Adam S. Bolton
Institute for Astronomy, University of Hawai`i
Strong Lensing in the
Spectroscopic Future
That of which I hope to convince you:
Strong lensing is good astronomy and good physics
Spectroscopy is better than imaging for finding lenses in principle
in practice
You get lenses almost for free from spectroscopic surveys
(Subjective/approximate terms)
An end-to-end proof of concept:
The Sloan Lens ACS (SLACS) Survey
o Two redshifts in one SDSS spectrum => lens candidates o High resolution HST-ACS imaging follow-up => lenses
o Strong-lens modeling and photometry => science Bolton, Burles, Koopmans, Treu, Moustakas, et al.
Adds direct mass observables to a complete set of photometric and dynamical observables for a significant (70--80) sample of massive (
σ
~ 200--400 km/s) early-type galaxies.Spectroscopic candidate selection
SDSS data:
Spectroscopic candidate selection
(Continuum model courtesy of D. J. Schlegel)
Spectroscopic candidate selection
Spectroscopic candidate selection
Spatially resolved follow-up
Integral-Field Spectroscopy (Gemini & Magellan telescopes)
(Bolton et al. 2006; Bolton & Burles 2007)
High-resolution imaging (HST-ACS WFC)
(SLACS: Bolton et al. 2005, 2006, 2007;
Treu et al. 2006; Koopmans et al. 2006;
Gavazzi et al. 2007)
Strong lens modeling
o Subtract smooth B-spline model for lens-galaxy light profile
o Parameterized models for (a) Lens mass
- Singular isothermal ellipsiod - Light-traces-mass
(b) Un-lensed galaxy light - Gaussian
- Sersic
- Multi-Gaussian or Multi-Sersic 1. Choose sensible starting parameters
2. Ray trace to view (b) through the potential of (a) 3. Compute
χ
2 relative to data4. Improve model parameters
5. Repeat from (2) to optimize.
Strong-lens modeling in action
Data Model Data - Model
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Strong-lens modeling in action
Data Model Data - Model
Strong-lens modeling in action
Data Model Data - Model
Strong-lens modeling in action
Data Model Data - Model
Strong-lens modeling in action
Data Model Data - Model
Strong-lens modeling in action
Data Model Data - Model
Strong-lens modeling in action
Data Model Data - Model
Strong-lens modeling in action
Data Model Data - Model
Strong-lens modeling in action
Data Model Data - Model
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Comparison of velocity dispersions:
strong-lensing v. stellar
One-to-one ratio is indicative of a total
mass-density profile
ρ ∝ r -2
(“Isothermal”)
This conclusion can be made more formally.
(Koopmans et al. 2006)
(Bolton et al. in prep; Treu et al. 2006)
Strong lensing confronts the fundamental plane
Jeans equation, a.k.a. virial theorem, a.k.a. dimensional analysis:
brightness
dimensionless constant Empirically:
size
velocity dispersion
mass-to-light ratio
(Dressler et al. 1987, Djorgovsi & Davis 1987;
these exponents from Bernardi et. al. 2003)
Conclusion: the “tilt” of the FP is a result of systematic
variation in either the structure (i.e., dimensionless constant) or the mass-to-light ratio of elliptical galaxies
Strong lensing confronts the fundamental plane
By defining a dimensional mass variable,
the FP can be described as
with
η
< 1But is this a true trend in M/L or is it a trend in c?
People go both ways...
(e.g. Faber et al. 1987)
Strong lensing confronts the fundamental plane
With strong lensing, we measure
within the Einstein radius aperture, in addition to
Thus we can examine both
and
If
δ
=η
, the FP is a trend in structure constant.If
δ
= 1, the FP is a trend in mass-to-light ratio.(Bolton et al. 2007; Bolton et al. in prep.)
Strong lensing confronts the fundamental plane
The data:
(Bolton et al. 2007; Bolton et al. in prep.)
Strong lensing confronts the fundamental plane
Result is
δ = 1
Indicating a systematic trend in mass-to-light ratio as the explanation for the tilt of the FP
(Bolton et al. 2007; Bolton et al. in prep.)
A quick smattering of other applications/results
o No evolution in mass-density slope since z = 1
o Confirmation of PPN
γ
= 1 on kiloparsec length scaleso Strong + weak lensing constraints on density profile to 100 Re
o Magnified views of “nano-galaxies”
o Strong-lens cosmological applications
(Koopmans et al. 2006)
(Bolton, Rappaport, & Burles 2006)
(Gavazzi et al. 2007, in press)
(Marshall et al. 2007)
(in the works...?)
Chronology of spectroscopic strong-lens discovery
o Q2237+0305 (Huchra et al. 1985)
o 0047-2808 (Warren et al. 1996, 1998, 1999) o Hewett et al. 2000; Willis 2000
o Johnston et al. 2003
o Bolton et al. 2004--2007 o Willis et al. 2005, 2006
State of the art in imaging strong-lens discovery
o Cabanac et al. 2007 (CFHTLS, lensed galaxies)
o Oguri & Inada et al. 2003--2007 (SDSS, lensed quasars) (Full chronology prohibitively long!)
(Radio wavelengths neglected in this talk!)
Why spectroscopy is better than imaging for finding lenses
o Atmosphere degrades imaging resolution but not spectral resolution
HST-ACS image:
SDSS image:
SDSS
Spectrum:
Why spectroscopy is better than imaging for finding lenses
o Both redshifts are known from the outset -- a necessary and sometimes prohibitive hurdle for lenses found in imaging.
observable
astrophysics redshifts and cosmology
Why spectroscopy is better than imaging for finding lenses
o Other objects (face-on spirals, ring galaxies, etc.) can mimic lenses in imaging; two redshifts in one spectrum harder to imitate.
o More resolution elements and fewer intrinsic modes of variation per object in spectroscopy than in imaging.
o Current and future surveys are not even close to the spectroscopic confusion limit.
o Easier to automate detection algorithm.
o Selection functions should ultimately be more straightforward.
o Greater demonstrated production of confirmed lenses!
(Almost) for free: lenses from redshift surveys
o To leading order, spectroscopic lens discovery requires
no specialized tweaking of the survey (e.g. SLACS from SDSS) o However, in comparison with redshift finding, lens
discovery is more demanding of wavelength calibration, flat-fielding, spectrophotometry, etc.
o Pure magnitude-based targeting is preferable; color-based targeting greatly complicates lens selection function.
o More straightforward with fibers than with slitlets.
o Spatially resolved multi-fiber surveys would allow even more efficient selection of lenses from among candidates.
o Current empirical incidence of ~1 in 1,000 only a lower limit:
more perfect data reduction should uncover several X more lenses.
(Dobler et al., submitted)
Beyond the low-hanging fruit:
o Future surveys need not be confined to lenses consisting of (early-type foreground) + (star-forming background) galaxies.
o Galaxy-quasar, quasar-galaxy, quasar-quasar... all possibilities
Strong lenses also look great on posters!
That of which I hope to have convinced you:
Strong lensing is good astronomy and good physics
Spectroscopy is better than imaging for finding lenses in principle
in practice
You get lenses almost for free from spectroscopic surveys
Go ahead -- give us a bullet in your survey justification.
You know you want to!
