ergs/s/A)
where LUV is the luminosity at 2000 A, for IMF slope 2.5, 0.1-120
(Boselli et al. 2001)
Hopkins 2004
SFR (Msun yr-1 Mpc-3)
1+z
Evolution of SFR density with redshift, using a common obscuration correction where necessary. The
points are color-coded by rest-frame wavelength as follows: Blue: UV; green: [O II]; red: H and
H ; pink: X-ray, FIR, submillimeter, and radio. The solid line shows the evolving 1.4 GHz LF derived
by Haarsma et al. (2000). The dot-dashed line shows the least-squares fit to all the z < 1 data points,
log( *) = 3.10 log(1 + z) - 1.80. The dotted lines show pure luminosity evolution for the Condon
(1989) 1.4 GHz LF, at rates of Q = 2.5 (lower dotted line) and Q = 4.1 (upper dotted line). The dashed
line shows the "fossil" record from Local Group galaxies (Hopkins et al. 2001b).
Low redshift example
Bell & Kennicutt 2001
High-z example: Lyman break galaxies (U-band dropouts)
Photometrically selected using rest frame UV colors
Steidel et al. 2003
Hopkins 2004
SFR (Msun yr-1 Mpc-3)
1+z
Evolution of SFR density with redshift, using a common obscuration correction where necessary. The
points are color-coded by rest-frame wavelength as follows: Blue: UV; green: [O II]; red: H and
H ; pink: X-ray, FIR, submillimeter, and radio. The solid line shows the evolving 1.4 GHz LF derived
by Haarsma et al. (2000). The dot-dashed line shows the least-squares fit to all the z < 1 data points,
log( *) = 3.10 log(1 + z) - 1.80. The dotted lines show pure luminosity evolution for the Condon
(1989) 1.4 GHz LF, at rates of Q = 2.5 (lower dotted line) and Q = 4.1 (upper dotted line). The dashed
line shows the "fossil" record from Local Group galaxies (Hopkins et al. 2001b).
SFR = 0.9 X 10-41 L(Hα) E(Hα) ergs/s
SFR = 2.0 X 10-41 L([OII]) E(Hα) ergs/s
SFR = 1.4 X 10-28 Lnu ergs/s/Hz (+ dust correction)
Remember extinction??? Lucido
SELECTIVE EXTINCTION
Stars spend the beginning of their
evolution deeply embedded in dusty
environments, later drifting away
from or dispersing the molecular
clouds where they were born.
Leitherer and collaborators –
STARBURST99
Selective extinction
1.
It is empirically motivated by observations
of star forming regions in nearby galaxies.
Giant panda http://www.wwf-uk.org
SELECTIVE EXTINCTION
Lower mean extinction for lower stellar effective temperature
(i.e. higher stellar age) in the Large Magellanic Cloud
Teff = 5500-6500 K
Teff > 12000 K
AV
Zaritsky 1999
Selective extinction
1.
It is empirically motivated by observations
of star forming regions in nearby galaxies.
2.
It is consistent with the fact that the
strongest starbursts are not characterized
by the strongest emission lines.
Giant panda http://www.wwf-uk.org
Normal galaxies (Kennicutt 1992)
Dusty starbursts (P. & Wu 2000)
Selective extinction
1.
It is empirically motivated by observations
of star forming regions in nearby galaxies.
2.
It is consistent with the fact that the
strongest starbursts are not characterized
by the strongest emission lines.
3.
It explains why different E(B-V) are
Giant panda http://www.wwf-uk.org
measured within the same spectrum when
using different features (ex. why extinction in
emission-lines is usually stronger than in the continuum).
SELECTIVE EXTINCTION
Numerous observational studies measure discrepant extinction values
when using different spectral ranges/features (and this is not due to the
uncertainty in the various extinction estimates).
Israel & Kennicutt 1980:
“Visual extinction of H II regions in nine nearby galaxies as derived from the
ratio of the radio continuum emission to H-alpha emission is systematically
larger than visual extinction deduced from the Balmer lines alone, if one
assumes a value Av/E(B-V) = 3.”
The reddening of the UV/optical stellar continuum in starburst galaxy spectra
is lower than the reddening of the ionized gas
The latter is lower than the one inferred from the comparison of Balmer fluxes
with the radio continuum
EXTINCTION
An age-dependent dust obscurations marks in a peculiar way the spectrum of a
dust-enshrouded starburst.
If dust is absent, each given portion of a galaxy spectrum is dominated by the
stellar population of a specific range of ages (eg emission lines and UV).
In presence of dust, if each stellar age is affected by a different amount of
obscuration, within the same spectrum we will measure different values of
extinction, depending on the spectral region/feature used to estimate it.
EXTINCTION
Dust effectively “steals” flux emission at short wavelengths
and gives it back at long wavelengths
Hopkins 2004
SFR (Msun yr-1 Mpc-3)
1+z
Evolution of SFR density with redshift, using a common obscuration correction where necessary. The
points are color-coded by rest-frame wavelength as follows: Blue: UV; green: [O II]; red: H and
H ; pink: X-ray, FIR, submillimeter, and radio. The solid line shows the evolving 1.4 GHz LF derived
by Haarsma et al. (2000). The dot-dashed line shows the least-squares fit to all the z < 1 data points,
log( *) = 3.10 log(1 + z) - 1.80. The dotted lines show pure luminosity evolution for the Condon
(1989) 1.4 GHz LF, at rates of Q = 2.5 (lower dotted line) and Q = 4.1 (upper dotted line). The dashed
line shows the "fossil" record from Local Group galaxies (Hopkins et al. 2001b).