29 research outputs found
A Direct Measurement of the IGM Opacity to HI Ionizing Photons
We present a new method to directly measure the opacity from HI Lyman limit
(LL) absorption k_LL along quasar sightlines by the intergalactic medium (IGM).
The approach analyzes the average (``stacked'') spectrum of an ensemble of
quasars at a common redshift to infer the mean free path (MFP) to ionizing
radiation. We apply this technique to 1800 quasars at z=3.50-4.34 drawn from
the Sloan Digital Sky Survey (SDSS), giving the most precise measurements on
k_LL at any redshift. From z=3.6 to 4.3, the opacity increases steadily as
expected and is well parameterized by MFP = (48.4 +/- 2.1) - (38.0 +/-
5.3)*(z-3.6) h^-1 Mpc (proper distance). The relatively high MFP values
indicate that the incidence of systems which dominate k_LL evolves less
strongly at z>3 than that of the Lya forest. We infer a mean free path three
times higher than some previous estimates, a result which has important
implications for the photo-ionization rate derived from the emissivity of star
forming galaxies and quasars. Finally, our analysis reveals a previously
unreported, systematic bias in the SDSS quasar sample related to the survey's
color targeting criteria. This bias potentially affects all z~3 IGM studies
using the SDSS database.Comment: 7 pages, 4 figures; Accepted to ApJ
A Definitive Survey for Lyman Limit Systems at z~3.5 with the Sloan Digital Sky Survey
We perform a semi-automated survey for tau>=2 Lyman Limit systems (LLSs) in
quasar spectra from the Sloan Digital Sky Survey, Data Release 7. From a
starting sample of 2473 quasars with zem=3.6-4.4, we analyze 469 spectra
meeting strict seletion criteria for a total redshift path Dz=93.8 and identify
192 intervening systems at z>3.3. The incidence of tau>=2 LLSs per unit
redshift, l(z), is well described by a single-power law at these redshifts:
l(z) = C_LLS [(1+z)/(1+z_*)]^gamma, with z_*=3.7, C_LLS = 1.9+/-0.2, and gamma
= 5.2+/-1.5 (68% c.l.). These values are systematically lower than previous
estimates (especially at z<4) but are consistent with recent measurements of
the mean free path to ionizing radiation. Extrapolations of this power-law to
z=0 are inconsistent with previous estimations of l(z) at z<1 and suggest a
break at z~2, similar to that observed for the Lya forest. Our results also
indicate that the systems giving rise to LLS absorption decrease by ~50% in
comoving number density and/or physical size from z=4 to 3.3, perhaps due to an
enhanced extragalactic ultraviolet background. The observations place an
integral constraint on the HI frequency distribution f(N_HI,X) and indicate
that the power-law slope beta= dln[f(N,X)]/dln[N] is likely shallower than beta
= -1 at N_HI=10^18 cm^-2. Including other constraints on f(N_HI,X) from the
literature, we infer that beta is steeper than beta = -1.7 at N_HI~10^15 cm^-2,
implying at least two inflections in f(N_HI,X). We also perform a survey for
proximate LLSs (PLLSs) and find that l(z)_PLLS is systematically lower ~25%
than intervening systems. Finally, we estimate that systematic effects impose
an uncertainty of 10-20% in the l(z) measurements; these effects may limit the
precision of all future surveys.Comment: 26 pages, 17 figures (most in color). Submitted to Ap
The HST/ACS+WFC3 Survey for Lyman Limit Systems II: Science
We present the first science results from our Hubble Space Telescope Survey
for Lyman limit absorption systems (LLS) using the low dispersion spectroscopic
modes of the Advanced Camera for Surveys and the Wide Field Camera 3. Through
an analysis of 71 quasars, we determine the incidence frequency of LLS per unit
redshift and per unit path length, l(z) and l(x) respectively, over the
redshift range 1 < z< 2.6, and find a weighted mean of l(x)=0.29 +/-0.05 for
2.0 < z < 2.5 through a joint analysis of our sample and that of Ribaudo et al.
(2011). Through stacked spectrum analysis, we determine a median (mean) value
of the mean free path to ionizing radiation at z=2.4 of lambda_mfp =
243(252)h^(-1) Mpc, with an error on the mean value of +/- 43h^(-1) Mpc. We
also re-evaluate the estimates of lambda_mfp from Prochaska et al. (2009) and
place constraints on the evolution of lambda_mfp with redshift, including an
estimate of the "breakthrough" redshift of z = 1.6. Consistent with results at
higher z, we find that a significant fraction of the opacity for absorption of
ionizing photons comes from systems with N_HI <= 10^{17.5} cm^(-2) with a value
for the total Lyman opacity of tau_lyman = 0.40 +/- 0.15. Finally, we determine
that at minimum, a 5-parameter (4 power-law) model is needed to describe the
column density distribution function f(N_HI, X) at z \sim 2.4, find that
f(N_HI,X) undergoes no significant change in shape between z \sim 2.4 and z
\sim 3.7, and provide our best fit model for f(N_HI,X).Comment: 36 pages, 20 figures, 10 tables, revision to match accepted ApJ
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The mean free path of ionizing photons at 5 < z < 6: evidence for rapid evolution near reionization
The mean free path of ionizing photons, λmfp, is a key factor in the photoionization of the intergalactic medium (IGM). At z ≳ 5, however, λmfp may be short enough that measurements towards QSOs are biased by the QSO proximity effect. We present new direct measurements of λmfp that address this bias and extend up to z ∼ 6 for the first time. Our measurements at z ∼ 5 are based on data from the Giant Gemini GMOS survey and new Keck LRIS observations of low-luminosity QSOs. At z ∼ 6 we use QSO spectra from Keck ESI and VLT X-Shooter. We measure λmfp=9.09+1.62−1.28 proper Mpc and 0.75+0.65−0.45 proper Mpc (68 per cent confidence) at z = 5.1 and 6.0, respectively. The results at z = 5.1 are consistent with existing measurements, suggesting that bias from the proximity effect is minor at this redshift. At z = 6.0, however, we find that neglecting the proximity effect biases the result high by a factor of two or more. Our measurement at z = 6.0 falls well below extrapolations from lower redshifts, indicating rapid evolution in λmfp over 5 < z < 6. This evolution disfavours models in which reionization ended early enough that the IGM had time to fully relax hydrodynamically by z = 6, but is qualitatively consistent with models wherein reionization completed at z = 6 or even significantly later. Our mean free path results are most consistent with late reionization models wherein the IGM is still 20 per cent neutral at z = 6, although our measurement at z = 6.0 is even lower than these models prefer
The Very Young Type Ia Supernova 2012cg: Discovery and Early-Time Follow-Up Observations
On 2012 May 17.2 UT, only 1.5 +/- 0.2 d after explosion, we discovered SN
2012cg, a Type Ia supernova (SN Ia) in NGC 4424 (d ~ 15 Mpc). As a result of
the newly modified strategy employed by the Lick Observatory SN Search, a
sequence of filtered images was obtained starting 161 s after discovery.
Utilizing recent models describing the interaction of SN ejecta with a
companion star, we rule out a ~1 M_Sun companion for half of all viewing angles
and a red-giant companion for nearly all orientations. SN 2012cg reached a
B-band maximum of 12.09 +/- 0.02 mag on 2012 June 2.0 and took ~17.3 d from
explosion to reach this, typical for SNe Ia. Our pre-maximum brightness
photometry shows a narrower-than-average B-band light curve for SN 2012cg,
though slightly overluminous at maximum brightness and with normal color
evolution (including some of the earliest SN Ia filtered photometry ever
obtained). Spectral fits to SN 2012cg reveal ions typically found in SNe Ia at
early times, with expansion velocities >14,000 km/s at 2.5 d past explosion.
Absorption from C II is detected early, as well as high-velocity components of
both Si II 6355 Ang. and Ca II. Our last spectrum (13.5 d past explosion)
resembles that of the somewhat peculiar SN Ia 1999aa. This suggests that SN
2012cg will have a slower-than-average declining light curve, which may be
surprising given the faster-than-average rising light curve.Comment: re-submitted to ApJL, 4 figures, 1 tabl