Table Of ContentGalaxy Formation and Evolution
in the Cold Universe
Andrew W. Blain
California Institute of Technology, Astronomy 105-24, Pasadena, CA 91125, USA
1 Introduction
Thispartoftheproceedingsdiscussesourknowledgeofthewaythatgalaxies
formed,andtheinformationthatwecangainabouttheprocessbyobserving
the radiation from the cold components of the Universe. The physical pro-
cesses involving the molecular and atomic gas and solid-phase components
of the interstellar medium (ISM) in galaxies are discussed in great detail in
the other contributions. Here we will concentrate on the continuum emission
processes that are relevant to studies of distant galaxies, mentioning briefly
the importance of molecular studies. Little new ground will be broken in de-
scribing the processes at work as galaxies form. This is an extremely large
and active research area, and one that has been described comprehensively
elsewhere, including at this school in past years, sometimes touching upon
the cold Universe theme [183]. Some current issues in the study of galaxy
formation at long wavelengths will be raised, and the detailed physics and
chemistry being discussed alongside [84,103] will be put into the context of
our rapidly developing observationally driven understanding of the way in
which galaxies formed. In addition, the discussion will be focused on how
observationsofthethermalemissionfromtheconstituentsoftheUniverseat
temperatures from a few 10’s to a few 100’s of degrees are moving into the
mainstream of observational cosmology, and the illumination of some of the
directions that these studies will take in the years ahead.
There are two main ways to probe the distant cold Universe. First, ob-
servingtheradiationfromgalaxiesatmid-infrared(mid-IR)andlongerwave-
lengths (λ>10µm) that is emitted by atomic and molecular gas and grains
of dust at temperatures of order 10−1000K. Line emission from molecular
andatomicgasplaysanimportantroleincoolingdensegascloudsintheISM,
although littleobservational information isavailableaboutthesephenomena
earlyinthehistoryoftheUniverse.Intermsofbolometricluminosity,thermal
emission from dust is certainly the dominant process at far-IR wavelengths
outtomoderateandhighredshifts,z (cid:1)1−2andz >2−3respectively.Unless
extreme redshifts are being investigated, the emission from stars makes only
a very small contribution to the signal detected at wavelengths longer than
10µm. Non-thermal synchrotron emission from high-energy electrons is also
unlikely to be significant, apart from in very rare powerful radio-loud active
galactic nuclei (AGN). Dust grains, the solid-phase of the ISM, and atomic
2 Andrew W. Blain
andmolecularISMgasallradiateatwavelengthsbetweenafewmicronsand
severalmillimetres.GrainsareheatedradiativelybyopticalandUVradiation
from young stars and AGN. They could also be excited by mechanical en-
ergy deposited by shocks generated by interactions between galaxies, clouds
within galaxies and supernova, but this is likely to provide only a very small
fractional correction to the luminosity inferred from radiative heating.
Second, observing the effects of absorption by neutral and low-excitation
gas along the line of sight to distant luminous objects. While this is pre-
dominantly a technique for optical observers, absorption line measurements
of molecules backlit by intense radio sources have been made, revealing very
detailedinformationaboutthepropertiesoftheISMingalaxiesatmoderate
redshift [268]. As the simultaneous spectral coverage of submillimetre-wave
instruments increases and their sensitivity steadily improves, absorption line
studies offer to become an important part of the armoury for studying the
cool parts of the Universe.
Additional probes can be provided by exploiting features that are visible
against the Cosmic Microwave Background (CMB) radiation at millimetre
wavelengths by scattering along the line of sight. Fluctuations in the CMB,
both spatial and spectral, typically at the 10−5 → 10−4 level, are critical
probesofcosmology.SpatialanisotropiesintheCMBemissionencodedirect
evidenceaboutthegalaxyformationprocess.Primordialfeaturesinthespec-
trum of the CMB trace activity in the early Universe, while foreground ob-
jects,whichpotentiallyincludetheveryearliestgalaxiesandclustersofgalax-
ies, can impose a variety of detectable characteristic ‘secondary’ anisotropies
to the CMB signal. Both primary and secondary features of the CMB are
already being detected. The most prominent is the Sunyaev–Zeldovich (SZ)
effect [255,70], produced by electrons at X-ray-emitting temperatures, of or-
der 108K/10keV, which has been clearly detected along the line of sight to
clustersofgalaxies,andmaydevelopintoanimportantprobeoflessmassive
structuresathighredshifts.TheintensityoftheCMBincreaseswithredshift
z as (1+z)4, exactly counteracting the reduction in surface brightness with
increasing redshift that affects all cosmological observations.
Direct observations of the submillimetre and far-IR emission from high-
redshiftgalaxiesareamongstthemostrecenttechniquestobeappliedtothe
studyofgalaxyformation,beginningin1997[57].Thisislargelybecausethe
technology to detect distant galaxies has matured later at submillimetre to
mid-IR wavelengths than at radio and optical wavelengths [215]. The results
discussed are generally all less than 5 years old, and so most of the relevant
informationwasgatheredsincetheextremelydeepopticalHubbleDeepField
(HDF) image was taken in 1995 [269]. Hopefully, the descriptions presented
here will remain valid for many years to come; however, this is a young, very
activefield,andsomeoftheresultsmaybeoverturnedwhenmoredata,secure
multiwavelengthgalaxyidentifications,andmoretimetocontemplatethere-
sults are available. Observational capabilities continue to develop rapidly, in
Galaxy Evolution 3
almost all the windows into the distant Universe. Some of the most relevant
developments outside of the far-IR/submillimetre sphere are at radio, near-
IR, optical and X-ray wavelengths. At radio wavelengths, the Very Large
Array (VLA) will soon be upgraded to provide significantly improved sensi-
tivityandresolution,anddrasticallyimprovedspectroscopiccapabilities.The
UK-basedMERLINradiointerferometerisalsobeingupgradeddramatically,
and the Indian Giant Metrewave Radio Telescope (GMRT) is being commis-
sioned with a very large collecting area at the longest radio wavelengths.
The number of 8-m-class optical/near-IR telescopes in service is increasing
steadily, and full suites of instruments are being delivered for them. Wide-
field multi-object spectroscopic capability at the Keck, Gemini, Subaru and
VLT telescopes are on the verge of generating a new level of detail in deep
galaxy redshift surveys, while the shallower, wide-area 2-degree-field (2dF)
redshift survey is complete with about 230,000 redshifts [83], and the Sloan
Digital Sky Survey (SDSS) [58] is well underway, generating galaxy redshifts
at the rate of over 105yr−1. The Chandra X-ray observatory has provided
the first arcsec-resolution images of the very opposite of the cold Universe,
andtogetherwiththegreatercollectingarea,butcoarserresolutionprovided
bytheXMM-Newton telescope,bothofferexcellentspectroscopicimagingof
the high-energy Universe.
Allofthesefacilitieswillhaveanimpactonourunderstandingofthedis-
tant Universe and the genesis of galaxies. The importance of observing over
as wide a range of wavelengths as possible, and the ways in which existing
observations and theories can be extended and improved by adding informa-
tionfromthecoolerpartsofgalaxies,willbeemphasized.Inthefinalsection
wewillreportondevelopmentsininstrumentationthatshouldleadtoaclear
high-resolution view of the formation of galaxies in the years ahead.
The other parts of this book describe the details of the radiation pro-
cesses that take place in the ISM. When studying galaxy evolution at long
wavelengths, it is generally difficult to make very detailed observations, both
because of the restrictions of spatial resolution, and due to there being only
afewfluxdensitymeasurementsavailablefromanentiredistantgalaxy.Itis
thereforeimportanttorememberthatextragalacticastronomyatthermalIR
to millimetre wavelengths lacks the precision of studies of molecular clouds
in the Milky Way and nearby galaxies.
Apracticallong-wavelengthlimittothespectralwindowinwhichgalaxy
evolutioncanbeprobedbyobservingtheISMisimposedbytheCMB,which
dominatestheenergydensityinradiationintheUniverseatthepresentepoch
by about two orders of magnitude. As the total energy output of galaxies
appears to be declining steadily with cosmic time at present, and it is likely
thatevenastheUniverseexpandsfurtherandredshiftsawaytheCMBenergy
density, CMB photons will not only remain the most numerous photons in
the Universe but will also always dominate the energy density in the diffuse
intergalactic cosmic radiation field, away from individual galaxies.
