Evidence of multiple stellar components in high-z elliptical galaxies from spectral indices and high resolution images

Transcription

1 Evidence of multiple stellar components in high-z elliptical galaxies from spectral indices and high resolution images ILARIA LONOCE ADRIANA GARGIULO Paolo Saracco Marcella Longhetti Sonia Tamburri

2 Outline AIM: studying the stellar population properties of early-type galaxies (ETGs) at high redshift with Spectral analysis of high redshift ( z ~1)ETGs based on the measure of blue spectral indices on a sample of about 15 ETGs Colour gradients analysis of a sample of cluster and field ETGs at high redshift ( z ~1.5)(A. Gargiulo) Combined analysis on the same galaxy: 1 Detect the younger component Localize it within the galaxy

3 The spectroscopic sample of ETGs Sample of 15 ETGs at 0.7 < z < 1.1 morphologically selected (see poster by S.Tamburri) from a catalogue complete to K~22 in the GOODS-South field. 14 photometric bands coverage: F435W, F606W, F775W, F850LP, U35, U38, Uvim, J, H, K, 3.6μm, 4.5μm, 5.8μm and 8.0μm Optical spectra with high S/N from: Van der Wel at al. 2005: VLT-FORS2 (MXU) GRISM600, resolution R=1390 Popesso et al. 2009: VLT-VIMOS (MOS) MR, resolution R=580 Mignoli et al (K20 Survey): VLT-FORS1/FORS2 (MXU) GRISM150, resolution=260 GRISM200, resolution=380 GRISM300, resolution=660

4 Age-dependent spectrophotometric indices: Δ4000 and H+K(CaII) Δ4000 index: Relative flux (arbitrary units) Age=0.6 Gyr Age=6 Gyr 1 range (Hamilton 1985) 2 range Δ4000 index provides a measure of the mean age of the stellar population Δ ( 4000)= Mean Flux ( Å) Mean Flux ( Å) Bruzual&Charlot 2003 models Changes among models with different star formation hystories are more evident for ages less than about 6-7 Gyr, which is the age of Universe at the redshift of the sample galaxies. τ = star formation time scale TREND WITH AGE 3

5 Age-dependent spectrophotometric indices: Δ4000 and H+K(CaII) H+K(CaII) index: (Rose 1985) Relative flux (arbitrary units) Age=0.6 Gyr Age=6 Gyr >1 K H <1 Index H+K(CaII) is more sensitive to the presence of young populations because H line is blended with Balmer Hε H+ K (CaII )= Minimum Flux ( H) Minimum Flux (K ) Bruzual&Charlot 2003 models H(CaII)+Hε Å K(CaII) Å τ = star formation time scale TREND WITH AGE 4

6 Age-dependent spectrophotometric indices: Δ4000 and H+K(CaII) H+K(CaII) index: H+ K (CaII )= K K K Minimum Flux ( H) Minimum Flux (K ) H H H Hδ H+K=1.13 Hδ H+K=0.87 Hδ H+K=0.77 Bruzual&Charlot 2003 models 5

7 Comparison with BC03 models Bruzual&Charlot 2003 models ~2Gyr Z=0.02 τ = 0.1 Gyr Many measured values are not consistent with models with different τ, neither with different metallicity. In particular, four objects have H+K(CaII) index > 1.2 which do not find any correspondences with models. 6

8 Comparison with BC03 models Bruzual&Charlot 2003 models 1.2 Z=0.02 τ = 0.1 Gyr Many measured values are not consistent with models with different τ, neither with different metallicity. In particular, four objects have H+K(CaII) index > 1.2 which do not find any correspondences with models. 6

9 The double-components hypothesis Comparison among age estimates deduced from: Age from Δ4000 [Gyr] Standard SED-fitting Index H+K(CaII) Index Δ(4000) Age from H+K(CaII) [Gyr] 7

10 The double-components hypothesis Comparison among age estimates deduced from: Age from Δ4000 [Gyr] Standard SED-fitting Index H+K(CaII) Index Δ(4000) NOT ALWAYS IN AGREEMENT Δ(4000) points to older ages Age from H+K(CaII) [Gyr] 7

11 The double-components hypothesis Comparison among age estimates deduced from: Age from Δ4000 [Gyr] Standard SED-fitting Index H+K(CaII) Index Δ(4000) NOT ALWAYS IN AGREEMENT Δ(4000) points to older ages Age from H+K(CaII) [Gyr] THE STAR FORMATION HISTORY OF THESE GALAXIES IS MORE COMPLEX Guess: presence of double stellar component with different ages (in agreement with Gargiulo et al. 2011,2012) 7

12 Double-components analysis Example: Few % of young component at 0.5 Gyr Bruzual&Charlot 2003 models Z=0.020 τ=0.1 Gyr 8

13 Double-components analysis Example: Few % of young component at 0.5 Gyr Bruzual&Charlot 2003 models Z=0.020 τ=0.1 Gyr ID-7424 example Age from SED fitting: 2.6 Gyr (Av=0.2) Age old component ~6.25 Gyr (83.3%) Age young component ~0.7 Gyr (16.7%) Av=0.5 8

14 Balmer Hε emission line Bruzual&Charlot 2003 models H+K=1.13 K H H+K=1.37 K H H+K=1.43 K H Some objects have a too high value of the index H+K(CaII) which have not a corrispondence with simple models: H+K(CaII)>1.2 This could be explained by the presence of emissions of gas around star forming regions. H line is filled by the presence of Hε emission 9 The value of the index H+K(CaII) increases

15 Double-components analysis with emissions Example: Few % of young component at Gyr Bruzual&Charlot 2003 models Z=0.020 τ=0.1 Gyr 10

16 Double-components analysis with emissions Example: Few % of young component at Gyr ID-9066 Bruzual&Charlot 2003 models Z=0.020 τ=0.1 Gyr example Age from SED fitting: 2 Gyr (Av=0.6) Age old component ~2 Gyr (99.25%) Age young component ~6 Myr (0.75%) Av=2.8 10

17 Comparison with high-z data With emissions Field galaxies 1.4 < z < 1.8 Also at higher z the younger component is required to explain the data at any bin of redshift 11

18 Results All the galaxies of the sample are well represented by a bulk of old stars with superimposed a small mass secondary younger component. Mean old component age = 4 Gyr Mean young component age = 0.4 Gyr 12 Few % In some cases the younger component is so young to show signs of recent star formation. The younger component is detected at any redshift and it is always younger than 1 Gyr.

19 Colour gradients The gradient of the colour X-Y is classically defined as the logarithmic slope of the color profile (Peletier et al. 1990) (X-Y)(r) = μx(r)-μy(r) μx(r), μy(r): light profiles in X and Y band gives quantitative information on the radial colour variation in galaxies. NEGATIVE COLOUR GRADIENT: galaxy redder toward the centre 13 POSITIVE COLOUR GRADIENT: galaxy bluer toward the centre

20 Colour gradients The gradient of the colour X-Y is classically defined as the logarithmic slope of the color profile (Peletier et al. 1990) (X-Y)(r) = μx(r)-μy(r) μx(r), μy(r): light profiles in X and Y band gives quantitative information on the radial colour variation in galaxies. NEGATIVE COLOUR GRADIENT: galaxy redder toward the centre POSITIVE COLOUR GRADIENT: galaxy bluer toward the centre COLOUR GRADIENTS: WHAT INFORMATION CAN WE DERIVE FROM? Colour variations 13 Variation of the properties of the underlying stellar population

21 Colour gradients & spectral indices Common object: ID-1950 z=1.044 Spectral analysis: Gradient analysis: Age main component ~ 5 Gyr Age younger component 0.5 Gyr Age younger component < 1 Gyr red blue U-B U-B Positive gradient Younger component < 1 Gyr Same conclusion: this galaxy shows a younger component with age < 1 Gyr localized in its inner region. 14

22 Colour gradients & spectral indices Common object: ID-1950 z=1.044 Spectral analysis: Gradient analysis: Age main component ~ 5 Gyr Age younger component 0.5 Gyr Age younger component < 1 Gyr red blue U-B U-B Positive gradient Younger component < 1 Gyr Same conclusion: this galaxy shows a younger component with age < 1 Gyr localized in its inner region. 14

23 Conclusions COLOUR GRADIENTS Localize the younger component in the centre of galaxies 15 SPECTRAL INDICES Presence of a younger stellar component (observed at any z and younger than 1 Gyr) Quantify the amount of the younger component

24 Conclusions COLOUR GRADIENTS Localize the younger component in the centre of galaxies SPECTRAL INDICES Presence of a younger stellar component (observed at any z and younger than 1 Gyr) Quantify the amount of the younger component WHAT IS ITS ORIGIN? 15 Merging? Constant weak star formation activity induced by cold accretion

25 Future prospects Extend the spectral analysis on a wider sample of high-z ETGs with high S/N spectra (see poster by A. Gargiulo) Combined analysis of spectral indices and colour gradients to give constraints also on metallicity Thank you 16

26 Comparison Jørgensen et al With emissions Cluster galaxies Z ~ 0.8 RXJ RXJ GEE3 Padova November 2014

27 Components age distributions OLD YOUNG < 1 Gyr Mean age 2 Gyr Mean age 5.4 Gyr There is no dependence on redshift

28 Components age distributions The SED fitting analysis extracts mean ages all younger than 3.25 Gyr. Respect to the results obtained from the doublecomponents analysis, the photometric analysis underestimates the age of the bulk of stars. This has conseguences also in the stellar mass estimate.