2,525 research outputs found
An Extremely Massive Dry Galaxy Merger in a Moderate Redshift Cluster
We have identified perhaps the largest major galaxy merger ever seen. While
analysing Spitzer IRAC images of CL0958+4702, an X-ray selected cluster at
z=0.39, we discovered an unusual plume of stars extending 110 kpc
outward from the bright central galaxy (BCG). Three galaxies 1-1.5 mag fainter
than the BCG lie within 17 kpc (projected) of the BCG and are probably
participating in the merger. The plume is detected in all four IRAC channels
and at optical wavelengths in images from the WIYN telescope; the surface
brightness is remarkably high (24.8 mag arcsec at 50 kpc).
The optical and infrared colors are consistent with those of other BCGs,
suggesting that the plume is composed of old stars and negligible recent star
formation (hence a "dry merger"). The luminosity in the plume is at least
equivalent to a 4L^* galaxy. A diffuse halo extending 110 kpc from the BCG in
one IRAC image suggests the total amount of diffuse light is L_r\sim
1.3x10^{11}h^{-2} L_sun. A Chandra observation shows an X-ray image and
spectrum typical of moderate-mass clusters. We use MMT/Hectospec to measure 905
redshifts in a 1 deg^2 region around the cluster. The velocities of two of the
BCG companions indicate a merger timescale for the companion galaxies of
110 Myr and 0.5-1 Gyr for the plume. We conclude that the BCG and
intracluster light of CL0958 is formed by major mergers at moderate redshifts.
After the major merger is complete, CL0958 will likely become a fossil cluster.Comment: 5 pages, 2 figures, to appear in ApJ
The H-alpha Luminosity Function and Star Formation Rate Volume Density at z=0.8 from the NEWFIRM H-alpha Survey
[Abridged] We present new measurements of the H-alpha luminosity function
(LF) and SFR volume density for galaxies at z~0.8. Our analysis is based on
1.18m narrowband data from the NEWFIRM H-alpha Survey, a comprehensive
program designed to capture deep samples of intermediate redshift emission-line
galaxies using narrowband imaging in the near-infrared. The combination of
depth ( erg s cm in H-alpha at
3) and areal coverage (0.82 deg) complements other recent H-alpha
studies at similar redshifts, and enables us to minimize the impact of cosmic
variance and place robust constraints on the shape of the LF. The present
sample contains 818 NB118 excess objects, 394 of which are selected as H-alpha
emitters. Optical spectroscopy has been obtained for 62% of the NB118 excess
objects. Empirical optical broadband color classification is used to sort the
remainder of the sample. A comparison of the LFs constructed for the four
individual fields reveals significant cosmic variance, emphasizing that
multiple, widely separated observations are required. The dust-corrected LF is
well-described by a Schechter function with L*=10^{43.00\pm0.52} ergs s^{-1},
\phi*=10^{-3.20\pm0.54} Mpc^{-3}, and \alpha=-1.6\pm0.19. We compare our
H-alpha LF and SFR density to those at z<1, and find a rise in the SFR density
\propto(1+z)^{3.4}, which we attribute to significant L* evolution. Our H-alpha
SFR density of 10^{-1.00\pm0.18} M_sun yr^{-1} Mpc^{-3} is consistent with UV
and [O II] measurements at z~1. We discuss how these results compare to other
H-alpha surveys at z~0.8, and find that the different methods used to determine
survey completeness can lead to inconsistent results. This suggests that future
surveys probing fainter luminosities are needed, and more rigorous methods of
estimating the completeness should be adopted as standard procedure.Comment: 19 pages (emulate-ApJ format), 16 figures, 5 tables, published in
ApJ. Modified to match ApJ versio
The effects of early years' childcare on child emotional and behavioural difficulties in lone and co-parent family situations
With targeted childcare initiatives and welfare-to-work programmes policy-makers have sought to address employment activation of lone mothers and negative outcomes for children in lone parent households. The present study examines non-parental childcare use and maternal employment among children living in lone and co-parent family situations at ages three and four and emotional and behavioural difficulties at ages four and five. The results demonstrate that negative outcomes associated with lone motherhood are explained largely by mother's age, education, material circumstances and area deprivation; and that maternal employment does not relieve lone mothersâ disadvantages in a way that alleviates the risks of difficulties to their children. However, in any family constellation, mainly group-based formal pre-school childcare does have a positive impact on child difficulties compared to drawing on informal childcare arrangements as main provider. In addition, and specifically for the difficulties of children in lone mother family situations, any non-parental childcare â formal or informal â for at least twenty-five hours per week is beneficial. Study findings support policy agendas which tackle familiesâ material hardship beyond promoting mothersâ employment, and through investment in formal childcare provision, and also through arrangements allowing lone mothers to divide their weekly load of childcare with another main provider
Properties of the KISS Green Pea Galaxies
Green peas (GPs) are a class of extreme star-forming galaxies (SFGs) at intermediate redshifts, originally discovered via color selection using multifilter, wide-field survey imaging data. They are commonly thought of as being analogs of high-redshift Lyα-emitting galaxies. The defining characteristic of GP galaxies is a high-excitation nebular spectrum with very large equivalent width lines, leading to the recognition that GP-like galaxies can also be identified in samples of emission-line galaxies. Here we compare the properties a sample of [O iii]-selected SFGs (z = 0.29â0.41) from the KPNO International Spectroscopic Survey (KISS) with the color-selected GPs. We find that the KISS [O iii]-selected galaxies overlap with the parameter space defined by the color-selected GPs; the two samples appear to be drawn from the same population of objects. We compare the KISS GPs with the full Hα-selected KISS SFG sample (z < 0.1) and find that they are extreme systems. Many appear to be young systems at their observed look-back times (3â4 Gyr), with more than 90% of their rest-frame B-band luminosity coming from the starburst population. We compute the volume density of the KISS red (KISSR) GPs at z = 0.29â0.41 and find that they are extremely rare objects. We do not see galaxies as extreme as the KISSR GPs in the local universe, although we recognize several lower-luminosity systems at z < 0.1
Properties of the KISS Green Pea Galaxies
Green Peas are a class of extreme star-forming galaxies at intermediate
redshifts, originally discovered via color-selection using multi-filter,
wide-field survey imaging data (Cardamone et al. 2009). They are commonly
thought of as being analogs of high-redshift Ly-emitting galaxies. The
defining characteristic of Green Pea galaxies is a high-excitation nebular
spectrum with very large equivalent width lines, leading to the recognition
that Green Pea-like galaxies can also be identified in samples of emission-line
galaxies. Here we compare the properties a sample of [O III]-selected
star-forming galaxies (z = 0.29-0.41) from the KPNO International Spectroscopic
Survey (KISS) with the color-selected Green Peas. We find that the KISS [O
III]-selected galaxies overlap with the parameter space defined by the
color-selected Green Peas; the two samples appear to be drawn from the same
population of objects. We compare the KISS Green Peas with the full
H-selected KISS star-forming galaxy sample (z 0.1) and find that
they are extreme systems. Many appear to be young systems at their observed
look-back times (3-4 Gyr), with more than 90% of their rest-frame B-band
luminosity coming from the starburst population. We compute the volume density
of the KISSR Green Peas at z = 0.29-0.41 and find that they are extremely rare
objects. We don't see galaxies as extreme as the KISSR Green Peas in the local
Universe, although we recognize several lower-luminosity systems at z 0.1.Comment: 21 pages, 12 figures. Accepted for publication in the Astrophysical
Journa
Halpha-Derived Star-Formation Rates For Three z ~ 0.75 EDisCS Galaxy Clusters
We present Halpha-derived star-formation rates (SFRs) for three z ~ 0.75
galaxy clusters. Our 1 sigma flux limit corresponds to a star-formation rate of
0.10-0.24 solar mass per year, and our minimum reliable Halpha + [N II]
rest-frame equivalent width is 10\AA. We show that Halpha narrowband imaging is
an efficient method for measuring star formation in distant clusters. In two
out of three clusters, we find that the fraction of star-forming galaxies
increases with projected distance from the cluster center. We also find that
the fraction of star-forming galaxies decreases with increasing local galaxy
surface density in the same two clusters. We compare the median rate of star
formation among star-forming cluster galaxies to a small sample of star-forming
field galaxies from the literature and find that the median cluster SFRs are
\~50% less than the median field SFR. We characterize cluster evolution in
terms of the mass-normalized integrated cluster SFR and find that the z ~ 0.75
clusters have more SFR per cluster mass on average than the z <= 0.4 clusters
from the literature. The interpretation of this result is complicated by the
dependence of the mass-normalized SFR on cluster mass and the lack of
sufficient overlap in the mass ranges covered by the low and high redshift
samples. We find that the fraction and luminosities of the brightest starburst
galaxies at z ~ 0.75 are consistent with their being progenitors of the
post-starburst galaxies at z ~ 0.45 if the post-starburst phase lasts several
(~5) times longer than the starburst phase.Comment: Accepted for publication in ApJ, 20 pages, 24 figure
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