2.4
Star formation rate density evolution
The Madau diagram
A famous constraint on the comoving-volume-averaged star formation rate history which was made before some of the recent discoveries of infrared astron- omy was the Madau diagram or Madau-Lilly diagram (Madau et al. [1996]; Lilly et al. [1996]; Madau et al. [1998]), which suggested that star formation peaked at z ∼ 1 (see Figure 2.3, left) based on semi-analytic hierarchical models20 and
optical/UV observations corrected for dust extinction. However, the discovery of the extent of extragalactic infrared radiation means that we now know that the Madau diagram underestimates star formation at earlier epochs. The latest consensus view (see recent review of cosmic star formation in Madau and Dicken- son [2014]) is that the peak of the star formation rate density occurred at z ∼ 2 (when the universe was about 3 billion years old), and that most of the stellar mass in the present-day Universe was formed between z ∼ 3 and z ∼ 1. A recent Madau diagram from Bouwens et al. [2011] is also shown in Figure 2.3 (right). The decline in star formation rate since then occurs at all masses (Sobral et al. [2014]). A star formation rate function prepared recently using UV luminosity for 4 < z < 7 galaxies found that a Schechter function21 can fit the data well
(Smit et al. [2012]).
Downsizing
The concept of “downsizing”- that the most massive galaxies formed earlier than less massive galaxies - was introduced by Cowie et al. [1996] in a study of 393 galaxies at 0.2 < z < 1.7. This pattern was supported by results from Brinchmann and Ellis [2000] whose study of a sample of 321 galaxies at 0 < z < 1 suggested that the most massive galaxies had formed their stars by z ∼ 1. This picture was confirmed by Juneau et al. [2005], whose sample of 207 galaxies extended back to z ∼ 2. This paper also makes the point that the downsizing of stellar mass assembly is the opposite of the accepted picture for dark matter assembly which is thought to proceed from small mass to large mass units. Dye
20Theoretical models of galaxy evolution are discussed in Section 2.8. 21Schechter functions are described in Section 4.2.3.
2.4 Star formation rate density evolution
Figure 2.3: Evolution of luminosity density: the Madau diagram. Left: The original Madau diagram from Madau et al. [1998] which used data from optically-selected galaxy studies and assumed a Salpeter IMF and dust extinction of E(B-V)=0.1. Right: A recent Madau diagram from Bouwens et al. [2011] with recent candidate discoveries at z ∼ 6, 7 and 10. The inferred star formation rate density is in red/orange and on the left axis; the observed rest-frame UV luminosity density is in blue and on the right axis. A Salpeter IMF was assumed and the conversion from luminosity density to SFR uses a dust extinction correction based on the estimated UV continuum slope.
et al. [2010] investigated the star formation histories of 92 BLAST sources and also found that high mass sources evolved more strongly and peaked at earlier times than low mass sources, which form a much higher fraction of their stellar mass at later times. Downsizing has also been confirmed by luminosity function studies (see Section 4.2.4).
The earliest galaxies, z � 5 (age of universe � 1 Gyr)
The timescale of cosmic reionization is still unknown, but is thought to have been completed by about z ∼ 6−7 (e.g. Zahn et al. [2012] and references therein). Since the installation of the Wide Field Camera 3 (WFC3) on the HST in May 2009, candidate galaxies at redshifts up to z ∼ 10 have been reported regularly. These are usually found by either by the Lyman Break method22(e.g. Oesch et al.
[2014]; Bouwens et al. [2011]), or by fitting an SED using multi-wavelength broad- band photometry, and calculating the probability of a high-redshift fit. These
22The Lyman limit (912 ˚A) is redshifted into optical bands at z > 3 and galaxies identified by
this method, the so-called Lyman Break Galaxies (LBGs), are currently the largest population of star-forming galaxies identified at very high redshift.
2.4 Star formation rate density evolution
candidates are mostly too faint for confirmation with detailed emission-line spec- troscopy. Finkelstein et al. [2013] recently reported identifying a Lyα emission line in a Lyman Break galaxy at z = 7.51. No candidate at z > 7.6 has yet been spectroscopically confirmed. Several major surveys are now underway to improve this situation. The Hubble Space Telescope Frontier Fields project was discussed in Section 2.3. Another deep survey is the Brightest of Reionizing Galaxies survey (BoRG; Trenti et al. [2011]). Another is the eXtreme Deep Field (XDF) in the Hubble Ultra-Deep Field, combining deep observations already taken with many previous surveys (Illingworth et al. [2013]).
Table 2.2 shows look-back time, the age of the universe, and the estimated peak of infrared flux from starburst galaxies for various redshifts, using the current concordance cosmological assumptions (see Section 2.9).
Galaxies at 5 � z � 3 (age of universe ∼ 1 to ∼ 2 Gyr)
Most galaxies at this distance have been identified with the Lyman Break tech- nique, as with higher-redshift sources. This tends to miss less-luminous sources: a search for Lyα emission in a field with LBGs found several times the num- ber of sources at z ∼ 3 (Steidel et al. [2000]). Most galaxies observed at high redshift are forming stars at a high rate, often hundreds of M⊙ per year, which contrasts with galaxies in the local universe where such high SFRs are rare (this was also found in the work in Chapter 3). A recent HerMES study found that star formation at z > 4 is significantly higher than predicted by models, finding 38 candidate galaxies above this redshift in 21 deg2 using SPIRE data at 250 µm,
350 µm and 500 µm (Dowell et al. [2014]). A study of Lyman Break Galaxies23
with SCUBA-2 (Coppin et al. [2015]) suggests SFRs of 40 - 200 M⊙ per year at 5 > z > 3.
Perhaps surprisingly, many relatively quiescent galaxies already exist at this early period, as shown in a recent major study of 14,200 galaxies in the COSMOS field at redshifts up to z ∼ 3 (Man et al. [2014]). Another recent paper which se- lected galaxies by fitting SEDs (using line breaks and bumps rather than emission
23Newly discovered populations of galaxies are usually called after the wavelength range of
discovery, such as radio galaxies, Ultra Luminous Infrared Galaxies (ULIRGs), Lyman Break Galaxies and submillimetre galaxies, and it is often some time before the connection between these newly discovered populations and other known populations becomes clear.
2.4 Star formation rate density evolution
Table 2.2: Lookback time and the age of the universe and the estimated far- infrared SED peak (assuming 100 µm peak at z = 0) by redshift. Assumptions: Ho= 72 km s−1Mpc−1, ΩM= 0.3, ΩΛ= 0.7.
Redshift 0 0.2 0.4 0.6 0.8 1.0 2 3 4 5 10
Lookback time / Gyr 0 2.4 4.2 5.6 6.6 7.5 10.0 11.0 11.6 12.0 12.6 Age of universe / Gyr 13.1 10.7 8.9 7.5 6.5 5.6 3.1 2.1 1.5 1.1 0.5 Est. FIR peak /µm 100 120 140 160 180 200 300 400 500 600 1100
lines) found a quiescent population (57 sources) of dusty galaxies at this redshift (Spitler et al. [2014]). A population of massive, quiescent galaxies at z > 4 has recently been reported in Straatman et al. [2014]. These results suggest an earlier obscured phase of rapid star formation.
Chapter 7 presents the detection of submillimetre-selected galaxies at z > 3.5 identified by their Hα emission lines. These comprise two radio galaxies and two submillimetre galaxies associated with one of them.
The role of galaxy mergers in star formation
Galaxy mergers are relatively common. Major mergers were once thought to be a significant cause of star formation, but recent work suggests that though they may well trigger starbursts, they do not make a dominant contribution to the total star formation rate density (e.g. Rodighiero et al. [2011]; Kaviraj et al. [2013]). Cold gas accretion and minor mergers, rather than major mergers, are now thought to be the main processes driving the evolution of star-formation.
A major new study (Schreiber et al. [2015]) from the GOODS-Herschel pro- gram combined with the CANDELS-Herschel program across both PACS and SPIRE wavelengths, using data selected primarily from Hubble’s ultra-deep H- band catalogues, found 10,497 sources (and, with stacking, 62,361 sources) in the Herschel data. They showed that most star formation is secular, i.e. at a rela- tively low, steady rate, corresponding to main-sequence mode (see Section 2.6), and that major mergers have had relatively little impact in triggering starbursts since z = 4. The Herschel data reached∼ 10 mJy at 250 µm. A study of a typical star-forming galaxy at z = 2.3 (Bouch´e et al. [2013]) found that the gas accretion rate was comparable to the star formation rate (∼33+40−11 M⊙ yr−1), supporting
2.5 Star formation and AGNs
Figure 2.4: The AGN unification model. Viewed along the jet axis, we observe blazers; perpendicular to the jet axis, we observe a morphology typical of radio galaxies. At an orientation of ∼30o we see Seyfert 1s and quasars; at larger angles, Seyfert 2s as the Broad Line Region is obscured by the torus. Figure from Torres and Anchordoqui [2004].
the idea that continuous accretion of gas plays the major role in galaxy growth.