1,489 research outputs found
Cosmic Star Formation History and the Future Observation of Supernova Relic Neutrinos
We investigate the flux and event rate of supernova relic neutrinos (SRNs)
and discuss their implications for the cosmic star formation rate. As reference
models, we adopt the supernova rate model based on recent observations and the
supernova neutrino spectrum numerically calculated by several groups. In the
detection energy range E_e>10 MeV, which will possibly be a background-free
region in the near future, the SRN event rate is found to be 1-2 yr^{-1} at a
water Cerenkov detector with a fiducial volume of 22.5 kton, depending on the
adopted neutrino spectrum. We also simulate the expected signal with one set of
the reference models by using the Monte Carlo method and then analyze these
pseudodata with several free parameters, obtaining the distribution of the
best-fit values for them. In particular, we use a parameterization such that
R_{SN}(z)=R_{SN}^0 (1+z)^\alpha, where R_{SN}(z) is the comoving supernova rate
density at redshift z and R_{SN}^0 and \alpha are free parameters, assuming
that the supernova neutrino spectrum and luminosity are well understood by way
of a future Galactic supernova neutrino burst or the future development of
numerical supernova simulations. The obtained 1\sigma errors for these two
parameters are found to be \delta\alpha /\alpha = 30% (7.8%) and \delta
R_{SN}^0/ R_{SN}^0 = 28% (7.7%) for a detector with an effective volume of 22.5
kton 5 yr (440 kton 5 yr), where one of the parameters is fixed. On the other
hand, if we fix neither of the values for these two parameters, the expected
errors become rather large, \delta\alpha /\alpha = 37% and \delta R_{SN}^0/
R_{SN}^0 = 55%, even with an effective volume of 440 kton 5 yr.Comment: 12 pages, 11 figures, corrected minor typos, accepted by Ap
The Stellar UV Background at z<1.5 and the Baryon Density of Photoionized Gas
We use new studies of the cosmic evolution of star-forming galaxies to
estimate the production rate of ionizing photons from hot, massive stars at low
and intermediate redshifts. The luminosity function of blue galaxies in the
Canada-France Redshift Survey shows appreciable evolution in the redshift
interval z=0-1.3, and generates a background intensity at 1 ryd of J_L~ 1.3 x
10^{-21} f_{esc} ergs cm^{-2} s^{-1} Hz^{-1} sr^{-1} at z~0.5, where f_esc is
the unknown fraction of stellar Lyman-continuum photons which can escape into
the intergalactic space, and we have assumed that the absorption is picket
fence-type. We argue that recent upper limits on the H-alpha surface brightness
of nearby intergalactic clouds constrain this fraction to be <~ 20%. The
background ionizing flux from galaxies can exceed the QSO contribution at z~
0.5 if f_{esc}>~ 6%. We show that, in the general framework of a diffuse
background dominated by QSOs and/or star-forming galaxies, the cosmological
baryon density associated with photoionized, optically thin gas decreases
rapidly with cosmic time. The results of a recent Hubble Space Telescope survey
of OVI absorption lines in QSO spectra suggest that most of this evolution may
be due to the bulk heating and collisional ionization of the intergalactic
medium by supernova events in young galaxy halos.Comment: 6 pages, Latex file, 2 figures, mn.sty, MNRAS in pres
Hypervelocity stars and the environment of Sgr A*
Hypervelocity stars (HVSs) are a natural consequence of the presence of a
massive nuclear black hole (Sgr A*) in the Galactic Center. Here we use the
Brown et al. sample of unbound and bound HVSs together with numerical
simulations of the propagation of HVSs in the Milky Way halo to constrain three
plausible ejection mechanisms: 1) the scattering of stars bound to Sgr A* by an
inspiraling intermediate-mass black hole (IMBH); 2) the disruption of stellar
binaries in the tidal field of Sgr A*; and 3) the two-body scattering of stars
off a cluster of stellar-mass black holes orbiting Sgr A*. We compare the
predicted radial and velocity distributions of HVSs with the limited-statistics
dataset currently available, and show that the IMBH model appears to produce a
spectrum of ejection velocities that is too flat. Future astrometric and deep
wide-field surveys of HVSs should shed unambiguous light on the stellar
ejection mechanism and probe the Milky Way potential on scales as large as 200
kpc.Comment: 5 pages, 5 figures, accepted for publication in MNRAS letter
On the Evolution of the Cosmic Supernova Rates
Ongoing searches for supernovae (SNe) at cosmological distances have recently
started to provide a link between SN Ia statistics and galaxy evolution. We use
recent estimates of the global history of star formation to compute the
theoretical Type Ia and Type II SN rates as a function of cosmic time from the
present epoch to high redshifts. We show that accurate measurements of the
frequency of SN events in the range 0<z<1 will be valuable probes of the nature
of Type Ia progenitors and the evolution of the stellar birthrate in the
universe. The Next Generation Space Telescope should detect of order 20 Type II
SNe per 4'x 4' field per year in the interval 1<z<4.Comment: LaTeX, 19 pages, 3 figures, to be published in the MNRA
Cosmic rays, lithium abundance and excess entropy in galaxy clusters
We consider the production of Li in spallation reactions by cosmic rays
in order to explain the observed abundance in halo metal-poor stars. We show
that heating of ambient gas by cosmic rays is an inevitable consequence of this
process, and estimate the energy input required to reproduce the observed
abundance of Li/H to be of order a few hundred eV per
particle. We draw attention to the possibility that this could explain the
excess entropy in gas in galaxy groups and clusters. The evolution of Li
and the accompanying heating of gas is calculated for structures collapsing at
the present epoch with injection of cosmic rays at high redshift. We determine
the energy required to explain the abundance of Li at
corresponding to the formation epoch of halo metal-poor stars, and also an
increased entropy level of keV cm necessary to explain X-ray
observations of clusters. The energy budget for this process is consistent with
the expected energy output of radio-loud AGNs, and the diffusion length scale
of cosmic-ray protons responsible for heating is comparable to the size of
regions with excess entropy. We also discuss the constraints imposed by the
extragalactic gamma-ray background.Comment: 5 pages, 1 Figure, Accepted for publication in MNRAS (Letters
Detectability of High Redshift Ellipticals in the Hubble Deep Field
Relatively few intensively star-forming galaxies at redshifts z>2.5 have been
found in the Hubble Deep Field (HDF). This has been interpreted to imply a low
space density of elliptical galaxies at high z, possibly due to a late (z<2.5)
epoch of formation, or to dust obscuration of the ellipticals that are forming
at z~3. I use HST UV (2300 Ang) images of 25 local early-type galaxies to
investigate a third option, that ellipticals formed at z>4.5, and were fading
passively by 2<z<4.5. Present-day early-types are faint and centrally
concentrated in the UV. If ellipticals formed their stars in a short burst at
z>4.5, and have faded passively to their present brightnesses at UV
wavelengths, they would generally be below the HDF detection limits in any of
its bands at z>2.5. Quiescent z ~ 3 ellipticals, if they exist, should turn up
in sufficiently deep IR images.Comment: AAS LaTex, 11 pages, 1 table, 1 figure, some corrections and
clarifications, accepted for publication in ApJ
Keck Deep Fields. II. The UV Galaxy Luminosity Function at z~4, 3, and 2
We use very deep UGRI multi-field imaging obtained at the Keck telescope to
study the evolution of the rest-frame 1700A galaxy luminosity function as the
Universe doubles its age from z~4 to z~2. The depth of our imaging allows us to
constrain the faint end of the luminosity function reaching M_1700A ~ -18.5 at
z~3 (equivalent to ~1M_sun/yr) accounting for both N^1/2 uncertainty in the
number of galaxies and for cosmic variance. We carefully examine many potential
sources of systematic bias in our LF measurements before drawing the following
conclusions. We find that the luminosity function of Lyman Break Galaxies
evolves with time and that this evolution is likely differential with
luminosity. The result is best constrained between the epochs at z~4 and z~3,
where we find that the number density of sub-L* galaxies increases with time by
at least a factor of 2.3 (11sigma statistical confidence); while the faint end
of the LF evolves, the bright end appears to remain virtually unchanged,
indicating that there may be differential, luminosity-dependent evolution
significant at the 97% level. Potential systematic biases restric our ability
to draw strong conclusions about continued evolution of the luminosity function
to lower redshifts, z~2.2 and z~1.7, but, nevertheless, it appears certain that
the number density of z~2.2 galaxies at all luminosities we studied,
-22<M_1700A<-18, is at least as high as that of their counterparts at z~3.
While it is not yet clear what mechanism underlies the observed evolution, the
fact that this evolution is differential with luminosity opens up new avenues
of improving our understanding of how galaxies form and evolve at high
redshift.Comment: Accepted for publication in ApJ. Updated preprint to reflect this
final versio
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