IMF variations in unresolved stellar populations: Challenges

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1 Department of Space and Climate Physics Mullard Space Science Laboratory IMF variations in unresolved stellar populations: Challenges Ignacio Ferreras Mullard Space Science Laboratory University College London XXIX IAU General Assembly Focus Meeting #7 Honolulu, Hawaii, USA August 13th, 2015

2 Collaborators Francesco La Barbera (INAF/OAC, IT) Alejandro Vazdekis (IAC, ES) Ignacio G. de la Rosa (IAC, ES) Reinaldo R. de Carvalho (INPE, BR) Carsten Weidner (IAC, ES) Marina Trevisan (INPE, BR) Ignacio Martín Navarro (IAC, ES) Jesús Falcón Barroso (IAC, ES) Elena Ricciardelli (Valencia, ES)

3 Universal IMF a cherished belief The stellar mass distribution at birth is an essential piece for the interpretation of light from unresolved populations: Stellar Mass Star formation history Chemical enrichment It is generally assumed that the IMF is universal Simple power law (Salpeter) With a cutof towards low masses (Kroupa, Chabrier)

4 Constraints on the IMF (unresolved stars) Spectroscopic Search for line strengths that are sensitive to the presence of lowmass stars. Early-epoch: Spinrad & Taylor 1971; Faber & French 1980; Carter et al. 1986; Couture & Hardy 1993) Late-epoch: Saglia+02,Cenarro+03, van Dokkum & Conroy10,Spiniello+12, Ferreras+13,LaBarbera+13 Dynamical Constraints from kinematics (rotation velocity and velocity dispersion profiles) and surface brightness distribution (e.g. Binney+90; Rix+97; Gerhard+01; Cappellari+12) Lensing Strong gravitational lensing over galaxy scales, compared with photometry (Treu+10; Ferreras+10) Dwarf to Giant mass ratio Stellar M/L

5 Spectral-based constraints in a nutshell The stellar populations of massive ETGs are dominated by old (8-12 Gyr) stars, therefore cool giant & dwarf stars are the only signifcant populations (controlling light and mass, respectively)

6 Challenge #0: Observational aspects In unresolved stellar populations, the diferences imprinted in the spectra are much more subtle. (Note these SEDs are normalized to the same fux within the spectral window) MIUSCAT models (Vazdekis et al. 2012, MNRAS, 424, 157)

7 Challenge #0: Observational aspects Most of the IMF-sensitive information is in the red/nir spectral window: airglow and tellurics. Redshift can help avoid particularly bad regions. La Barbera, IF, et al. (2013, MNRAS, 433, 3017)

8 Challenge #0: Observational aspects Spectral lines in galaxies are heavily blended versions of stellar atomic+molecular absorption lines. This is especially troublesome in massive galaxies (higher velocity dispersion). SDSS / UVESPOP / MILES Ferreras et al. (2013, MNRAS, 429, L15)

9 Challenge #1: Interpretation of line strengths The blending of lines results in a very complex dependence of the spectral indices on the properties of the stellar populations, mainly Age Metallicity General [α/fe] Individual abundance variations IMF Several line strengths can be combined to break the strong degeneracies (Ferreras et al. 2013; La Barbera et al. 2013; Spiniello et al. 2014)

10 city i l l a t Me IMF Slop e Adding gravity-sensitive line strengths: At the high velocity dispersion end, a standard SSP grid requires a bottom-heavy IMF e Ag Challenge #1: Interpretation of line strengths Metallicity La Barbera, IF, et al. (2013, MNRAS, 433, 3017)

11 Challenge #1: Interpretation of line strengths A combination of several indices with diferent IMF sensitivities associated to the same element can break the multiple degeneracies. Examples: Na: NaD + Na0.82um + Na1.14um TiO: TiO1, TiO2, TiO0.89um La Barbera et al. (in prep) XSHOOTER high S/N spectrum Fe: <Fe>, FeH0.99

12 Challenge #2: Driving mechanisms of IMF variations Although noisier, the results for stacks assembled at fxed velocity dispersion (σ) AND [α/fe] show that the main trend is with σ (in contrast with Conroy-van Dokkum 2012; see also Smith 2014). Other trends: Anticorrelation with mass density (Spiniello et al. 2015); Metallicity (Martín-Navarro et al. 2015) Smith (2014, MNRAS, 443, L69) La Barbera et al. (2015, MNRAS, 449, L137)

13 Challenge #2: Driving mechanisms of IMF variations Integral Field Spectroscopy gives a more local view of the driver of IMF variations. SDSS gives a trend with respect to the integrated velocity dispersion within the 3 arcsec fbre. CALIFA (IFU) data from 24 ETGs show that locally the IMF slope does not correlate with velocity dispersion. Metallicity (in high velocity dispersion regions) may be the local driver of IMF variations. (However, see McDermid et al. 2015) Martín-Navarro et al. (2015, ApJ, 806, L31)

14 Challenge #3: Constraints on stellar M/L log Φ The observations mostly constrain the fraction in stars with mass below some threshold (~0.5-1M ), but diferent functional forms of the IMF can result in diferent values of M/L log Φ log M log M La Barbera, IF, et al. (2013, MNRAS, 433, 3017)

15 Challenge #3: Constraints on stellar M/L Strong gravitational lensing constrains the line-of-sight(total) mass with special accuracy inside the Einstein radius. Ferreras et al. (2010, MNRAS, 409, L30)

16 Challenge #3: Constraints on stellar M/L A few strong gravitational lenses seem to feature MWlike stellar M/L that rule out even the Salpeter case (Smith & Lucey 2013; Smith, Lucey & Conroy 2015). However, the SLACS lenses suggest increased stellar M/L with vel.disp. (Posacki et al. 2014). Are the Smith lenses special? Smith et al. (2015)

17 Challenge #4: Chemical enrichment A time-invariant bottomheavy IMF is incapable of reaching solar metallicity Microphysics : Star forming regions with a high Machnumber ISM could trigger fragmentation (e.g. Padoan & Nordlund 2002; Hopkins 2012; Chabrier et al. 2014) Observed on-going starbursts suggest a top-heavy IMF. If IMF is bottom-heavy in massive ETGs, it must be time-dependent Weidner, IF et al. (2013. MNRAS, 435, 2274) Ferreras et al (2015, MNRAS, 448, L82)

18 Challenges to overcome Challenge #0: Observational Careful selection of sample, better data Challenge #1: Interpretation of line strengths Better models, new line strengths (NUV-optical-NIR). Selection of (many) spectral lines with varying dependencies Challenge #2: Driving mechanisms of IMF variation Contrast integrated and spatially resolved (IFU/long slit) measurements. Challenge #3: Stellar M/L constraints IMF functional forms: beyond simple power law(s) Challenge #4: Chemical enrichment Search for independent probes in high-z star forming systems