Age-redshift relation. The time since the big bang depends on the cosmological parameters.

Transcription

1 Age-redshift relation The time since the big bang depends on the cosmological parameters.

2 Lyman Break Galaxies High redshift galaxies are red or absent in blue filters because of attenuation from the neutral intergalactic medium. A galaxy which is visible at infrared, but not optical, wavelengths could be a high redshift galaxy.

3 Lyman Alpha Emitters Lyman alpha emitters are galaxies where the dominant observed feature is the Lyman alpha emission line at 1216 Angstroms (121.6 nm). A single emission line in the infrared with no continuum shortward and weak continuum longward of the line could be a high redshift galaxy. We can probe to greater distances by looking in the direction of massive lensing clusters.

4 Submillimetre Galaxies Submillimetre galaxies are the most luminous galaxies in the Universe in terms of total energy output. Most of their radiation is thermal emission from dust heated by OB stars. As the redshift increases, we are moving further up the peak of the spectral energy distribution, so the galaxies become brighter!

5 GN-z11 The most distant galaxy discovered is GN-z11 in the Hubble Ultra deep Field at redshift z=11.9, about 600 million years after the Big Bang. GN-z11 has about 1%, the stellar mass of the Milky Way and is forming stars at about 20 times the current rate. The estimated stellar age is about 40 million years, about 10% of the age of the Universe in the rest frame of the galaxy.

6 Growth of fluctuations Baryonic fluctuations are damped or oscillate Dark matter fluctuations continue to grow between matter-radiation equilibrium and recombination After recombination the baryonic material falls into the gravitational potential wells generated by the dark matter

7 Minimum mass needed to collapse The solid diagonal line represents the average fluctuation mass for dark matter fluctuations as a function of redshift The upper envelope to the shaded area represents the minimum mass that will collapse as a function of redshift Tegmark et al ApJ

8 Fluctuation amplitude distribution The first peeks to collapse are maxima on several scales

9 Molecular hydrogen cooling Masses greater than 10 6 solar masses. Molecular hydrogen from interactions between H and H -, not on dust grains like in galaxies. The H - is made from the small number of electrons left over from recombination. Inefficient because H - is rare. the arrows represent an equilibrium heavily shifted towards the right

10 Atomic cooling Once the mass scale of the most massive fluctuations becomes greater than 10 8 solar masses, atomic hydrogen(the red line) dominates over molecular hydrogen (the blue line) as a cooling mechanism. The difference in cooling rate between the two mechanisms is greater than three orders of magnitude and consequently any relic signatories of the very first galaxies formation is quickly erased since the fluctuation mass spectrum is a steep function of redshift. Barkana & Loeb (2001)

11 Cooling by metals In present-day galaxies, the dominant cooling mechanism is metallic cooling. Metals are much better than hydrogen at losing energy because so much energy is transferred when a metal atom is cooled to its ground state. Once sufficient metals have been generated by the first one or two generations of stars, galaxy formation can begin in earnest. Even trace amounts are sufficient because metals are such good coolants. This probably happened by the time GN-z11 started forming stars. Conti et al. 1996

12 Population II metalicity distribution iron abundance [Fe/H] = log(ratio of densities of iron and hydrogen relative to the Sun) [Fe/H] = 0 represents solar metalicity. The lowest metalicity stars in the halo and bulge of the Milky Way have abundance 1/100 solar. This is too much have been generated by Population III before there were enough metals to generate significant metallic cooling. M31

13 Carbon enhanced metal poor stars A small number of stars in the Milky Way can be discovered with [Fe/H] < -5.0 All of these are carbon enhanced stars with [C/Fe] greater than 10 times solar These could be generated by anomalous feedback during star formation driving differential amounts of different elements towards the surface. Unlikely that this could be Population III because carbon enhanced metal poor stars are frequently overabundant in s (slow) process elements. Salvadori et al. 2015

14 Stellar initial mass function The stellar initial mass function is the number of stars between mass m and m+dm. It is poorly constrained that masses about one solar mass in Milky Way open clusters and unconstrained in globular clusters. Any relic or signature of Population III will have features that depend on the most massive stars as these contribute most energy on short timescales. Bastian et al. 2010

15 Population III simulations Simulation of star formation from zero metalicity material indicates massive stars form. Massive low metalicity stars experience rapid mass loss. Stellar winds can trigger convective mixing and alters surface abundances. Stars much greater than 150 solar masses have very short dynamical times and never reach the main sequence. Pair production during causes an immediate collapse in the most massive stars.

16 Lyman alpha forest metalicities The line between quasars and the Earth passes through many intergalactic medium clouds. Each cloud absorbs blue continuum from the quasar at a wavelength 1216 A multiplied by (1+z) where z is the redshift of the cloud. Every cloud also absorbs red continuum from the quasar at a wavelength equal to the rest frame wavelength of each metal feature like C[IV] 1548 multiplied by (1+z). The metalicity of the neutral IGM is obtained by Cross-correlating the positions of all these small absorption features on the red side of the quasar Lyman alpha line with the Lyman alpha forest on the blue side. The neutral IGM has a typical metalicity of 0.02 solar at high redshift.

17 Damped Lyman alpha systems metalicities Damped Lyman alpha systems represent massive accumulations of neutral gas at high redshift that act as reservoirs for star formation. Pettini 2011

18 Gamma ray bursts Every few days the normally-bright gamma-ray sky flares up in a bright flash. Gamma Ray Bursts are energetic explosions at extragalactic distances. These are the most energetic explosions in the Universe. A GRB releases as much energy in a few seconds as the Sun in 10 billion years. The most energetic GRBs last longer than one second and hypothesized to form as a rapidly rotating supernova collapses to form a black hole. After the initial flash of gamma rays, an afterglow is detected at longer wavelengths from x-ray to radio.

19 GRB supernova GRB showed evidence for an optical supernova immediately following the gamma ray burst. The progenitor star mass was 20 solar masses, large enough to generate a black hole. This is consistent with GRBs being the result of a shock breakout during massive star collapse.

20 GRB GRB was one of the most energetic gamma ray bursts. Within one minute of the burst a 9th magnitude optical flash was observed at a rapid response telescope in New Mexico. An amateur astronomer reported observing the flash with binoculars. The burst was at redshift z = 1.6, so the absolute magnitude was M = This was the most optically luminous object observed. A number of luminous prompt optical afterglows have since been observed, for example the naked eye burst GRB B. These bursts would be luminous enough to detect from Population III stars exploding. The optical emission would be redshifted into the mid-infrared and would be time-dilated.

21 Stellar rotation at low metallicities A rapidly forming massive star with no metals will not have an appreciable stellar wind so rotation braking will be negligible. If fast spin rates are a common feature of core collapses that generate a GRB, these explosions may be a common feature of Population III. (The host galaxy of GRB looks similar to low metallicity dwarf galaxies)

22 L M t/s

23 Redshift z