High redshift galaxies

• Because of studies of local stellar pops, we anticipate that we really have to go to high redshift, say $z>2$, before we can really see galaxy formation, at least of current massive objects. Three possible ways of finding these have been tried:
  • look for faint objects (consider distance modulus as f(redshift)),
  • look for ly$\alpha$ emission,
  • look for galaxies with strong Ly breaks (from intrinsic stellar, gas within galaxy, intergalactic gas).

• Latter has been most successful to date. Why?
  • too many relatively nearby FBG's,
  • presence of dust strongly reduces ly alpha because resonant scattering makes ly alpha path lengths much longer and consequently much more prone to absorption by dust,
  • presence of intergalactic H makes dropout present independent of galaxy spectrum, presence of interstellar H within galaxy.

• Ly break method (steidel et al AJ 110, 2519, Fig 1 and Fig 2; HDF Madau et al Fig 7).
  • Initial test: look around lines of sight to QSOs which show intervening absorption: find several objects within a few arcsec with colors consistent for object at z of absorption feature (Steidel et al AJ 110, 2519).

• Lyman break galaxies: early results
  • Keck spectra: Steidel et al ApJ 462, L17 Fig 1. Many objects currently confirmed, with $>70\%$ success rate (poss as high as 90% and essentially no contaminators.
  • Colors consistent with moderately unreddened models of star forming galaxies, with limits on reddening corresponding to $E(B-V)<0.3$; with K magnitudes, colors somewhat redder than instanteous bursts, possibly giving age as much as 1Gyr. Discovery of these high z galaxies at all suggests that dust may not be an important limiting factor.
  • Stellar absorption lines generally weaker than those of present day starbursts, possibly suggesting lower metallicity. Line widths still uncertain, but very important!
  • SF rates can be estimated from UV flux and give 4-75 solar masses per year for various cosmologies.
  • Surface densities around $0.4 arcmin^{-1}$, making these objects about 1.3% of all objects to R=25 and 2% of objects in range $23.5<r<25$.
  • Space density, asssuming uniform sampling over redshift range is about 0.1-0.5 space density (lower limit) of current galaxies with $L>L^*$.

  • HST imaging resolves these objects (pictures from Giavalisco et al ApJ 470, 189).
    • Radial profiles (Giavalisco et al fig 2).
    • Objects are compact, cores $<0.7 arcsec$, with some exceptions. At $z=3.25, 1.5 arcsec = 10.5 h_{50}^{-1} (18 h_{50}^{-1}) kpc$ for $q_0=0.5 (0.05)$. half light radii of a few kpc.
    • In general, morphologies show less dispersion in morphology than medium redshift galaxies. However, beware surface brightness dimming $(1+z)^4$. Perhaps high z objects are SF in objects which can retain gas, while intermediate z objects are SF in lower mass objects. Maybe high z objects are bulges of current day galaxies. But maybe they're just components which will merge to form current day spheroids.

• More recent surveys/results:
  • GOODS, ACS imaging in HDF-N and CDF-S ($316 arcmin^2$) in $B_{435}$, $V_{606}$, $i_{775}$, $z_{850}$
    • 1409 U-dropouts at $z\sim 3.1$, 2440 B-dropouts at $z\sim 3.8$, and 845 V-dropouts at $z\sim 4.9$.
    • morphological analysis, e.g. Ravindranath et al ApJ 652, 963 (2006
      • more than half have profiles fit well by Sersic
      • large fraction of exponential-like systems (40% exponential, 30% even less centrally concentrated, 30% de Vaucouleurs,)
      • Even high-n systems have larger ellipticities than local galaxies
      • ellipticity distribution suggests that it's not just disks: filaments or bars?
  • HST UDF: ACS imaging ($11.5 arcmin^2$), F435W ($B_{435}$), F606W ($V_{606}$), F775W ($i_{775}$), and F850LP ($z_{850}$)
    • Deeper (probes lower luminosities), but smaller field
    • Number counts fairly well fit by Schecter functions, e.g. Beckwith et al Fig 15; if we associate these with present day galaxies, implies strong evolution in number density (but see also Bouwens et al - if significant numbers of galaxies aren't being missed!
    • Range of morphologies, e.g. Elmgreen et al, Fig 1. Star formation only in disks?
  • COSMOS: multiwavelength, wide field (2 degree square) survey, ground-based plus HST imaging + Xray + radio

• Global SF history extended to higher redshift - the ``Madau plot''. Discussion of advantages of using heavy element formation rate as relative estimator of SF rate which is less sensitive to possible variations/uncertainties in IMF and also insensitive to cosmological model
  • Madau et al Fig 9.
  • More recent results by Steidel et al from ground based higher Z studies (ApJ 519, 1 Figs 8 and Fig 9).
  • UDF results
  • Can one account for present day density of heavy elements from derived history of metal production? Can one get enough production at early enough times for an early epoch of formation of spheroids?

• Limitations of using LBGs as a probe of galaxy formation: what objects are being missed?

• If one considers strongly star forming galaxies, one expects significant dust emission (c.f. the nearby ultraluminous IR galaxies). At higher redshift, one would look in the submm for the dust emission. In fact, observable submm wavelengths are longward of peak of dust emission in galaxies, which gives a positive K-correction, potentially allowing observations out to very high redshift (Smail et al Fig 1).
  • Initial 2D instrument used for submm surveys was SCUBA on JCMT observes at 450 and 850 microns, and many sources ($>$300) have now been detected; this is an active area with lots of instrument development (note ALMA).
  • Converting dust emission to star formation rate suggests very high star formation rates and that these objects are responsible for a significant fraction of star formation at high redshift. Space density comparable to that of present day ellipticals, although association with these is by no means clear.
  • Unfortunately, beam of SCUBA is 15 arcsec, making optical identification of sources very difficult (Smail et al Fig 6); may be possible to identify objects by looking for corresponding radio emission. There appears to be an association of the submm sources with high redshift objects, however, although this conclusion is certainly not completely secure. If submm are at high redshift, they could represent a significant population, perhaps even outnumbering the Lyman break galaxies, and representing an obscured population. This could have significant implications for the high redshift SF rates.
  • Optical redshifts difficult, but appear that significant fraction are in in $z\sim 2-3$ range, e.g. Chapman et al
  • Note implications of ALMA

• Is there a population with intermediate dust content which escapes detecting both by submm and optical searches?