711 research outputs found
The Evolving Faint-End of the Luminosity Function
We investigate the evolution of the faint-end slope of the luminosity
function, , using semi-analytical modeling of galaxy formation. In
agreement with observations, we find that the slope can be fitted well by
, with a=-1.13 and b=-0.1. The main driver for the evolution
in is the evolution in the underlying dark matter mass function.
Sub-L_* galaxies reside in dark matter halos that occupy a different part of
the mass function. At high redshifts, this part of the mass function is steeper
than at low redshifts and hence is steeper. Supernova feedback in
general causes the same relative flattening with respect to the dark matter
mass function. The faint-end slope at low redshifts is dominated by field
galaxies and at high redshifts by cluster galaxies. The evolution of
in each of these environments is different, with field galaxies
having a slope b=-0.14 and cluster galaxies b=-0.05. The transition from
cluster-dominated to field-dominated faint-end slope occurs roughly at a
redshift , and suggests that a single linear fit to the overall
evolution of might not be appropriate. Furthermore, this result
indicates that tidal disruption of dwarf galaxies in clusters cannot play a
significant role in explaining the evolution of at z< z_*. In
addition we find that different star formation efficiencies a_* in the
Schmidt-Kennicutt-law and supernovae-feedback efficiencies generally
do not strongly influence the evolution of .Comment: 4 pages, replaced with version accepted to ApJL, minor changes to
figure
Evidence for extended, obscured starbursts in submm galaxies
We compare high-resolution optical and radio imaging of 12 luminous submm
galaxies at z=2.2+/-0.2 observed with HST and the MERLIN and VLA at comparable
spatial resolution, 0.3" (2kpc). The radio emission traces the likely
far-infrared morphology of these dusty, luminous galaxies. In ~30% of the
sample the radio appears unresolved, suggesting that the emission is compact:
either an obscured AGN or nuclear starburst. However, in the majority, ~70%
(8/12), the radio emission is resolved by MERLIN/VLA on scales of ~1" (10 kpc).
For these galaxies the radio morphologies are broadly similar to their
restframe UV emission seen by HST. We discuss the probable mechanisms for the
extended emission and conclude that their luminous radio and submm emission
arises from a large, spatially-extended starburst. The median SFRs are
1700Mo/yr occuring within a ~40kpc^2 region, giving a star formation density of
45Mo/yr/kpc^2. Such vigorous and extended starbursts appear to be uniquely
associated with the submm population. A more detailed comparison of the
distribution of UV and radio emission shows that the broad similarities on
large scales are not carried through to smaller scales, where there is rarely a
one-to-one correspondance. We interpret this as resulting from highly
structured internal obscuration, suggesting that the vigorous activity is
producing wind-blown channels through the obscuration in these galaxies. If
correct this underlines the difficulty of using UV morphologies to understand
structural properties of this population and also may explain the surprising
frequency of Ly-alpha emission in their spectra. [Abridged]Comment: 7 pages, 3 figure
NICMOS Imaging of the Dusty Microjansky Radio Source VLA J123642+621331 at z = 4.424
We present the discovery of a radio galaxy at a likely redshift of z = 4.424
in one of the flanking fields of the Hubble Deep Field. Radio observations with
the VLA and MERLIN centered on the HDF yielded a complete sample of microjansky
radio sources, of which about 20% have no optical counterpart to I < 25 mag. In
this Letter, we address the possible nature of one of these sources, through
deep HST NICMOS images in the F110W (J) and F160W (H) filters. VLA
J123642+621331 has a single emission line at 6595-A, which we identify with
Lyman-alpha at z = 4.424. We argue that this faint (H = 23.9 mag), compact (r =
0.2 arcsec), red (I - K = 2.0) object is most likely a dusty, star-forming
galaxy with an embedded active nucleus.Comment: Accepted for publication in Astrophysical Journal Letters. 11 pages,
4 figures, uses aastex v5.0 and psfi
Compact Lyman-alpha Emitting Candidates at z~2.4 in Deep Medium-band HST WFPC2 Images
Medium-band imaging with HST/WFPC2 in the F410M filter has previously
revealed a population of compact Lyman-alpha emission objects around the radio
galaxy 53W002 at z~2.4. We report detections of similar objects at z~2.4 in
random, high-latitude HST parallel observations of three additional fields,
lending support to the idea that they constitute a widespread population at
these redshifts. The three new fields contain 18 Lyman-alpha candidates, in
contrast to the 17 detected in the deeper exposure of the single WFPC2 field
around 53W002. We find substantial differences in the number of candidates from
field to field, suggesting that significant large-scale structure is already
present in the galaxy distribution at this cosmic epoch. The likely existence
of z~2.4 sub-galactic clumps in several random fields shows that these objects
may have been common in the early universe and strengthens the argument that
such objects may be responsible for the formation of a fraction of the luminous
present-day galaxies through hierarchical merging.Comment: Uses slightly modified AASTeX preprint style file (included).
Contains 22 pages, including 5 figures and 2 tables. Accepted for the
December issue of the Astronomical Journa
"GiGa": the Billion Galaxy HI Survey -- Tracing Galaxy Assembly from Reionization to the Present
In this paper, we review the Billion Galaxy Survey that will be carried out
at radio--optical wavelengths to micro--nanoJansky levels with the telescopes
of the next decades. These are the Low-Frequency Array, the Square Kilometer
Array and the Large Synoptic Survey Telescope as survey telescopes, and the
Thirty Meter class Telescopes for high spectral resolution+AO, and the James
Webb Space Telescope (JWST) for high spatial resolution near--mid IR follow-up.
With these facilities, we will be addressing fundamental questions like how
galaxies assemble with super-massive black-holes inside from the epoch of First
Light until the present, how these objects started and finished the
reionization of the universe, and how the processes of star-formation, stellar
evolution, and metal enrichment of the IGM proceeded over cosmic time. We also
summarize the high-resolution science that has been done thus far on high
redshift galaxies with the Hubble Space Telescope (HST). Faint galaxies have
steadily decreasing sizes at fainter fluxes and higher redshifts, reflecting
the hierarchical formation of galaxies over cosmic time. HST has imaged this
process in great structural detail to z<~6. We show that ultradeep
radio-optical surveys may slowly approach the natural confusion limit, where
objects start to unavoidably overlap because of their own sizes, which only SKA
can remedy with HI redshifts for individual sub-clumps. Finally, we summarize
how the 6.5 meter James Webb Space Telescope (JWST) will measure first light,
reionization, and galaxy assembly in the near--mid-IR.Comment: 8 pages, LaTeX2e requires 'aip' style (included), 8 postscript
figures. To appear in the proceedings of the `The Evolution of Galaxies
through the Neutral Hydrogen Window' conference, Arecibo Observatory Feb 1-3,
2008; Eds. R. Minchin & E. Momjian, AIP Conf Pro
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