Ci concentreremo per lo piu’ su galassie con formazione stellare in atto…..

1. 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.

2. 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)

3. Evoluzione cosmica della formazione stellare 1 + z SFR (Msun yr-1 Mpc-3) Hopkins 2004 Evolution of SFR densitywith redshift, using acommon obscuration correction wherenecessary. The points arecolor-coded by rest-frame wavelengthas follows: Blue: UV;green: [O II]; red:H   and H  ; pink:X-ray, FIR, submillimeter, andradio. The solid lineshows the evolving 1.4GHz LF derived byHaarsma et al. (2000).The dot-dashed line showsthe least-squares fit toall the z <1 data points, log(  *)= 3.10 log(1 +z) - 1.80. Thedotted lines show pureluminosity evolution for theCondon (1989) 1.4 GHzLF, at rates ofQ = 2.5 (lower dotted line)and Q = 4.1(upper dotted line). The dashed lineshows the "fossil" recordfrom Local Group galaxies(Hopkins et al. 2001b).

4. BEST INDICATOR IN THE OPTICAL: THE Hα LINE (6563 A) IN EMISSION Elliptical Sa Sc Sm/Irr Kennicutt 1992

5. LUCIDI

6. 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…

7. WHY IT WORKS: • The calibration LHalpha-SFR can be used because we know with sufficient approximation: • The stellar ionizing emission • The physics of the recombination lines of hydrogen • 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

8. Other members of the Balmer family: 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 For a gas with T=10000 K

9. 100 MO ZAMS 2.5 MO 20 MO PAGB0.6 MO PN 5 MO 5 MO 2.5 MO 2.5 MO RGB ZAHB To WD 1 MO 1MO 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 Padova 94 set Z=Zo Y=0.28

10. AGN EMISSION Different source of photoionization, different ionizing spectrum (power-law, the spectrum of the ionizing radiation extends to much higher energies) Conventional method to distinguish between photoionization from O,B stars and non-thermal processes are the so-called “diagnostic diagrams”, using line intensity ratios OIII(5007)/Hbeta(4861) NII(6583)/Halpha SII(6716)/Halpha Veilleux & Osterbrock, 1987, Baldwin et al. 1981

11. OTHER LIMITATIONS • Need to be corrected for underlying stellar absorption and NII emission • 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

12. 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

13. OTHER LIMITATIONS • Need to be corrected for underlying stellar absorption and NII emission • 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

14. 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 Hαwith 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)

15. 1 + z Hopkins 2004 SFR (Msun yr-1 Mpc-3) Evolution of SFR densitywith redshift, using acommon obscuration correction wherenecessary. The points arecolor-coded by rest-frame wavelengthas follows: Blue: UV;green: [O II]; red:H   and H  ; pink:X-ray, FIR, submillimeter, andradio. The solid lineshows the evolving 1.4GHz LF derived byHaarsma et al. (2000).The dot-dashed line showsthe least-squares fit toall the z <1 data points, log(  *)= 3.10 log(1 +z) - 1.80. Thedotted lines show pureluminosity evolution for theCondon (1989) 1.4 GHzLF, at rates ofQ = 2.5 (lower dotted line)and Q = 4.1(upper dotted line). The dashed lineshows the "fossil" recordfrom Local Group galaxies(Hopkins et al. 2001b).

16. [OII]3727 FORBIDDEN-LINE DOUBLET The next best thing: widely used as a substitute for Hαat 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

17. [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

18. [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

19. 1 + z Hopkins 2004 SFR (Msun yr-1 Mpc-3) Evolution of SFR densitywith redshift, using acommon obscuration correction wherenecessary. The points arecolor-coded by rest-frame wavelengthas follows: Blue: UV;green: [O II]; red:H   and H  ; pink:X-ray, FIR, submillimeter, andradio. The solid lineshows the evolving 1.4GHz LF derived byHaarsma et al. (2000).The dot-dashed line showsthe least-squares fit toall the z <1 data points, log(  *)= 3.10 log(1 +z) - 1.80. Thedotted lines show pureluminosity evolution for theCondon (1989) 1.4 GHzLF, at rates ofQ = 2.5 (lower dotted line)and Q = 4.1(upper dotted line). The dashed lineshows the "fossil" recordfrom Local Group galaxies(Hopkins et al. 2001b).

20. 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

21. THE ULTRAVOLET EMISSION AS INDICATOR OF ONGOING AND RECENT STAR FORMATION Leitherer and collaborators – STARBURST99

22. SPECTROPHOTOMETRIC MODELS Simply adding up the light of all stars: a Single Stellar Population (SSP) stellar IMF Monochromatic luminosity emitted by a star with mass m, metallicity Z and age T

23. SPECTROPHOTOMETRIC MODELS Simply adding up the light of all stars: a galaxy (composite spectrum)

24. THE ULTRAVOLET EMISSION AS INDICATOR OF ONGOING AND RECENT STAR FORMATION Leitherer and collaborators – STARBURST99

25. 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 (?)

26. 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. SFR = 1.4 X 10-28 Lnu (ergs/s/Hz) Lnu at 1500 A (flat between 1500-2800 A) Log SFR For young starbursts, the proportionality constant can be significantly different (using this calibration the SFR would be overestimated) Madau et al. 1998 L(1550, 2800 A)

27. 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)

28. 1 + z Hopkins 2004 SFR (Msun yr-1 Mpc-3) Evolution of SFR densitywith redshift, using acommon obscuration correction wherenecessary. The points arecolor-coded by rest-frame wavelengthas follows: Blue: UV;green: [O II]; red:H   and H  ; pink:X-ray, FIR, submillimeter, andradio. The solid lineshows the evolving 1.4GHz LF derived byHaarsma et al. (2000).The dot-dashed line showsthe least-squares fit toall the z <1 data points, log(  *)= 3.10 log(1 +z) - 1.80. Thedotted lines show pureluminosity evolution for theCondon (1989) 1.4 GHzLF, at rates ofQ = 2.5 (lower dotted line)and Q = 4.1(upper dotted line). The dashed lineshows the "fossil" recordfrom Local Group galaxies(Hopkins et al. 2001b).

29. Low redshift example Bell & Kennicutt 2001

30. High-z example: Lyman break galaxies (U-band dropouts) Photometrically selected using rest frame UV colors Steidel et al. 2003

31. 1 + z Hopkins 2004 SFR (Msun yr-1 Mpc-3) Evolution of SFR densitywith redshift, using acommon obscuration correction wherenecessary. The points arecolor-coded by rest-frame wavelengthas follows: Blue: UV;green: [O II]; red:H   and H  ; pink:X-ray, FIR, submillimeter, andradio. The solid lineshows the evolving 1.4GHz LF derived byHaarsma et al. (2000).The dot-dashed line showsthe least-squares fit toall the z <1 data points, log(  *)= 3.10 log(1 +z) - 1.80. Thedotted lines show pureluminosity evolution for theCondon (1989) 1.4 GHzLF, at rates ofQ = 2.5 (lower dotted line)and Q = 4.1(upper dotted line). The dashed lineshows the "fossil" recordfrom Local Group galaxies(Hopkins et al. 2001b).

32. 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)

33. Remember extinction??? Lucido

34. 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

35. Selective extinction 1. It is empirically motivated by observations of star forming regions in nearby galaxies. Giant panda http://www.wwf-uk.org

36. 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

37. 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

38. Normal galaxies (Kennicutt 1992) Dusty starbursts (P. & Wu 2000)

39. 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 measured within the same spectrum when using different features (ex. why extinction in emission-lines is usually stronger than in the continuum). Giant panda http://www.wwf-uk.org

40. 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

41. 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.

42. EXTINCTION Dust effectively “steals” flux emission at short wavelengths and gives it back at long wavelengths

43. 1 + z Hopkins 2004 SFR (Msun yr-1 Mpc-3) Evolution of SFR densitywith redshift, using acommon obscuration correction wherenecessary. The points arecolor-coded by rest-frame wavelengthas follows: Blue: UV;green: [O II]; red:H   and H  ; pink:X-ray, FIR, submillimeter, andradio. The solid lineshows the evolving 1.4GHz LF derived byHaarsma et al. (2000).The dot-dashed line showsthe least-squares fit toall the z <1 data points, log(  *)= 3.10 log(1 +z) - 1.80. Thedotted lines show pureluminosity evolution for theCondon (1989) 1.4 GHzLF, at rates ofQ = 2.5 (lower dotted line)and Q = 4.1(upper dotted line). The dashed lineshows the "fossil" recordfrom Local Group galaxies(Hopkins et al. 2001b).