revisiting the classics

Revisiting the Classics: Is [Mg/Fe] a good proxy forgalaxy formation timescales?

Akinsanmi A. Babatunde

Department of AstronomyUniversity 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

Outline

1 INTRODUCTION

2 MOTIVATION

3 ETGs

4 DATA

5 ANALYSIS & RESULTS

6 DISCUSSION

7 CONCLUSIONS

8 REFERENCES


INTRODUCTION

Introduction

Early-Type Galaxies(ETGs) are interesting for stellar formation studiesdue to their relatively simple formation.

ETGs have old and α-enhanced populations with more massivesystems being older and possessing increasing [Mg/Fe] ratio

This is related to the correlation found between Mg features andgalaxy velocity dispersion(Worthey & Collobert 2003.)


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 thanobserved in the Milky Way.

3 selective mass loss.


ETGs

Prelude

Massive ETGs form (τ ≤ 1GYr) at high redshifts with intense starformation within massive halos giving rise to old & [Mg/Fe] enhancedsystem.

With the Galaxy mass, the age, [Mg/Fe], σ can be naturallyexplained.Assuming a Milky-Way like IMF, the formation timescale can also beretrieved from spectrum of ETG(Thomas et al. 2005).


ETGs

ETG

But IMF of ETGs are not universal having a systematic departure fromMilky-Way like IMF as galaxy mass increases.

Massive ETGs (σ ∼ 300kms−1) have steep IMF leading to anenhanced 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 theSFR.


DATA

Impact of non-universal IMF on [Mg/Fe] ratio

[Mg/Fe] is traditionally considered as good proxy for formationtimescale

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


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 IMFparameterization.


ANALYSIS & RESULTS

Current ETG formation

SFH of ETGs with σ ∼ 190, 230, 270, 300kms−1.


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


ANALYSIS & RESULTS

Dependence on IMF

2. Interpret the observed [Mg/Fe] ratios on the basis of these updatedIMF values.re-calculate the typical formation time-scale of each galaxy inthe sample.


ANALYSIS & RESULTS

Dependence on IMF

Therefore, the expected formation time-scale for massive galaxies isgiven by the intersection corresponding to τ = 0.007Gyr .

We see then that a high [Mg/Fe] can be reached either in a fastformation event or assuming a flat IMF (Γ = 0.7).

But we know IMF in massive galaxies is not flat but even steeperthan that of Milkyway Standard (Γ = 1.3)

Therefore, the more massive an ETG is, the steeper the IMF and theshorter the time-scale is compared to the standard view.


ANALYSIS & RESULTS

ETG formation if [Mg/Fe] and IMF are taken into account


ANALYSIS & RESULTS

Results

The comparison between Fig1 and Fig4 shows the strong influence ofthe IMF on the chemical evolution of ETGs

While low-mass ETGs, well modelled with a universal IMF remainsame, massive ETGs require now extremely short and intensestar-formation episodes.

ETG of σ ≥ 300kms−1 with IMF slope Γ = 2.1 and [Mg/Fe]=0.3dexwould form as fast as τ = 0.007Gyr


ANALYSIS & RESULTS

Analysis

Despite the results, cooling and recycling all ejected material in only7Myr seems unreasonable.(Thomas et al. 2005).

And this predicted SFR is orders of magnitude larger than thatobserved in most extreme sub-millimeter galaxies. (Barger et al. 2014)


DISCUSSION

Discussion

We see steep IMF slope and the enhanced [Mg/Fe] ratio observed inmassive ETGs would imply unrealistically short formation time-scales.

So the use of [Mg/Fe] as a proxy for galaxy formation warrantscaution.


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 thetrend with galaxy mass though.

If models used to explain this trend are correct then we can invoke 2scenarios to reconcile [Mg/Fe] and IMF observations:


DISCUSSION

Scenarios: Non-canonical IMF shape

Nearby massive ETGs are old systems, so are populated only by stellarmasses < 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 thehigh-mass end of the IMF.

construct IMF1 with enhanced massive star fraction to match observed [Mg/Fe]2 also with enhanced fraction of low-mass stars to reproduce the IMF

observations.


DISCUSSION

Issues

High fraction of low-mass stars would prevent massive ETGs fromreaching their observed high-[Mg/Fe].

The steep IMF slope required might lead to extremely high M/Lratios, inconsistent with dynamical and lensing studies(Ferreras et al. 2013a,2015).


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 thechemical imprint of the first generation of stars(Vazdekis et al. 1997; Weidner et al. 2013; Ferreras et al. 2013a, 2015)


DISCUSSION

Scenarios

These two scenarios would lead to old stellar populations, withsuper-solar [Mg/Fe] ratios and an enhanced fraction of low-mass starsas observed in nearby massive ETGs.

uncertainties in the IMF shape prevents favouring one explanationover the other.

Hence, to understand the chemical evolution of massive ETGs, it isnecessary to observe them at high redshift while forming.


DISCUSSION

Back to the Question

Is it safe then to interpret the [Mg/Fe] ratio as an indicator of theformation time-scale of a galaxy?

Surprisingly,[Mg/Fe]-based time scales are in great agreement withthose obtained through the analysis of SFH (de La Rosa et al. 2011; McDermid et al.2015)


CONCLUSIONS

conclusions

Without a proper knowledge of the chemical evolution of galaxies, anaive interpretation of z ∼ 0 observations could result into misleadingconclusions.

Joint consideration of [Mg/Fe] ratio and IMF slope observed in nearbymassive ETGs would lead to unrealistic formation time-scales andSFR. Therefore,one has to be careful while interpreting the [Mg/Fe]ratio purely as a tracer of the formation time-scale of a galaxy.


CONCLUSIONS

conclusions

The non-universality of the IMF in massive ETGs must be furtherinvestigated.

Direct probe of the IMF during the formation epoch of massivegalaxies will clarify the debate between a non-canonical or atime-varying IMF.


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 CarvalhoR. R., Trevisan M., Falcon-Barroso J., Ricciardelli E., 2013, MNRAS,433, 3017

Weidner et al. 2013


REFERENCES


revisiting the classics

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