1,576 research outputs found
Resurrection of Traditional Luminosity Evolution Models to Explain Faint Field Galaxies
We explore the nature of the evolution of faint field galaxies by assuming
that the local luminosity function is not well defined. We use a non-negative
least squares technique to derive a near optimal set of local luminosity
functions for different spectral types of galaxies by fitting to the observed
optical and near-infrared counts, B-R colors, and redshift distributions for
galaxies with 15 < B < 27. We report here the results of using only traditional
luminosity evolution (ie. the photometric evolution of stars in a galaxy over
time given reasonable assumptions of the form of the star formation history for
various galaxy types), and simple galaxy reddening and find excellent fits to
the observed data to B = 27. We conclude that models more exotic than
traditional luminosity evolution are not yet required to explain existing faint
field galaxy data and thus the need for contributions by mergers or new
populations of galaxies is at least 5x less than previously estimated.Comment: 9 pages + 1 table + 4 figures; uuencoded tar compressed postscript;
to be published in The Astrohysical Journal Letter
Evidence for Spatially Compact Lyman Alpha Emission in z=3.1 Lyman-Alpha-Emitting Galaxies
We present the results of a high-spatial-resolution study of the line
emission in a sample of z=3.1 Lyman-Alpha-Emitting Galaxies (LAEs) in the
Extended Chandra Deep Field-South. Of the eight objects with coverage in our
HST/WFPC2 narrow-band imaging, two have clear detections and an additional two
are barely detected (~2-sigma). The clear detections are within ~0.5 kpc of the
centroid of the corresponding rest-UV continuum source, suggesting that the
line-emitting gas and young stars in LAEs are spatially coincident. The
brightest object exhibits extended emission with a half-light radius of ~1.5
kpc, but a stack of the remaining LAE surface brightness profiles is consistent
with the WFPC2 point spread function. This suggests that the Lyman Alpha
emission in these objects originates from a compact (<~2 kpc) region and cannot
be significantly more extended than the far-UV continuum emission (<~1 kpc).
Comparing our WFPC2 photometry to previous ground-based measurements of their
monochromatic fluxes, we find at 95% (99.7%) confidence that we cannot be
missing more than 22% (32%) of the Lyman Alpha emission.Comment: 4 pages, 3 figures, accepted to ApJ letter
A Population of Metal-Poor Galaxies with ~L* Luminosities at Intermediate Redshifts
We present new spectroscopy and metallicity estimates for a sample of 15
star-forming galaxies with redshifts in the range 0.29 - 0.42. These objects
were selected in the KPNO International Spectroscopic Survey via their strong
emission lines seen in red objective-prism spectra. Originally thought to be
intermediate-redshift Seyfert 2 galaxies, our new spectroscopy in the far red
has revealed these objects to be metal-poor star-forming galaxies. These
galaxies follow a luminosity-metallicity (L-Z) relation that parallels the one
defined by low-redshift galaxies, but is offset by a factor of more than ten to
lower abundances. The amount of chemical and/or luminosity evolution required
to place these galaxies on the local L-Z relation is extreme, suggesting that
these galaxies are in a very special stage of their evolution. They may be
late-forming massive systems, which would challenge the current paradigm of
galaxy formation. Alternatively, they may represent intense starbursts in
dwarf-dwarf mergers or a major infall episode of pristine gas into a
pre-existing galaxy. In any case, these objects represent an extreme stage of
galaxy evolution taking place at relatively low redshift.Comment: 6 pages, 3 figures; to appear in 10 April 2009 ApJ
Comparing Infrared Star-Formation Rate Indicators with Optically-Derived Quantities
We examine the UV reprocessing efficiencies of warm dust and polycyclic
aromatic hydrocarbons (PAHs) through an analysis of the mid- and far-infrared
surface luminosity densities of 85 nearby H-selected star-forming
galaxies detected by the volume-limited KPNO International Spectroscopic Survey
(KISS). Because H selection is not biased toward continuum-bright
objects, the KISS sample spans a wide range in stellar masses
(-), as well as H luminosity
(-), mid-infrared 8.0m luminosity
(-), and [Bw-R] color (-.1-2.2). We find that
mid-infrared polycyclic aromatic hydrocarbon (PAH) emission in the Spitzer IRAC
8.0m band correlates with star formation, and that the efficiency with
which galaxies reprocess UV energy into PAH emission depends on metallicity. We
also find that the relationship between far-infrared luminosity in the Spitzer
MIPS 24m band pass and H-measured star-formation rate varies from
galaxy to galaxy within our sample; we do not observe a metallicity dependence
in this relationship. We use optical colors and established mass-to-light
relationships to determine stellar masses for the KISS galaxies; we compare
these masses to those of nearby galaxies as a confirmation that the
volume-limited nature of KISS avoids strong biases. We also examine the
relationship between IRAC 3.6m luminosity and galaxy stellar mass, and
find a color-dependent correlation between the two.Comment: 15 pages, 10 figure
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