The Cosmic Near-Infrared Background: Remnant light form early stars Journal Club talk A. B. Fry Ferdinand & Komatsu 2006 (F&K06) also Ferdinand & Komatsu et al (F&K10)

The Cosmic Near-Infrared Background: Remnant light form early stars The cosmic infrared background radiation is the diffuse light from faint galaxies that remains after Milky Way and zodiacal light is subtracted. The near-infrared (1-3 um) background light (NIRB) from redshifted stars at z~10 contributes to this backgorund. The NIRB offers invaluable information regarding the physics of cosmic reionization that is difficult to probe by other means. Fernandez & Komatsu explore the uncertainty in the background intensity from these stars due to metallicity, the mass spectrum, and other parameters. The authors show that contrary to previous results that the stellar component of the NIRB could come from stars with some metals (Z=1/50 solar). A follow up paper examines intensity fluctuations of the mean NIRB which could further constrain high-z galaxy populations.

The cosmic infrared background radiation (CIBR) is the diffuse light from faint galaxies that remains after Milky Way and zodiacal light is subtracted.

R/ The black data points between 1 and 300 microns on this graph come from the DIRBE experiment on the COBE satellite. The red data points are from Wright, E.L (ApJ, 553, 538) which use a different zodiacal light model than the one used by Hauser et al. (1998, ApJ, 508, 25). The blue lower limit symbols are based on integrating galaxy counts, while the purple upper limit symbols are based on limits on photon-photon collisions from gamma-ray astronomy. The black data points at wavelengths shorter than 1 microns come from Dube, Wickes & Wilkinson (1979, ApJ, 232, 333), Toller (1983, ApJL, 266, 79), and Hurwitz, Martin & Bowyer (1991, ApJ, 372, 167). The curve is the Lambda CDM model with the Salpeter IMF from Primack et al., multiplied by a factor of 1.84, and with modifications for wavelengths longer than 300 microns to fit the FIRAS distortion limits, and for wavelengths shorter than 0.8 microns to fit the optical and UV data.

Lets focus on the near infrared (1-3 µm) background radiation (NIRB) where redshifted ultraviolet light from early stars at z~10 contributes.

The observed NIRB seems too large to be accounted for by the integrated light from galaxies, it could come from early stars…

For example, suppose that most reionization occurred at z=9. Then ultraviolet photons (λ~1000 Å ) produced at this redshift during reionizaiton will then be redshifted to the near infrared regime (λ~1µm).

The NIRB offers invaluable information regarding the physics of cosmic reionization that is difficult to probe by other means.

In F&K06 they discuss the simplified physics of the NIRB, explore different metallicities and initial mass spectra of the first stars, and provide a relation to the NIBR and star formation rate. They predict the average intensity at 1-2 µm (units of nW m -2 sr -1, just like previous plots) which is a function of the mass spectrum of early stars, the star formation rate, metallicity. Etc.

The mean background intensity is computed in terms of the volume emissivity, p( ʋ,z ), which is a function of the mass spectrum of early stars, the star formation rate, metallicity. Etc. The background intensity* (Peacock 1999, p. 91): * Redshift affects the flux density in several ways Photon energies and arrival rates are redshfited reducing the flux density by a factor of (1+z) 2 The bandwidth d ʋ is reduced by a factor of 1+z so the energy flux per unit bandwidth does down by one power of 1+z The observed photons at frequency ʋ 0 were emitted at a frequency ʋ 0 (1+z) si the flux density is the luminosity at this frequency divided by the total area divided by 1+z

p( ʋ,z ) is the volume emissivity in units of energy per unit time per unit frequency and unit commoving volume: The sum over α takes into account the various radiative process contributions to the emissivity…

p( ʋ,z ) is the volume emissivity in units of energy per unit time per unit frequency and unit commoving volume: p * is the continuum emission form the stars themselves p line is the emission from recombination lines p cont is free-free and free-bound continuum emissions p 2 γ is the two-photon emission

p( ʋ,z ) is the volume emissivity in units of energy per unit time per unit frequency and unit commoving volume: The dimensionless quantity represents a ratio of the mass-weighted average total radiative energy to the stellar rest-mass energy in a unit frequency interval…

p( ʋ,z ) is the volume emissivity in units of energy per unit time per unit frequency and unit commoving volume: dρ * /dt is the mean star formation rate at the redshift of interest in units of M ⊙ yr -1 Mpc -3. It is very uncertain! L ʋ is a time averaged luminosity for the radiative process α τ(m) is the stellar main-sequence lifetime

p( ʋ,z ) is the volume emissivity in units of energy per unit time per unit frequency and unit commoving volume: Finally, f(m) is the mass spectrum. They use three different versions…

Salpeter Larson: Top-heavy:

From 6 to 8 M ⊙, the O/Ne/Mg core of the star collapses, or the star ejects its outer envelope, leaving a white dwarf or neutron star. From 8 to 25 M ⊙, the iron core collapses, the star explodes as a supernova, and a neutron star is left as a remnant. A significant amount of metals are ejected. From 25 to 40 M ⊙, there is a weak supernova and a black hole is created by fallback. The amount of metals that are ejected into the IGM decreases sharply, leaving most of the metals locked in the black hole. From 40 to 100 M ⊙, the star directly collapses into a black hole. The only metals produced are from mass loss during the star’s life. From 100 to 140 M ⊙, a pulsational pair instability supernova results. This ejects the outer envelope of the star, and then the core collapses into a black hole. Metals in the outer envelope pollute the IGM. From 140 to 260 M ⊙, a pair instability supernova results, which completely disrupts the star and leaves no remnant. All the metals are ejected into the IGM. Above 260 M ⊙, the star collapses directly into a black hole, and there is no enrichment of the IGM. The Fate of Massive Stars

The previous equations 1-3 allow us to predict the average intensity (in units of nW m -1 sr -1 ) at 1-2 µm:

Equations 1-3 allow us to predict the average intensity (in units of nW m -1 sr -1 ) at 1-2 µm:

3 to 1 µm corresponds to.414 to 1.24 eV The spectrum of the NIRB

The dependence on the initial mass spectrum f(m) is such that heaver mass spectra tend to give higher background intensities. For metal- rich stars Ly α emission dominates. For metal-poor stars there is a significant contribution form the stars themselves. The predicted sensitivity is not sensitive to stellar metallicity The spectrum of the NIRB

Metallicity changes the hardness of the stellar spectrum, it affects the ratio of energy in the Ly α and two-photon emission to stellar emission energy: the harder the spectrum is the more ionizing photons are emitted and thus the more the Ly α and two photon-emission energies. The spectrum of the NIRB

F&K06 find that a population of metal-poor stars do not overproduce metals that we observe in the universe today, except for the Larson mass function upper 1 σ value for the star formation rate.

The uncertainty in measurements of the NIRB are massive. They vary from 2-50 nW m -2 sr -1 at 1- 2 µ m including the upper and lower 1 σ bounds. The mean SFR dρ * /dt is constrained to.3-12 M ⊙ yr -1 Mpc -3.

Measuring the absolute value of the NIRB is very difficult due to systematic uncertainty in zodiacal light subtraction, however, fluctuations could be measured without this absolute calibration!

F&K 2010 et al. present calculations of the power spectrum and metallicity/initial-mass- spectrum dependence of the NIRB fluctuations, as well as dependence on the star formation efficiency and the escape fraction of ionizing photons. Fluctuations

F&K 2010 et al. present calculations of the power spectrum and metallicity/initial-mass- spectrum dependence of the NIRB fluctuations, as well as dependence on the star formation efficiency and the escape fraction of ionizing photons. Fluctuations

Luminosity-density power spectrum of halos with Pop II stars with an initial mass spectrum, f esc = 0.19, and f* = 0.5, assuming a rectangular bandpass from 1 − 2 μm.

The luminosity-density power spectra of halos are approximately power-laws over the entire range of wave numbers that the simulation covers. The clustering of halos is highly non-linearly biased relative to the underlying matter distribution. The growth of the power spectrum is partly driven by the growth of linear matter fluctuations as well as that of halo bias. Fluctuations

The NIRB intensity contribution from early stars is essentially determined by the mass-weighted mean nuclear burning energy of the stars and the cosmic star formation rate. The intensity is not sensitive to stellar metallicity. Variations in the NIRB can tell us details about the first stars, reionization, and the high redshift host galaxies/IGM. The amplitude of the (observable) angle power spectrum of the mean NIRB is determined by f * while the angular power spectrum of the IGM can probe the ionization history. Conclusions

Image credit: Robert Hurt, SSC, JPL, CalTech, NASA ‘T’he first stars may have lighted up the cosmos within 200 to 400 million years after the Big Bang, and then clustered together into what later became galaxies’SSCJPLCalTechNASA

*A note about the escape fraction for Ly α which varies widely in the literature: the fraction of ionizing photons escaping the nebula does not alter the Ly α luminosity very much because all of the ionizing photons will eventually be converted to Ly α photons that in turn will escape freely via the cosmological redshift therefore these predictions should be free from uncertainty in the escape fraction.

The amplitude of Cl is, among other things, a sensitive probe of the nature of high-z galaxies.

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The authors find if the NIRB has a stellar origin… metal-free stars are not the only explanation of the excess NIRB; stars with significant metals (e.g., Z = 1/50 Z ʘ ) can produce the same amount of background intensity as metal- free stars. We predict ʋ * /σ ~4–8nWm −2 sr −1, where * is the mean star formation rate at z = 7–15 (solar masses per year per cubic megaparsec) for stars more massive than 5 solar masses While the star formation rate at z = 7–15 inferred from the current data is significantly higher than the local rate at z<5, it does not rule out the stellar origin of the cosmic near-infrared background. In addition, we show that a reasonable initial mass function, coupled with this star formation rate, does not overproduce metals in the universe in most cases and may produce as little as less than 1% of the metals observed in the universe today.

The authors find if the NIRB has a stellar origin… This is because the average intensity at 1–2 microns is determined by the efficiency of nuclear burning in stars, which is not very sensitive to metallicity. We have very little knowledge about the form of the mass spectrum of early stars, the uncertainty in the average intensity due to the mass spectrum could be large. An accurate determination of the near-infrared background allows us to probe the formation history of early stars, which is difficult to constrain by other means. A reasonable initial mass function, coupled with this star formation rate, does not overproduce metals in the universe in most cases and may produce as little as less than 1% of the metals observed in the universe today.

o When and how was the universe reionized? o What does (the calculated bound on stellar mass density) imply? o Would metal-poor stars overproduce metals that we observe in the universe today? Questions they asked

more The Cosmic Infrared Background ExpeRiment (CIBER) is a rocket-borne absolute photometry imaging and spectroscopy experiment optimized to detect signatures of first-light galaxies present during reionization in the unresolved IR background. CIBER-I consists of a wide-field two-color camera for fluctuation measurements, a low-resolution absolute spectrometer for absolute EBL measurements, and a narrow-band imaging spectrometer to measure and correct scattered emission from the foreground zodiacal cloud. CIBER-I was successfully flown on February 25th, 2009 and has one more planned flight in early We propose, after several additional flights of CIBER-I, an improved CIBER-II camera consisting of a wide-field 30 cm imager operating in 4 bands between 0.5 and 2.1 microns. It is designed for a high significance detection of unresolved IR background fluctuations at the minimum level necessary for reionization. With a FOV 50 to 2000 times largerthan existing IR instruments on satellites, CIBERII will carry out the definitive study to establish the surface density of sources responsible for reionization. Cooray et al

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