TY - JOUR
T1 - Using Quasars as Standard Candles for Studying Dark Energy
AU - Denney, Kelly D.
AU - Vestergaard, Marianne
AU - Watson, D.
AU - Davis, T.
PY - 2012/1/1
Y1 - 2012/1/1
N2 - We have recently demonstrated (Watson et al. 2011, ApJ, 740, L49) that
quasars, or more generally active galactic nuclei (AGNs), can be used as
standard candles for measuring distances in the universe, similar to
Type Ia supernovae (SNe). Here, we present the initial findings of this
new method, which relies on the technique of reverberation mapping to
measure time delays between the quasar continuum and emission line
variability signatures. Measuring this time delay effectively measures
the radius between the central source and the emission-line gas. The
emission line gas is photo-ionized by the continuum photons, and the
radius to this emission-line region scales tightly with the nuclear
luminosity - a consequence of the photoionization physics responsible
for regulating the production of line-emitting photons. Hence, measuring
the radius of the emission-line gas provides a measure of the intrinsic
luminosity of the AGN, allowing for the determination of the AGN
distance. Since AGNs are luminous and the emission line spectrum is
easily visible at high redshifts (e.g., out to z 4), AGN-based distances
can be used to extend the distance ladder past the SNe cutoff (z 1.5).
This regime is where the power to distinguish the possible time
dependence of the dark energy equation of state lies. We present our
initial discovery AGN Hubble diagram of nearby reverberation mapped
sources and discuss ways (1) to extend this in redshift and (2) to
reduce the current scatter. We also present simulated forecasts
demonstrating the power this method can have over, e.g., SNe, to
constrain dark energy parameters by extending to higher redshifts than
can currently be probed with any other technique.
AB - We have recently demonstrated (Watson et al. 2011, ApJ, 740, L49) that
quasars, or more generally active galactic nuclei (AGNs), can be used as
standard candles for measuring distances in the universe, similar to
Type Ia supernovae (SNe). Here, we present the initial findings of this
new method, which relies on the technique of reverberation mapping to
measure time delays between the quasar continuum and emission line
variability signatures. Measuring this time delay effectively measures
the radius between the central source and the emission-line gas. The
emission line gas is photo-ionized by the continuum photons, and the
radius to this emission-line region scales tightly with the nuclear
luminosity - a consequence of the photoionization physics responsible
for regulating the production of line-emitting photons. Hence, measuring
the radius of the emission-line gas provides a measure of the intrinsic
luminosity of the AGN, allowing for the determination of the AGN
distance. Since AGNs are luminous and the emission line spectrum is
easily visible at high redshifts (e.g., out to z 4), AGN-based distances
can be used to extend the distance ladder past the SNe cutoff (z 1.5).
This regime is where the power to distinguish the possible time
dependence of the dark energy equation of state lies. We present our
initial discovery AGN Hubble diagram of nearby reverberation mapped
sources and discuss ways (1) to extend this in redshift and (2) to
reduce the current scatter. We also present simulated forecasts
demonstrating the power this method can have over, e.g., SNe, to
constrain dark energy parameters by extending to higher redshifts than
can currently be probed with any other technique.
M3 - Journal article
SN - 0002-7537
VL - 219
JO - American Astronomical Society. Bulletin
JF - American Astronomical Society. Bulletin
ER -