971 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
Thermal Emission from HII Galaxies: Discovering the Youngest Systems
We studied the radio properties of very young massive regions of star
formation in HII galaxies, with the aim of detecting episodes of recent star
formation in an early phase of evolution where the first supernovae start to
appear. Our sample consists of 31 HII galaxies, characterized by strong
Hydrogen emission lines, for which low resolution VLA 3.5cm and 6cm
observations were obtained. The radio spectral energy distribution has a range
of behaviours; 1) there are galaxies where the SED is characterized by a
synchrotron-type slope, 2) galaxies with a thermal slope, and, 3) galaxies with
possible free-free absorption at long wavelengths. The latter SEDs were found
in a few galaxies and represent a signature of heavily embedded massive star
clusters closely related to the early stages of massive star formation. Based
on the comparison of the star formation rates determined from the recombination
lines and those determined from the radio emission we find that SFR(Ha) is on
average five times higher than SFR(1.4GHz). We confirm this tendency by
comparing the ratio between the observed flux at 20 cm and the expected one,
calculated based on the Ha star formation rates, both for the galaxies in our
sample and for normal ones. This analysis shows that this ratio is a factor of
2 smaller in our galaxies than in normal ones, indicating that they fall below
the FIR/radio correlation. These results suggest that the emission of these
galaxies is dominated by a recent and massive star formation event in which the
first supernovae (SN) just started to explode. We conclude that the systematic
lack of synchrotron emission in those systems with the largest equivalent width
of Hb can only be explained if those are young starbursts of less than 3.5Myr
of age.Comment: Accepted for publication in Ap
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
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
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
The color of health: Skin color, ethnoracial classification, and discrimination in the health of Latin Americans
Latin America is one of the most ethnoracially heterogeneous regions of the world. Despite this, health disparities research in Latin America tends to focus on gender, class and regional health differences while downplaying ethnoracial differences. Few scholars have conducted studies of ethnoracial identification and health disparities in Latin America. Research that examines multiple measures of ethnoracial identification is rarer still. Official data on race/ethnicity in Latin America are based on self-identification which can differ from interviewer-ascribed or phenotypic classification based on skin color. We use data from Brazil, Colombia, Mexico, and Peru to examine associations of interviewer-ascribed skin color, interviewer-ascribed race/ethnicity, and self-reported race/ethnicity with self-rated health among Latin American adults (ages 18-65). We also examine associations of observer-ascribed skin color with three additional correlates of health – skin color discrimination, class discrimination, and socio-economic status. We find a significant gradient in self-rated health by skin color. Those with darker skin colors report poorer health. Darker skin color influences self-rated health primarily by increasing exposure to class discrimination and low socio-economic status
Baade's red sheet resolved into stars with HST in the Blue Compact Dwarf Galaxy VII Zw 403
HST WFPC2 observations of the nearby Blue Compact Dwarf Galaxy VII~Zw~403 (=
UGC 6456) resolve single stars down to M-2.5, deep enough to
identify red giants. This population has a more uniform spatial distribution
than the young main-sequence stars and supergiants, forming the structure known
as "Baade's red sheet". We conclude that VII~Zw~403 is not a primeval galaxy.Comment: submitted to: ApJ Letter
Out-of-phase oscillation between superfluid and thermal components for a trapped Bose condensate under oscillatory excitation
The vortex nucleation and the emergence of quantum turbulence induced by
oscillating magnetic fields, introduced by Henn E A L, et al. 2009 (Phys. Rev.
A 79, 043619) and Henn E A L, et al. 2009 (Phys. Rev. Lett. 103, 045301), left
a few open questions concerning the basic mechanisms causing those interesting
phenomena. Here, we report the experimental observation of the slosh dynamics
of a magnetically trapped Rb Bose-Einstein condensate (BEC) under the
influence of a time-varying magnetic field. We observed a clear relative
displacement in between the condensed and the thermal fraction center-of-mass.
We have identified this relative counter move as an out-of-phase oscillation
mode, which is able to produce ripples on the condensed/thermal fractions
interface. The out-of-phase mode can be included as a possible mechanism
involved in the vortex nucleation and further evolution when excited by time
dependent magnetic fields.Comment: 5 pages, 5 figures, 25 reference
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