1,126 research outputs found
Collision rates in the present-day Kuiper Belt and Centaur Regions: Applications to surface activation and modification on Comets, Kuiper Belt Objects, Centaurs, and Pluto-Charon
We extend previous results showing that the surfaces of Edgeworth-Kuiper Belt
objects are not primordial and have been moderately to heavily reworked by
collisions. Objects smaller than about km have collisional disruption
lifetimes less than 3.5 Gyr in the present-day collisional environment and have
been heavily damaged in their interiors by large collisions. In the 30--50 AU
region, impacts of 1 km radius comets onto individual 100 km radius objects
occur on -- yr timescales, cratering the surfaces of
the larger objects with 8--54 craters 6 km in diameter over 3.5 Gyr.
Collision time scales for impacts of 4 meter radius projectiles onto 1 km
radius comets range from 3--5 yr. The cumulative fraction of the
surface area of 1 and 100 km radius objects cratered by projectiles with radii
larger than 4 m ranges from a few to a few tens percent over 3.5 Gyr. The flux
of EKO projectiles onto Pluto and Charon is also calculated and is found to be
3--5 times that of previous estimates. Our impact model is also applied
to Centaur objects in the 5--30 AU region. We find the collisional/cratering
histories of Centaurs are dominated by the time spent in the Edgeworth-Kuiper
Belt rather than the time spent on planet-crossing orbits. Hence, the
predominant surface activity of Centaur objects like Chiron is almost certainly
not impact-induced.Comment: 17 pages, 8 figures. Icarus, 2000, in pres
Collisional evolution in the Vulcanoid region: Implications for present-day population constraints
We explore the effects of collisional evolution on putative Vulcanoid
ensembles in the region between 0.06 and 0.21 AU from the Sun, in order to
constrain the probable population density and population structure of this
region today. Dynamical studies have shown that the Vulcanoid Zone (VZ) could
be populated. However, we find that the frequency and energetics of collisional
evolution this close to the Sun, coupled with the efficient radiation transport
of small debris out of this region, together conspire to create an active and
highly intensive collisional environment which depletes any very significant
population of rocky bodies placed in it, unless the bodies exhibit orbits that
are circular to ~10^-3 or less, or highly lossy mechanical properties that
correspond to a fraction of impact energy significantly less than 10% being
imparted to ejecta. The most favorable locale for residual bodies to survive in
this region is in highly circular orbits near the outer edge of the dynamically
stable Vulcanoid Zone (i.e., near 0.2 AU), where collisional evolution and
radiation transport of small bodies and debris proceed most slowly. If the mean
random orbital eccentricity in this region exceeds ~10^-3, then our work
suggests it is unlikely that more than a few hundred objects with radii larger
than 1 km will be found in the entire VZ; assuming the largest objects have a
radius of 30 km, then the total mass of bodies in the VZ down to 0.1 km radii
is likely to be no more than ~10^-6Mearth, <10^-3 the mass of the asteroid
belt. Despite the dynamical stability of large objects in this region (Evans &
Tabachnik 1999), it is plausible that the entire region is virtually empty of
km-scale and larger objects.Comment: text plus 7 .ps figures, gzipped. Icarus, 2000, in pres
One-Line Redshifts and Searches for High-Redshift Lyman-Alpha Emission
We report the serendipitous discovery of two objects close in projection with
fairly strong emission lines at long wavelength (~9190 A). One (A) seems not to
be hosted by any galaxy brighter than V(555)=27.5, or I(814)=26.7 (Vega-based
3-sigma limits in 1.0 arcsec diameter apertures), while the other line is
associated with a faint (I(814)~24.4) red galaxy (B) offset by 2.7 arcsec and 7
A spectrally. Both lines are broad (FWHM 700 km/s), extended spatially, and
have high equivalent widths (W(A,obs)>1225 A, 95% confidence limit;
W(B,obs)~150 A). No secondary spectral features are detected for galaxy A. Blue
continuum and the marginal detection of a second weak line in the spectrum of
galaxy B is consistent with [OII] (the strong line) and MgII (the weak line) at
z=1.466. By association, galaxy A is likely at z=1.464, implying a rest-frame
equivalent width of the [OII] emission line in excess of 600 A and a projected
separation of 30 kpc for the galaxy pair. Conventional wisdom states that
isolated emission lines with rest-frame equivalent widths larger than ~200 A
are almost exclusively Lyman-alpha. This moderate-redshift discovery therefore
compromises recent claims of high-redshift Lyman-alpha emitters for which other
criteria (i.e., line profile, associated continuum decrements) are not
reported. We discuss observational tests to distinguish Lyman-alpha emitters at
high redshift from foreground systems.Comment: 16 pages, 3 figures; to appear in The Astrophysical Journal (July 1,
2000
A Serendipitous Search for High-Redshift Lyman alpha Emission: Two Primeval Galaxy Candidates at z~3
In the course of our ongoing search for serendipitous high-redshift Lyman
alpha (LyA) Emissionin deep archival Keck spectra, we discovered two very high
equivalent width (W_{obs} ~ 450A, 2-sigma) LyA emission line candidates at z ~3
in a moderate dispersion (R~1200) spectrogram. Both lines have low velocity
dispersions (sigma_v ~ 60 km/s) and deconvolved radii r ~ 1 kpc (h = 0.5). We
argue that the lines are LyA, and are powered by stellar ionization. The
surface density of robust, high equivalent width LyA candidates is estimated to
be ~3 \pm 2 per arcmin^2 per unit redshift at z ~ 3, consistent with the
estimate of Cowie etal (1998). The LyA emission line source characteristics are
consistent with the galaxies undergoing their first burst of star formation,
ie, with being primeval. Source sizes and velocity dispersions are comparable
to the theoretical primeval galaxy model of Lin and Murray (1992) based on the
inside-out, self-similar collapse of an isothermal sphere. In this model, star
formation among field galaxies is a protracted process. Galaxies are thought to
be able to display high equivalent widths for only the first few x 10 Myr. This
time is short in relation to the difference in look back times between z=3 and
z=4, and implies that a substantial fraction of strong line-emitting galaxies
at z=3 were formed at redshifts z < 4. We discuss the significance of
high-equivalent width LyA-emitting galaxies in terms of the emerging picture of
the environment, and the specific characteristics of primeval galaxy formation
at high redshift.Comment: 17 pages, 3 figures, one table. To appear in the Astrophysical
Journa
First Results from the Large Area Lyman Alpha Survey
We report on a new survey for z=4.5 Lyman alpha sources, the Large Area Lyman
Alpha (LALA) survey. Our survey achieves an unprecedented combination of volume
and sensitivity by using narrow-band filters on the new 8192x8192 pixel CCD
Mosaic Camera at the 4 meter Mayall telescope of Kitt Peak National
Observatory.
Well-detected sources with flux and equivalent width matching known high
redshift Lyman alpha galaxies (i.e., observed equivalent width above 80
Angstroms and line+continuum flux between 2.6e-17 and 5.2e-17 erg/cm^2/sec in
an 80 Angstrom filter) have an observed surface density corresponding to 11000
+- 700 per square degree per unit redshift at z=4.5. Spatial variation in this
surface density is apparent on comparison between counts in 6561 and 6730
Angstrom filters.
Early spectroscopic followup results from the Keck telescope included three
sources meeting our criteria for good Lyman alpha candidates. Of these, one is
confirmed as a z=4.52 source, while another remains consistent with either
z=4.55 or z=0.81. We infer that 30 to 50% of our good candidates are bona fide
Lyman alpha emitters, implying a net density of about 4000 Lyman alpha galaxies
per square degree per unit redshift.Comment: 10 pages, 2 figures (3 .ps files), uses AASTeX 4. Submitted to The
Astrophysical Journal Letter
Spectroscopic Properties of the z=4.5 Lyman-alpha Emitters
We present Keck/LRIS optical spectra of 17 Lya-emitting galaxies and one
Lyman break galaxy at z=4.5 discovered in the Large Area Lyman Alpha (LALA)
survey. The survey has identified a sample of ~350 candidate Lya-emitting
galaxies at z=4.5 in a search volume of 1.5 x 10^6 comoving Mpc^3. We targeted
25 candidates for spectroscopy; hence, the 18 confirmations presented herein
suggest a selection reliability of 72%. The large equivalent widths (median
W(rest)~80 A) but narrow physical widths (v < 500 km/s) of the Lya emission
lines, along with the lack of accompanying high-ionization state emission
lines, suggest that these galaxies are young systems powered by star formation
rather than by AGN activity. Theoretical models of galaxy formation in the
primordial Universe suggest that a small fraction of Lya-emitting galaxies at
z=4.5 may still be nascent, metal-free objects. Indeed, we find with 90%
confidence that 3 to 5 of the confirmed sources show W(rest) > 240 A, exceeding
the maximum Lya equivalent width predicted for normal stellar populations.
Nonetheless, we find no evidence for HeII 1640 emission in either individual or
composite spectra, indicating that though these galaxies are young, they are
not truly primitive, Population III objects.Comment: 12 pages, Accepted to Ap
The X-Ray and Mid-infrared Luminosities in Luminous Type 1 Quasars
Several recent studies have reported different intrinsic correlations between the AGN mid-IR luminosity (LMIR) and the rest-frame 2-10 keV luminosity (LX) for luminous quasars. To understand the origin of the difference in the observed LX−LMIR relations, we study a sample of 3,247 spectroscopically confirmed type 1 AGNs collected from Bo\ {o}tes, XMM-COSMOS, XMM-XXL-North, and the SDSS quasars in the Swift/XRT footprint spanning over four orders of magnitude in luminosity. We carefully examine how different observational constraints impact the observed LX−LMIR relations, including the inclusion of X-ray non-detected objects, possible X-ray absorption in type 1 AGNs, X-ray flux limits, and star formation contamination. We find that the primary factor driving the different LX−LMIR relations reported in the literature is the X-ray flux limits for different studies. When taking these effects into account, we find that the X-ray luminosity and mid-IR luminosity (measured at rest-frame 6μm, or L6μm) of our sample of type 1 AGNs follow a bilinear relation in the log-log plane: logLX=(0.84±0.03)×logL6μm/1045ergs−1+(44.60±0.01) for L6μm\u3c1044.79ergs−1, and logLX=(0.40±0.03)×logL6μm/1045ergs−1+(44.51±0.01) for L6μm≥1044.79ergs−1. This suggests that the luminous type 1 quasars have a shallower LX−LMIR correlation than the approximately linear relations found in local Seyfert galaxies. This result is consistent with previous studies reporting a luminosity-dependent LX−LMIR relation, and implies that assuming a linear LX−LMIR relation to infer the neutral gas column density for X-ray absorption might overestimate the column densities in luminous quasar
The X-Ray and Mid-infrared Luminosities in Luminous Type 1 Quasars
Several recent studies have reported different intrinsic correlations between the AGN mid-IR luminosity (LMIR) and the rest-frame 2-10 keV luminosity (LX) for luminous quasars. To understand the origin of the difference in the observed LX−LMIR relations, we study a sample of 3,247 spectroscopically confirmed type 1 AGNs collected from Bo\ {o}tes, XMM-COSMOS, XMM-XXL-North, and the SDSS quasars in the Swift/XRT footprint spanning over four orders of magnitude in luminosity. We carefully examine how different observational constraints impact the observed LX−LMIR relations, including the inclusion of X-ray non-detected objects, possible X-ray absorption in type 1 AGNs, X-ray flux limits, and star formation contamination. We find that the primary factor driving the different LX−LMIR relations reported in the literature is the X-ray flux limits for different studies. When taking these effects into account, we find that the X-ray luminosity and mid-IR luminosity (measured at rest-frame 6μm, or L6μm) of our sample of type 1 AGNs follow a bilinear relation in the log-log plane: logLX=(0.84±0.03)×logL6μm/1045ergs−1+(44.60±0.01) for L6μm\u3c1044.79ergs−1, and logLX=(0.40±0.03)×logL6μm/1045ergs−1+(44.51±0.01) for L6μm≥1044.79ergs−1. This suggests that the luminous type 1 quasars have a shallower LX−LMIR correlation than the approximately linear relations found in local Seyfert galaxies. This result is consistent with previous studies reporting a luminosity-dependent LX−LMIR relation, and implies that assuming a linear LX−LMIR relation to infer the neutral gas column density for X-ray absorption might overestimate the column densities in luminous quasar
Spectroscopic Confirmation of Three Redshift 5.7 Lyman-alpha Emitters from the Large Area Lyman Alpha Survey
Narrow-band searches for Lyman alpha emission are an efficient way of
identifying star-forming galaxies at high redshifts. We present Keck telescope
spectra confirming redshifts z = 5.7 for three objects discovered in the Large
Area Lyman Alpha (LALA) survey at Kitt Peak National Observatory.
All three spectra show strong, narrow emission lines with the asymmetric
profile that is characteristically produced in high redshift Lyman alpha
emitters by preferential HI absorption in the blue wing of the line. These
objects are undetected in deep Bw, V, R, and 6600A narrow-band images from the
NOAO Deep Wide-Field Survey and from LALA, as expected from Lyman break and
Lyman alpha forest absorption at redshift z = 5.7. All three objects show large
equivalent widths (>= 150A in the rest-frame), suggesting at least one of the
following: a top-heavy initial mass function, very low stellar metallicity, or
the presence of an active nucleus. We consider the case for an active nucleus
to be weak in all three objects due to the limited width of the Lyman alpha
emission line (< 500 km/s) and the absence of any other indicator of quasar
activity.
The three confirmed high redshift objects were among four spectroscopically
observed targets drawn from the sample of 18 candidates presented by Rhoads and
Malhotra (2001). Thus, these spectra support the Lyman alpha emitter population
statistics from our earlier photometric study, which imply little evolution in
number density from z=5.7 to z=4.5 and provide strong evidence that the
reionization redshift is greater than 5.7.Comment: Submitted to AJ, June 2002. 15 pages, AASTe
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