# Presentation: 325724

## The Cosmic Near Infrared Background:

***The Mass Spectrum of the First Stars***
- We used three possibilities for the mass spectrum:
- Salpeter (Salpeter 1955)
- Larson (Larson 1998)
- Top Heavy

***Light from an Early Population of Stars***
- The intensity is given by (Peacock 1999):

-  is the observed frequency, H(z) is the expansion rate, and p(,z) is the volume emissivity, given by:

- It depends on the star formation rate,     , and the mass weighted average of the total radiative energy to the stellar rest mass energy per frequency, given by:

- Where m is the stellar mass and m* is the mean stellar mass.  Therefore, the intensity depends on the luminosity of the source L(m), the lifetime of the source (m), the mass spectrum f(m), and the star formation rate.

***The Cosmic Near Infrared Background:***
***Remnant Light from the Early Stars***

***Elizabeth Fernandez and           Eiichiro Komatsu***
***The University of Texas at Austin***
***beth@astro.as.utexas.edu***

***Introduction***
- We observe a background in the infrared that is above and beyond the galactic contribution.  Where does this excess light come from?  It is quite possible that this light is a remnant from the era of reionization.
- The Near Infrared Background (NIRB) can be a powerful tool to observe the era of reionization.  Ultraviolet photons that ionized the universe at a redshift of around 10 are redshifted into what we observe as the near infrared.  Therefore, some light from in the near infrared background *must* come from the era of reionization – and perhaps the first few generations of stars.

***Conclusions***
-  If the Near Infrared Background comes wholly or in part from the first stars, the intensity from these stars is determined from the mass-weighted mean nuclear burning energy of the stars and the star formation rate.
-  The process is not sensitive to stellar metallicity!
-  The NIRB from 1-2 microns can lead to the star formation rate at z = 7-15.
-  NIRB another window to cosmic reionization!
- We found a star formation rate of ~0.3-12 M yr-1 Mpc-3 at z=7-15.
-  Uncertainties dominated by observations and by the uncertainty in the mass spectrum of the early stars.
-  This is significantly higher than low-z rates, but does not rule out the stellar origin of the NIRB.
-  Right now, we need better observations of the NIRB to truly ascertain the star formation rate.
-  Using these star formation rates, as little as 1% of the metals in the universe would be produced by stars making up the NIRB.  Therefore, metals are not necessarily overproduced in the IGM.

***The Luminosity of the First Stars***
- We see luminosity from:
- Blackbody radiation from the star
- Emission lines
- Two photon emission
- Free-free  and free-bound radiation

***The Radiative Efficiency Spectrum***
- Here we show the efficiency of each of the sources of emission.
- Previous work assumed Population III (metal-free) formed up to a redshift of ~7 to produce the NIRB.  However, as stars die, the IGM becomes polluted with metals and at some point, metal-poor stars form.
- My model takes into account two limiting cases of stars: metal-free (Z=0) and metal-poor (Z=1/50Z).

- Observations from 1-2 microns correspond to a stellar population at 7<z<15.
- Light is dominated by Lyman-alpha emission.
- For metal-poor stars, there is also a significant stellar contribution.
- The emitted energy is about the same in both cases – a consequence of the conservation of energy.
- Therefore, the overall intensity of metal-free and metal-poor stars is about the same! The prediction for the star formation rate will be very robust, *not depending on metallicity of the population of stars!  *Our model shows that* both *populations of stars can produce the NIRB equally well.
- Intensity does increase with “heaviness” of mass spectrum.

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***Deriving the Star Formation Rate***
- Observations from
- Diffuse Infrared Background Explorer (DIRBE) on the Cosmic Background Explorer (COBE)
- I = 14.6+/- 12.4 nW m-2 sr-1 (Wright 2001) from 1-2 microns
- I = 30.9+/- 12.8 nW m-2 sr-1 (Cambresy et al. 2001) from 1-2 microns
- Difference from zodiacal light subtraction model
- Near Infrared Spectrometer (NIRS) on the InfraRed Telescope in Space (IRTS)
- I = 41.6+/- 9.7 nW m-2 sr-1 (Matsumoto et al. 2005) from 1-2 microns
- From these observations, we derive a star formation rate of **0.3 to 12 solar masses per year per cubic megaparsec**.
- Error is dominated by observations, not by theory!
- By modeling metals produced by various mass spectra, metals are not overproduced in the IGM in most cases.

- References:
- Cooray, A., & Yoshida, N. 2004, MNRAS, 351, L71
- Dwek, E., & Arendt, R.G. 1998, ApJ, 508, L9
- Madau, P., & Silk, J. 2005, MNRAS, 359, L22
- Salvaterra, R., & Ferrara, A. 2003, MNRAS, 339, 973
- Santos, M.R., Bromm, V., & Kamionkowski, M. 2002, MNRAS, 336, 1082

***Abstract***
- The redshifted ultraviolet light from early stars at z~10 contributes to the cosmic near infrared background.  We present detailed calculations of its spectrum with various assumptions about metallicity and mass spectrum of the early stars.  We show that if the near infrared background has a stellar origin, metal-free stars are *not* the only explanation of the excess near infrared background; stars with metals (e.g., Z=1/50 Z) can produce the same amount of background intensity as metal-free stars.  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 predict I/~ 4-8 nW m-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. On the other hand, since 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. 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.

- At low energies, the majority of the contribution is from nebular emission (free-free and free-bound emission).
- Stellar, Lyman-alpha, and two-photon emission dominate at high energy.
- Lyman-alpha and two-photon emission is *greater *for the metal-free case because the spectrum is harder and more ionizing photons are emitted.
- However, metal-poor stars have more stellar emission, bringing the total amount of efficiency for metal-poor stars to be about equal to that of metal-free stars.
- The total radiative efficiency is about the *same *for the metal-free and metal-poor case!

***The Spectrum of the Near Infrared Background***
- By integrating the volume emissivity, p(,z), over redshift, we attain the intensity spectrum from the early stars.  We assume a constant star formation rate (therefore attaining a “time-averaged” star formation rate).

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