Galaxies grow by accreting gas from the intergalactic medium (IGM), and through mergers
of smaller progenitors. Cosmological hydrodynamic simulations suggest that the former process, the infall of gas into galaxies, dominates the growth for most galaxies throughout most of their history, whereas mergers (infall of entire galaxies) are more important for the most massive galaxies, and at late times.
The in-falling gas is generally very dilute (with its density a small multiple of
the mean density of the universe) and highly ionized (at temperatures
of several 10,000 K), so any radiation emitted by it will be very weak. So far,
the inflow of gas has been mainly detected in absorption against bright background
light sources like QSOs (see the research described in the section on using metals in the IGM as tracers of infalling gas).
The argument, that particular absorption features arise in gas flowing into galaxies is based on a statistical comparison between the actually observed properties of these
absorption lines and absorption lines produced in simulations of forming galaxies.
Because each galaxy can only be probed by one, or, at most, a few passing lines of sight (using gravitationally lensed QSOs or close QSO pairs; see
the section on the IGM on small scales ), each galactic environment is only very sparsely sampled.
The geometry and the physical detail of the inflow process can only be studied
with a more detailed two or three-dimensional view of the spatial distribution of the gas and stars. To achieve a sufficiently close sampling, we need to observe the gas in emission.
For a number of years my collaborators and I have been obtaining very deep
long slit spectroscopy of "blank" regions of the sky to detect faint emission, chiefly by
the hydrogen Lyman alpha line, from the gas surrounding high redshift galaxies. The spectra where taken in regions where previous
surveys with the Hubble Space Telescope (HST) had obtained very deep
imaging data in various broad band colors. The images mostly contain information
about the spatial distribution, luminosities and colors of the stars in those high redshift
galaxies. Our spectroscopic data add measurements of the spatial distribution, and velocity of the gas surrounding the galaxies, as far as they can be derived from
the properties of its emission lines.
These projects have led to a number of interesting and partly unexpected results:
Most galaxies appear to be surrounded by a symmetric "glow", a halo of Lyman alpha
line emission, but there seems to be a subset of galaxies where the emission is
very disturbed, spatially anisotropic, and much more extended (see the figure).
Spectra of two faint, disturbed Lyman alpha emitters (left column), and two
standard Lyman alpha halos from previously known Lyman-break-selected galaxies (LBG; right column),
all of which were (re-)discovered serendipitously on the spectrograph slits of our blind surveys. The dispersion direction is horizontal, the spatial direction vertical.
Even though the Lyman alpha emission from the Lyman break galaxies is much stronger (due
to their stronger star formation), the two messier objects on the left appear more extended for a given surface brightness, and spatially asymmetric. In the latter cases, anisotropic flows of gas may facilitate the escape of Lyman alpha photons in certain directions. Alternatively, their gaseous halos may contain cracks or holes through which ionizing radiation from the stars can escape, causing Lyman alpha fluorescence in the more distant gas (from Rauch et al 2013).
Looking at the extended Lyman alpha emitters in more detail, we seem to be able to
discern a number of different physical scenarios where Lyman alpha emission occurs.
Among those there is at least one case where several strands of evidence suggest that
we are indeed seeing the accretion of gas onto a galaxy in action.
A redshift 3.3 galaxy illuminating its own accretion stream
Probably the first direct observation of gas accretion by a high redshift galaxy, the figure below shows a situation, where gas moving toward a redshift 3.3
galaxy appears to be illuminated by ionizing radiation escaping from that galaxy.
The discovery spectrum of a peculiar extended Lyman alpha emitter (left panel),
and two HST images (V and I band) of the same region (center and right panel).
The left panel shows that
a highly asymmetric Lyman alpha emission region (dark patches) can be traced in the spatial (vertical direction) out to about 35 kpc from the galaxy, the position of which is indicated by a horizontal dotted line.
The HST images of the same region reveal a peculiarly shaped galaxy offset to the edge of the nebula.
In the central panel the outlines of the blue and red component of the Lyman alpha nebula are overplotted (Rauch et al 2011, 2016).
A closer study of the velocity structure of the above system suggests that the Lya emitting gas is moving toward the galaxy from the backside. The illumination of the nebula appears to be provided
by escaping ionizing radiation, a conclusion supported by the large extent of the nebula,
the strong anisotropy,
and the presence of apparently gaseous tentacles, connected to a hotspot of star formation
at one end of the galaxy.
The galaxy exhibits tentacles that originate from a bright spot of star formation and extend for 5 kpc into the Lyman alpha emitting region. The occurrence of the tentacles
in two different filters shows that its light is dominated by continuum emission, most
likely cooling or recombination radiation that accompanies the emission of Lyman alpha
photons.
How do we know that we are really seeing accretion at work ? Cosmological hydro-simulations predict that galaxies are fed by bundles of thin, cold streams of gas,
that descend onto the existing galactic body in some fashion. At the point of impact,
large amounts of gas cool rapidly and turn into stars that produce copious amounts
of ionizing radiation and stellar winds. Smaller existing galactic halos may fall
in from the cosmic web together with the gas and disturb or punch through the accreting
galaxy, creating holes in its gas distribution and possibly pulling out some of the
gas and stars from the main galaxy.
This plot shows an image from a cosmological hydro-simulation by Agertz et al (top),
and the various features in our observed system corresponding to certain predictions
of the cold accretion scenario. If this interpretation is correct, the asymmetric Lyman alpha nebula should correspond to the incoming cold streams, the tentacles to the pinched
streams descending onto the galaxy. The accretion feeds a hot-spot of star-formation
where it enters the galaxy, that, in our case, appears to leak ionizing radiation up-stream, illuminating the incoming filaments in Lyman alpha.
The presence of young, recently formed stars is also indicated by the galaxy exhibiting
a broad HeII 1640 emission line (here not shown).
At least one faint satellite galaxy can be seen to have recently passed by (or through)
the main object, which is another finding consistent
with the assumption that this is a cosmic web filament accreting onto a high z galaxy.
