Friday. My happiness is depending too much on finding a good place to live in Bonn. I'm going to have to rely on Ellen and Laura's help (and of many others) to get me through this apartment hunting (and stop killing myself over this).
1109.4632
A new cosmological distance measure using AGN
Watson, Denny, Vestergaard, Davis
Discovery of an accurate luminosity distance measure using AGN: Use the tight relationship between the luminosity of an AGN and the radius of its broad line region established via reverberation mapping to determine the luminosity distances to a sample of 38 AGN. All reliable distance measures have (up to now) been limited to moderate redshifts; AGN will allow distances to be estimated to z~4, where variations of DE and alternate gravity theories can be probed.
2.1) reverberation mapping:
* Physics: SMBH is surrounded at a distance by high velocity gas clouds that produce the broad emission lines characteristic of the spectra of near-face-on AGN (quasars and Seyfert 1 galaxies). Size of BLR is determined by the depth to which the surrounding gas can be photo-ionized by the central source. Ionizing flux drops with distance according to the inverse square law, so the radius of the BLR (broad-line emitting region), "r", is expected to be proportional to the square root of the luminosity L. Establish r and the flux to measure luminosity distance.
* Method: Photons emitted by GLR gas are reprocessed continuum photons; the flux in the broad lines varies in response to variations in the luminosity of the central source with a time-delay, tau, governed by the light travel time, tau=r/c. Measuring the time delay thus allows a determination of the BLR radius--i.e., "reverberation mapping". Radius is effectively determined by measuring the time lag between changes in the continuum luminosity of the AGN and the luminosity of a bright emission line.
* Systematics: luminosities: remove the contaminating effects of the host galaxy. lag: reobserving AGN with previously had poorly sampled light-curves. Populating the low luminosity regime of the existing sample. The r ~ sqrt(L) relation followed to good accuracy (how good?) for 4 orders of magnitude in luminosity.
3.) Data
* important to remove host galaxy component; done primarily using images from HST to model the underlying galaxy contribution (i.e., it's expensive). (It doesn't say exactly how)
* some AGNs are close enough to be directly calibrated by Cepheids.
4.) Results
* rms scatter in AGN Hubble diagram: 0.2 dex (0.5 mag in distance modulus). Observational uncertainty account for ~50% of the total scatter in the relation, or 0.14 dex (0.36 mag).
* repeated observation of given sources will reduce the scatter related to observational uncertainty to a level of NGC5548 (i.e., ~<0.3 mag).
* extinction can be measured (and hence corrected for) using the "Balmer decrement metnod, Na I D or K I line equivalent widths, or a calibration based on several methods. Scatter here can be lowered from the current 0.2 mag to 0.1 mag.
* possible improvements in lag measurements. past lag measurements are likely to be less uncertain
5) Discussion
* Prospects for extension to high z: it's much brighter than SNe, so can go to z~4; but requires longer temporal baselines (redshift time dilation + brighter (and hence larger r) AGNs). Time lags can be ~2 years for AGN at z~2. Different lines have different lags (apparently) though. Some lines don't require host correction (i.e., C IV). But is less resource intensive because the lightcurve can be measured over a long time (i.e., leisurely).
* AGN method may not require calibration to absolute distances, like SNe.
* Intrinsic diversity: Assumed constant ionisation parameters across the sample--requires density close to the same value in a given region of the BLR across sources. ** the ultimate limit to the accuracy of AGN Hubble diagram. (degree unknown; current measurements have larger scatter)
* Competitiveness: should be achievable in less than a decade at redshifts up to z=3.
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