1. Revisiting the Classics: Is [Mg/Fe] a good proxy for
galaxy formation timescales?
Akinsanmi A. Babatunde
Department of Astronomy
University of Porto
Paper by: Ignacio Martin Navarro (2015)
AST 4001, June 2016
Akinsanmi A. Babatunde (University of Porto)Revisiting the Classics: Is [Mg/Fe] a good proxy for galaxy formation timescales?AST 4001, June 2016 1 / 26

2. Outline
1 INTRODUCTION
2 MOTIVATION
3 ETGs
4 DATA
5 ANALYSIS & RESULTS
6 DISCUSSION
7 CONCLUSIONS
8 REFERENCES
Akinsanmi A. Babatunde (University of Porto)Revisiting the Classics: Is [Mg/Fe] a good proxy for galaxy formation timescales?AST 4001, June 2016 2 / 26

3. INTRODUCTION
Introduction
Early-Type Galaxies(ETGs) are interesting for stellar formation studies
due to their relatively simple formation.
ETGs have old and α-enhanced populations with more massive
systems being older and possessing increasing [Mg/Fe] ratio
This is related to the correlation found between Mg features and
galaxy velocity dispersion(Worthey & Collobert 2003.)
Akinsanmi A. Babatunde (University of Porto)Revisiting the Classics: Is [Mg/Fe] a good proxy for galaxy formation timescales?AST 4001, June 2016 3 / 26

4. MOTIVATION
Motivation
How do we explain the over abundant Mg/Fe ratio in massive ETGs?
Akinsanmi A. Babatunde (University of Porto)Revisiting the Classics: Is [Mg/Fe] a good proxy for galaxy formation timescales?AST 4001, June 2016 4 / 26

5. MOTIVATION
Motivation
How do we explain the over abundant Mg/Fe ratio in massive ETGs?
1 different formation time-scales compared to the solar neighbourhood.
2 a non-universal stellar initial mass function (IMF), flatter than
observed in the Milky Way.
3 selective mass loss.
Akinsanmi A. Babatunde (University of Porto)Revisiting the Classics: Is [Mg/Fe] a good proxy for galaxy formation timescales?AST 4001, June 2016 4 / 26

6. ETGs
Prelude
Massive ETGs form (τ ≤ 1GYr) at high redshifts with intense star
formation within massive halos giving rise to old & [Mg/Fe] enhanced
system.
With the Galaxy mass, the age, [Mg/Fe], σ can be naturally
explained.
Assuming a Milky-Way like IMF, the formation timescale can also be
retrieved from spectrum of ETG(Thomas et al. 2005).
Akinsanmi A. Babatunde (University of Porto)Revisiting the Classics: Is [Mg/Fe] a good proxy for galaxy formation timescales?AST 4001, June 2016 5 / 26

7. ETGs
ETG
But IMF of ETGs are not universal having a systematic departure from
Milky-Way like IMF as galaxy mass increases.
Massive ETGs (σ ∼ 300kms−1) have steep IMF leading to an
enhanced population of M-dwarf stars while
Low-mass (σ ∼ 100kms−1) have Milky-Way like IMF.
A steep IMF would impact the chemical evolution of the galaxy and the
SFR.
Akinsanmi A. Babatunde (University of Porto)Revisiting the Classics: Is [Mg/Fe] a good proxy for galaxy formation timescales?AST 4001, June 2016 6 / 26

8. DATA
Impact of non-universal IMF on [Mg/Fe] ratio
[Mg/Fe] is traditionally considered as good proxy for formation
timescale
Massive ETGs have bottom-heavy IMF & enhanced [Mg/Fe]
This would lead to a τ ≤ 0.01GYr formation & SFR ∼ 105M Yr−1 at
high z
Akinsanmi A. Babatunde (University of Porto)Revisiting the Classics: Is [Mg/Fe] a good proxy for galaxy formation timescales?AST 4001, June 2016 7 / 26

9. DATA
How?
Taking 124 ETGs covering stellar mass range 100kms−1 ≤ σ ≤ 300kms−1.
Start with chemical evolutionary model (Thomas et al. 2005):
explored IMF, τ, and [Mg/Fe]
Using Thielemann et al. (1996) stellar yield on clumpy collapse model.
Finally, reference IMF-σ relation of La Barbera et al. (2013) for IMF
parameterization.
Akinsanmi A. Babatunde (University of Porto)Revisiting the Classics: Is [Mg/Fe] a good proxy for galaxy formation timescales?AST 4001, June 2016 8 / 26

10. ANALYSIS & RESULTS
Current ETG formation
SFH of ETGs with σ ∼ 190, 230, 270, 300kms−1.
Akinsanmi A. Babatunde (University of Porto)Revisiting the Classics: Is [Mg/Fe] a good proxy for galaxy formation timescales?AST 4001, June 2016 9 / 26

11. ANALYSIS & RESULTS
Dependence on IMF
1. Use IMF-σ relation to translate σ of each Galaxy to expected IMF slope.
σ ∼150km/s are represented by MW IMF but more massive ETGs require steeper IMF
Akinsanmi A. Babatunde (University of Porto)Revisiting the Classics: Is [Mg/Fe] a good proxy for galaxy formation timescales?AST 4001, June 2016 10 / 26

12. ANALYSIS & RESULTS
Dependence on IMF
2. Interpret the observed [Mg/Fe] ratios on the basis of these updated
IMF values.re-calculate the typical formation time-scale of each galaxy in
the sample.
Akinsanmi A. Babatunde (University of Porto)Revisiting the Classics: Is [Mg/Fe] a good proxy for galaxy formation timescales?AST 4001, June 2016 11 / 26

13. ANALYSIS & RESULTS
Dependence on IMF
Therefore, the expected formation time-scale for massive galaxies is
given by the intersection corresponding to τ = 0.007Gyr.
We see then that a high [Mg/Fe] can be reached either in a fast
formation event or assuming a flat IMF (Γ = 0.7).
But we know IMF in massive galaxies is not flat but even steeper
than that of Milkyway Standard (Γ = 1.3)
Akinsanmi A. Babatunde (University of Porto)Revisiting the Classics: Is [Mg/Fe] a good proxy for galaxy formation timescales?AST 4001, June 2016 12 / 26

14. ANALYSIS & RESULTS
Dependence on IMF
Therefore, the expected formation time-scale for massive galaxies is
given by the intersection corresponding to τ = 0.007Gyr.
We see then that a high [Mg/Fe] can be reached either in a fast
formation event or assuming a flat IMF (Γ = 0.7).
But we know IMF in massive galaxies is not flat but even steeper
than that of Milkyway Standard (Γ = 1.3)
Therefore, the more massive an ETG is, the steeper the IMF and the
shorter the time-scale is compared to the standard view.
Akinsanmi A. Babatunde (University of Porto)Revisiting the Classics: Is [Mg/Fe] a good proxy for galaxy formation timescales?AST 4001, June 2016 12 / 26

15. ANALYSIS & RESULTS
ETG formation if [Mg/Fe] and IMF are taken into account
Akinsanmi A. Babatunde (University of Porto)Revisiting the Classics: Is [Mg/Fe] a good proxy for galaxy formation timescales?AST 4001, June 2016 13 / 26

16. ANALYSIS & RESULTS
Results
The comparison between Fig1 and Fig4 shows the strong influence of
the IMF on the chemical evolution of ETGs
While low-mass ETGs, well modelled with a universal IMF remain
same, massive ETGs require now extremely short and intense
star-formation episodes.
ETG of σ ≥ 300kms−1 with IMF slope Γ = 2.1 and [Mg/Fe]=0.3dex
would form as fast as τ = 0.007Gyr
Akinsanmi A. Babatunde (University of Porto)Revisiting the Classics: Is [Mg/Fe] a good proxy for galaxy formation timescales?AST 4001, June 2016 14 / 26

17. ANALYSIS & RESULTS
Analysis
Despite the results, cooling and recycling all ejected material in only
7Myr seems unreasonable.(Thomas et al. 2005).
And this predicted SFR is orders of magnitude larger than that
observed in most extreme sub-millimeter galaxies. (Barger et al. 2014)
Akinsanmi A. Babatunde (University of Porto)Revisiting the Classics: Is [Mg/Fe] a good proxy for galaxy formation timescales?AST 4001, June 2016 15 / 26

18. DISCUSSION
Discussion
We see steep IMF slope and the enhanced [Mg/Fe] ratio observed in
massive ETGs would imply unrealistically short formation time-scales.
So the use of [Mg/Fe] as a proxy for galaxy formation warrants
caution.
Akinsanmi A. Babatunde (University of Porto)Revisiting the Classics: Is [Mg/Fe] a good proxy for galaxy formation timescales?AST 4001, June 2016 16 / 26

19. DISCUSSION
Discussion
It is widely accepted that massive ETGs have super-solar [Mg/Fe]
ratios.
A universal IMF on the other hand is debatable.
Dynamical and stellar population IMF determinations reports the
trend with galaxy mass though.
If models used to explain this trend are correct then we can invoke 2
scenarios to reconcile [Mg/Fe] and IMF observations:
Akinsanmi A. Babatunde (University of Porto)Revisiting the Classics: Is [Mg/Fe] a good proxy for galaxy formation timescales?AST 4001, June 2016 17 / 26

20. DISCUSSION
Scenarios: Non-canonical IMF shape
Nearby massive ETGs are old systems, so are populated only by stellar
masses < 1M
∴ IMF studies are actually probing the IMF low-mass end. while
[Mg/Fe] ratio is settled by massive stars that is sensitive only to the
high-mass end of the IMF.
construct IMF
1 with enhanced massive star fraction to match observed [Mg/Fe]
2 also with enhanced fraction of low-mass stars to reproduce the IMF
observations.
Akinsanmi A. Babatunde (University of Porto)Revisiting the Classics: Is [Mg/Fe] a good proxy for galaxy formation timescales?AST 4001, June 2016 18 / 26

21. DISCUSSION
Issues
High fraction of low-mass stars would prevent massive ETGs from
reaching their observed high-[Mg/Fe].
The steep IMF slope required might lead to extremely high M/L
ratios, inconsistent with dynamical and lensing studies(Ferreras et al. 2013a,
2015).
Akinsanmi A. Babatunde (University of Porto)Revisiting the Classics: Is [Mg/Fe] a good proxy for galaxy formation timescales?AST 4001, June 2016 19 / 26

22. DISCUSSION
Scenarios: Time-Varying IMF
IMF evolving with subsequent generation of stars formed
Flat IMF for 1st generation giving high fraction of massive stars.
subsequent Steeper IMF producing lesser massive stars.
∴ In the local universe we would observe the latter generation but with the
chemical imprint of the first generation of stars
(Vazdekis et al. 1997; Weidner et al. 2013; Ferreras et al. 2013a, 2015)
Akinsanmi A. Babatunde (University of Porto)Revisiting the Classics: Is [Mg/Fe] a good proxy for galaxy formation timescales?AST 4001, June 2016 20 / 26

23. DISCUSSION
Scenarios
These two scenarios would lead to old stellar populations, with
super-solar [Mg/Fe] ratios and an enhanced fraction of low-mass stars
as observed in nearby massive ETGs.
uncertainties in the IMF shape prevents favouring one explanation
over the other.
Hence, to understand the chemical evolution of massive ETGs, it is
necessary to observe them at high redshift while forming.
Akinsanmi A. Babatunde (University of Porto)Revisiting the Classics: Is [Mg/Fe] a good proxy for galaxy formation timescales?AST 4001, June 2016 21 / 26

24. DISCUSSION
Back to the Question
Is it safe then to interpret the [Mg/Fe] ratio as an indicator of the
formation time-scale of a galaxy?
Akinsanmi A. Babatunde (University of Porto)Revisiting the Classics: Is [Mg/Fe] a good proxy for galaxy formation timescales?AST 4001, June 2016 22 / 26

25. DISCUSSION
Back to the Question
Is it safe then to interpret the [Mg/Fe] ratio as an indicator of the
formation time-scale of a galaxy?
Surprisingly,[Mg/Fe]-based time scales are in great agreement with
those obtained through the analysis of SFH (de La Rosa et al. 2011; McDermid et al.
2015)
Akinsanmi A. Babatunde (University of Porto)Revisiting the Classics: Is [Mg/Fe] a good proxy for galaxy formation timescales?AST 4001, June 2016 22 / 26

26. CONCLUSIONS
conclusions
Without a proper knowledge of the chemical evolution of galaxies, a
naive interpretation of z ∼ 0 observations could result into misleading
conclusions.
Joint consideration of [Mg/Fe] ratio and IMF slope observed in nearby
massive ETGs would lead to unrealistic formation time-scales and
SFR. Therefore,one has to be careful while interpreting the [Mg/Fe]
ratio purely as a tracer of the formation time-scale of a galaxy.
Akinsanmi A. Babatunde (University of Porto)Revisiting the Classics: Is [Mg/Fe] a good proxy for galaxy formation timescales?AST 4001, June 2016 23 / 26

27. CONCLUSIONS
conclusions
The non-universality of the IMF in massive ETGs must be further
investigated.
Direct probe of the IMF during the formation epoch of massive
galaxies will clarify the debate between a non-canonical or a
time-varying IMF.
Akinsanmi A. Babatunde (University of Porto)Revisiting the Classics: Is [Mg/Fe] a good proxy for galaxy formation timescales?AST 4001, June 2016 24 / 26

28. REFERENCES
References
Barger et al.2014, ApJ, 784, 9
de La Rosa et al. 2011, MNRAS, 418, L74
Ferreras et al. 2013a, preprint, (arXiv:1312.5317) 2015
McDermid et al. 2015 MNRAS, 448, 3484
Thomas D., Maraston C., Bender R., Mendes de Oliveira C., 2005,
ApJ, 621, 673
Vazdekis et al., 2015, MNRAS, 449, 1177
La Barbera F., Ferreras I., Vazdekis A., de la Rosa I. G., de Carvalho
R. R., Trevisan M., Falcon-Barroso J., Ricciardelli E., 2013, MNRAS,
433, 3017
Weidner et al. 2013
Akinsanmi A. Babatunde (University of Porto)Revisiting the Classics: Is [Mg/Fe] a good proxy for galaxy formation timescales?AST 4001, June 2016 25 / 26

29. REFERENCES
Akinsanmi A. Babatunde (University of Porto)Revisiting the Classics: Is [Mg/Fe] a good proxy for galaxy formation timescales?AST 4001, June 2016 26 / 26