Testing claims of IMF variation with strong lensing (and dynamics)

Transcription

1 IAU311, Oxford, July 2014 Testing claims of IMF variation with strong lensing (and dynamics) Russell Smith University of Durham

2 A Heavy IMF in Ellipticals: Consensus? Spectroscopy: Gravity-sensitive features require significant flux contributions from dwarf stars - a Bottom-Heavy IMF. Conroy & van Dokkum (2012); Ferreras et al. 2012; RJS et al. 2012; Spiniello et al. (2013) La Barbera et al. (2013) etc. Na I Lensing: Strong lens + velocity dispersions of distant (z~0.3) lenses require heavier IMF at high σ if DM halo is Universal NFW. Treu et al. (2010); Auger et al. (2010); Spiniello et al. (2012) etc. Dynamics: ATLAS3D dynamical models require heavier IMF at high σ for a variety of halo models. Cappellari et al. (2012, 2013); Lasker et al. (2013) etc.

3 A Heavy IMF in Ellipticals: Consensus? All three methods have different sensitivities/biases/ degeneracies but appear to converge on a progressively heavier IMF in more massive ellipticals. α = (M/L) (M/L) Kroupa IMF αmw "heavyweight" Salpeter Kroupa Chabrier Treu et al. (2010) Conroy & van Dokkum (2012) Cappellari et al. (2013) ESO325 G (km/s) σ (km/s) = M/L factor = IMF mismatch factor

4 Spectroscopy vs Lensing Strong lensing yields the most accurate extragalactic masses. Total mass projected inside R Ein is good to ~5% at most. But need to subtract DM contribution. For z~0.3 lenses (e.g. SLACS) this can be ~50% of total mass (e.g. Auger et al. 2010) Minimise DM fraction inside R Ein by selecting low-z lenses where R Ein R Eff REin Use low-z lenses to verify/calibrate spectroscopic IMF indicators.

5 A low-z elliptical lens: ESO325-G004 Unusual (unique so far!) low-redshift stronglensing massive elliptical (RJS et al. 2005). RJS et al. (2005) RJS & Lucey (2013) Redshift = 0.034; Vel. disp. = 330 km/s; [Mg/Fe]=+0.3. Apparently just the sort of galaxy where CvD find heavyweight IMFs. Robust system to measure stellar mass, R Ein ~ R eff /4

6 A low-z elliptical lens: ESO325-G rest wavelength (Ang) 4950 rest wavelength (Ang) [OIII] 1.0 Unusual (unique so far!) low-redshift strong lensing massive elliptical H 4 1. (RJS et al. 2005).s=2 z arc Redshift = 0.034; RJS et al. (2005) RJS & Lucey (2013) [NII] [OIII] H [NII] flux (arbitrary units) 0.5 [OIII] (b) km/s; Vel. disp. = [Mg/Fe]=+0.3. Apparently just the sort of galaxy where CvD find observed wavelength (Ang) observed wavelength (Ang) heavyweight IMFs. Fig 2: Extracts from the X-shooter spectrum, showing the emission lines detected from the arcs. The inse Higha source redshift implies a small shows portion of the 2d spectrum, confirming that OIIIdeflection emission is seen in both arcs covered by the slit. Lin positions are so marked for the derivedsmall source redshift of z= angle... a relatively projected mass. 5 Robust system to Total I-band M/L~3.7 afrom lensing, including DM. Treu et al. (2010) expected 3.0-ish for old, metal rich SSP. & van Dokkum (2012) measure stellar an Conroy 4 from F814W, F625W and F475WCf. observations. van Dokkum & Conroy (2010) after subtracting a smooth model for the lens mass, ~ Reff/4 Cappellari et al. (2012) entation used R in Ein our X-shooter observation. See Russell Smith ky- 2 e/releases/2007/08/image/b/ "heavyweight" IMF IAU311, Oxford, July 2014

7 Lightweight IMF in ESO325-G004 To isolate IMF effect, need to: (1) estimate (small) DM contribution (2) normalize out age and metallicity effects on M/L by fitting optical spectrum. (see the paper!) RJS & Lucey 2013 Pr(α MW ) Probability density Contracted DM halos F475W MW-like Chabrier Kroupa Salpeter Heavyweight Default (F814W) Age from Hγ F only Age from Hβ only Salpeter No dark matter Conroy & van Dokkum α MW 2 IMF mass factor relative to MW Redshift of arcs implies M/L consistent with a Milky Way IMF: M stel /L = 1.05±0.15 (M stel /L) Kroupa IMF Various systematic uncertainties at the 10-15% level. Heavier-than-Salpeter IMFs excluded at >3σ and heavyweight IMFs firmly ruled out for this galaxy.

8 Just an outlier? At face value much smaller α MW than SLACS / CvD12 galaxies at same σ or [Mg/Fe]... αmw "heavyweight" Salpeter Kroupa Chabrier Treu et al. (2010) Conroy & van Dokkum (2012) Cappellari et al. (2013) ESO325 G004 Maybe ESO325-G004 is an intrinsic outlier: i.e. giant Es on average have bottom-heavy IMFs, but this one doesn t? If so, presumably it has spectrum of a normal, non-dwarf-enriched IMF... αmw "heavyweight" Salpeter Kroupa Chabrier (km/s) σ (km/s) Conroy & van Dokkum (2012) ESO325 G [Mg/Fe] [Mg/Fe]

9 IMF sensitive lines in E325-G004 Conroy & van Dokkum (2012) In fact, FORS2 spectrum spectro does show "heavyweight" the typical dwarf-starenriched features as seen by Salpeter CvD in giant Es! αmw Kroupa Chabrier ESO325 G004 In particular, strong NaI doublet and weak Ca II triplet [Mg/Fe] Full-spectrum fit with Conroy s code: (µm) αmw 2 Na I IMPORTANT: this fit as shown [Mg/Fe] lensing in my talk did not use the very residuals (%) latest versions of CvD models. Ca II (µm) Working to update this soon. CvD fitting code applied to VIMOS+FORS2 spectrum with a three-part broken-power-law IMF yields α MW 2.0.

10 SINFONI Low-z Lens Search Aim: find more stellar-mass lenses Beat low yield in SLACS-like surveys by: 1) pre-select the most massive lens candidates (cross-section ~σ 4, and high-σ targets are the most interesting in current debate context. 2) search for background emitters in the IR (much more volume at higher z; Hα is brightest line). 3) use integral field data for search (much increased contrast of lines over flux from lens galaxy). ESO325-G004 ESO325-G004 Combined effect: Total predicted yield (HiZELS Hα LFs) with 1hr exposure is one detectable lens in every ~10 candidate galaxies. P93 search targets 35 candidates.

11 SINFONI Low-z Lens Search Aim: find more stellar-mass lenses Beat low yield in SLACS-like surveys by: 1) pre-select the most massive lens candidates (cross-section ~σ 4, and high-σ targets are the most interesting in current debate context. 2) search for background emitters in the IR (much more volume at higher z; Hα is brightest line). 3) use integral field data for search (much increased contrast of lines over flux from lens galaxy). ESO325-G004 Combined effect: Total predicted yield (HiZELS Hα LFs) with 1hr exposure is one detectable lens in every ~10 candidate galaxies. P93 search targets 35 candidates.

12 New SINFONI lens candidate z=0.031 σ 6dF 330 km/s Likely background line emitter, probably Halpha at z~ but is it lensed? Deeper data needed to confirm the faint counter-image. 2MASS J Sinfoni FoV 4.9 A preliminary analysis: lensing mass is M Ein = 9x10 10 M sun (Do not take too seriously!) For z src =0.93, z lens =0.03 and theta θ Ein ~2.4 (half separation of the arcs ): luminosity is L Ein = 5x10 10 L sun (2MASS J-band) so (M/L) J = 1.8 (M/L) sun (before subtracting any DM) Contours of 1.264μm line emission (Hα at z=0.93?) [NII] M05 models with 10-13Gyr ages, 1-2x Z sun have: (M/L) J = (M/L) sun (Salpeter IMF) or (M/L) J = (M/L) sun (Kroupa IMF) Hα

13 Another promising candidate NGC 5532 σ 340 km/s z=0.024 SDSS/i from STIS image shows arc-like feature at 4 from nucleus (Hardcastle et al. 2005) HST/STIS ( nm) No SINFONI data yet: still need to secure the source redshift.

14 Lens Search By-product CN NaI KI Stacked data KI NaI Paβ J-band: N=18 Models H-band: N=17 MgI MgI CO MgI IR spectra for 36 very massive (>300 km/s) ellipticals in J and H bands, over < z < ideal for stacking. Equivalent to 12 hrs on one galaxy, but without sky/telluric residuals.

15 Spectroscopy vs Dynamics Since benchmark lenses are rare, alternative approach is to compare spectroscopic mass excess against dynamical results. In particular, the CvD12 sample is drawn from SAURON/ATLAS3D, so we can compare spectroscopy vs dynamics for identical galaxy sample. Do they agree?

16 Spectroscopy vs Dynamics IMF mass factors from ATLAS3D and CvD12 for identical sample of 34 galaxies. E325-G004 Do they agree? YES! On average, both methods find evidence for heavierthan-mw IMF. RJS 2014 NO! Reported mass excess for individual galaxies disagrees significantly in many cases. No correlation between spectroscopic and dynamical estimates.

17 Spectroscopy vs Dynamics RJS 2014 Mass excess correlates strongly with Mg/Fe when measured from absorption line strengths by CvD... but only with velocity dispersion when measured from dynamics by A3D.

18 Spectroscopy vs Dynamics RJS 2014 Mass excess correlates strongly with Mg/Fe when measured from absorption line strengths by CvD... but only with velocity dispersion when measured from dynamics by A3D.

19 What is going on? Two possible responses to these results: Skeptical interpretation: At least one of the methods is being biased by unidentified systematic effects. E.g. synthesis models don t fully capture effect of non-solar abundances on cool stars. Sanguine interpretation: Perhaps both methods correctly measure different aspects of the IMF; if so, we can learn more about the details of IMF variation by combining them.

20 What kind of IMF variation? Spectroscopic method doesn t uniquely determine the M/L CvD α = M/L relative to Kroupa IMF N(M) M/Msun NB: strictly CvD use two lowmass slopes, i.e. a 3-part IMF Grey is the range for the 3- part IMF Given the spectrum, if age etc are known, the implied M/L depends on form of IMF variation. Black = low-mass slopes identical NaI 0.82µm

21 What kind of IMF variation? Spectroscopic method doesn t uniquely determine the M/L Single PL remnant- dominated α N(M) M/Msun Given the spectrum, if age etc are known, the implied M/L depends on form of IMF variation. NaI 0.82µm

22 What kind of IMF variation? Spectroscopic method doesn t uniquely determine the M/L La Barbera+13 remnant- dominated α N(M) broken PL; lowmass slope fixed M/Msun Given the spectrum, if age etc are known, the implied M/L depends on form of IMF variation. NaI 0.82µm

23 What kind of IMF variation? Spectroscopic method doesn t uniquely determine the M/L Barnabe+13 α N(M) Single slope, varying cut-off M/Msun Given the spectrum, if age etc are known, the implied M/L depends on form of IMF variation. NaI 0.82µm

24 Translation between IMF models α = M/L relative to Kroupa IMF Spectroscopic Information (most features degenerate w.r.t. IMF shape, to 1st order)

25 Putting it all together Only expect a 1:1 correlation if IMF variation really follows the CvD prescription (variable low-mass slope) Can use previous argument to derive lines for other types of variation: variable single slope variable high-mass slope variable low-mass cutoff Implied M/L factor (Spectroscopy, CvD-IMF) E325-G004 CvD IMF variation M/L factor (Dynamics) varying single-slope PL

26 Consensus restored? To match the worst outliers (including E325-G004) requires BOTH: 1) *very* steep (x~4) IMF at M sun to provide the dwarf spectral signatures. AND 2) rapid cut-off at ~0.5 M sun to prevent low-mass stars dominating M/L. Implied M/L factor (Spectroscopy, CvD-IMF) M/L factor (Dynamics)

27 Summary Comparing M/L (dynamics/lensing) versus spectroscopic dwarf-star signal tests not only the data, but also the assumed IMF variation model. Even the most discrepant cases can be reconciled if given enough freedom in the IMF. (Which is not to say that such a baroque IMF is actually plausible.) Apparent tension between methods suggests either: (1) The IMF variation is more complex than a simple one-parameter variation... or... (2) One (or more) of the methods is biased by incomplete accounting for confounding factors Another possibility: are heavy IMFs limited to innermost core of galaxies? - affects nuclear spectrum more than dynamics (e.g. Martín-Navarro et al. 2014) See: Smith & Lucey (2013MNRAS S), Smith (2014MNRAS.443L..69S)