1,090 research outputs found
Stellar Populations Found in the Central kpc of Four Luminous Compact Blue Galaxies at Intermediate Redshift
We investigate the star formation history of the central regions of four
Luminous Compact Blue Galaxies (LCBGs). LCBGs are blue (B-V<0.6), compact
(MU_B<21.5 mag arcsec^-2) galaxies with absolute magnitudes M_B brighter than
-17.5. The LCBGs analyzed here are located at 0.436<z<0.525. They are among the
most luminous (M_B < -20.5), blue (B-V < 0.4) and high surface brightness (MU_B
< 19.0 mag arcsec^-2) of this population. The observational data used were
obtained with the HST/STIS spectrograph, the HST/WF/PC-2 camera and the
HST/NICMOS first camera. We find evidence for multiple stellar populations. One
of them is identified as the ionizing population, and the other one corresponds
to the underlying stellar generation.
The estimated masses of the inferred populations are compatible with the
dynamical masses, which are typically 2--10x 10^9 M_sun. Our models also
indicate that the first episodes of star formation the presented LCBGs
underwent happened between 5 and 7 Gyr ago.
We compare the stellar populations found in LCBGs with the stellar
populations present in bright, local HII galaxies, nearby spheroidal systems
and Blue Compact Dwarf Galaxies. It turns out that the underlying stellar
populations of LCBGs are similar yet bluer to those of local HII galaxies. It
is also the case that the passive color evolution of the LCBGs could convert
them into local Spheroidal galaxies if no further episode of star formation
takes place. Our results help to impose constraints on evolutionary scenarios
for the population of LCBGs found commonly at intermediate redshifts.Comment: 35 pages, 10 Figures. Accepted for publication in AJ. Compile with
pdflatex. Contains png figure
Light-Induced Atomic Desorption for loading a Sodium Magneto-Optical Trap
We report studies of photon-stimulated desorption (PSD), also known as
light-induced atomic desorption(LIAD), of sodium atoms from a vacuum cell glass
surface used for loading a magneto-optical trap (MOT). Fluorescence detection
was used to record the trapped atom number and the desorption rate. We observed
a steep wavelength dependence of the desorption process above 2.6 eV photon
energy, a result significant for estimations of sodium vapor density in the
lunar atmosphere. Our data fit well to a simple model for the loading of the
MOT dependent only on the sodium desorption rate and residual gas density. Up
to 3.7x10^7 Na atoms were confined under ultra-high vacuum conditions, creating
promising loading conditions for a vapor cell based atomic Bose-Einstein
condensate of sodium.Comment: Sodium LIAD loaded MOT, 7 pages, 5 figures. Revised submitted
manuscript with minor corrections, new data presented, Fig.5 change
On the Upper Cretaceous age and affinities of Neocyprideis lusitanicus (Ostracoda)
Guernet & Lauverjat (1986) described a new species, Neocyprideis lusitanicus, from sediments deposited near Aveiro, Portugal. For these authors, some associated fossils (Molluscs, planktonic Foraminifera) indicated a Pliocene age. That seemingly
was the first record of Neocyprideis in post-Miocene sediments in Europe.
A recent study of Upper Cretaceous material from the same region showed an abundant Neocyprideis fauna, associated with
Charophyta. These Neocyprideis could be assigned without any doubt to N. lusitanicus. Therefore, N. lusitanicus appears as an Upper Cretaceous species, reworked in much later sediments, not Pliocene but Quaternary, as indicated by the planktonic Foraminifera assemblage. This interpretation is supported by:
1 - the incompatibility of the Neocyprideis (restricted to lacustrine-lagoonal environments) with abundant planktic
Foraminifera;
2 - the occurrence of N. lusitanicus with Charophytes and non marine, cretaceous vertebrates but without the same
Foraminifera.
Neocyprideis lusitanicus is a valid species, clearly different from the other late Cretaceous species (N. coudouxensis and
N. murciensis) as well as the Early Miocene described species (N. aquitanica, N. janoscheki)
On the Maximum Luminosity of Galaxies and Their Central Black Holes: Feedback From Momentum-Driven Winds
We investigate large-scale galactic winds driven by momentum deposition.
Momentum injection is provided by (1) radiation pressure produced by the
continuum absorption and scattering of UV photons on dust grains and (2)
supernovae. UV radiation can be produced by a starburst or AGN activity. We
argue that momentum-driven winds are an efficient mechanism for feedback during
the formation of galaxies. We show that above a limiting luminosity, momentum
deposition from star formation can expel a significant fraction of the gas in a
galaxy. The limiting, Eddington-like luminosity is , where is the galaxy velocity dispersion and is the
gas fraction. A starburst that attains moderates its star formation
rate and its luminosity does not increase significantly further. We argue that
ellipticals attain this limit during their growth at and that
this is the origin of the Faber-Jackson relation. We show that Lyman break
galaxies and ultra-luminous infrared galaxies have luminosities near . Star formation is unlikely to efficiently remove gas from very small
scales in galactic nuclei, i.e., scales much smaller than that of a nuclear
starburst. This gas is available to fuel a central black hole (BH). We argue
that a BH clears gas out of its galactic nucleus when the luminosity of the BH
itself reaches . This shuts off the fuel supply to the BH
and may also terminate star formation in the surrounding galaxy. As a result,
the BH mass is fixed to be , where is the electron scattering opacity. This
limit is in accord with the observed relation. (Abridged)Comment: 21 pages, emulateapj, accepted to ApJ, minor changes to discussio
Chemical Properties of Star-Forming Emission Line Galaxies at z=0.1 - 0.5
We measure oxygen and nitrogen abundances for 14 star-forming emission line
galaxies (ELGs) at 0.11<z<0.5 using Keck/LRIS optical spectroscopy. The targets
exhibit a range of metallicities from slightly metal-poor like the LMC to
super-solar. Oxygen abundances of the sample correlate strongly with rest-frame
blue luminosities. The metallicity-luminosity relation based on these 14
objects is indistinguishable from the one obeyed by local galaxies, although
there is marginal evidence (1.1sigma) that the sample is slightly more
metal-deficient than local galaxies of the same luminosity. The observed
galaxies exhibit smaller emission linewidths than local galaxies of similar
metallicity, but proper corrections for inclination angle and other systematic
effects are unknown. For 8 of the 14 objects we measure nitrogen-to-oxygen
ratios. Seven of 8 systems show evidence for secondary nitrogen production,
with log(N/O)> -1.4 like local spirals. These chemical properties are
inconsistent with unevolved objects undergoing a first burst of star formation.
The majority of the ELGs are presently ~4 magnitudes brighter and ~0.5 dex more
metal-rich than the bulk of the stars in well-known metal-poor dwarf
spheroidals such as NGC 205 and NGC 185, making an evolution between some ELGs
and metal-poor dwarf spheroidals improbable. However, the data are consistent
with the hypothesis that more luminous and metal-rich spheroidal galaxies like
NGC 3605 may become the evolutionary endpoints of some ELGs. [abridged]Comment: 41 pages, w/12 figures, uses AASTeX aaspp4.sty, psfig.sty; To appear
in The Astrophysical Journa
Emission-Line Galaxy Surveys as Probes of the Spatial Distribution of Dwarf Galaxies. I. The University of Michigan Survey
Objective-prism surveys which select galaxies on the basis of line-emission
are extremely effective at detecting low-luminosity galaxies and constitute
some of the deepest available samples of dwarfs. In this study, we confirm that
emission-line galaxies (ELGs) in the University of Michigan (UM)
objective-prism survey (MacAlpine et al. 1977-1981) are reliable tracers of
large-scale structure, and utilize the depth of the samples to examine the
spatial distribution of low-luminosity (M -18.0) dwarfs relative to
higher luminosity giant galaxies (M -18.0) in the Updated Zwicky
Catalogue (Falco et al. 1999). New spectroscopic data are presented for 26 UM
survey objects. We analyze the relative clustering properties of the overall
starbursting ELG and normal galaxy populations, using nearest neighbor and
correlation function statistics. This allows us to determine whether the
activity in ELGs is primarily caused by gravitational interactions. We conclude
that galaxy-galaxy encounters are not the sole cause of activity in ELGs since
ELGs tend to be more isolated and are more often found in the voids when
compared to their normal galaxy counterparts. Furthermore, statistical analyses
performed on low-luminosity dwarf ELGs show that the dwarfs are less clustered
when compared to their non-active giant neighbors. The UM dwarf samples have
greater percentages of nearest neighbor separations at large values and lower
correlation function amplitudes relative to the UZC giant galaxy samples. These
results are consistent with the expectations of galaxy biasing.Comment: 17 pages, 4 tables, 10 figures. Accepted for publication in the Ap
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