The Mid-Infrared Luminosities of Normal Galaxies over Cosmic Time (discussion of arXiv: ) Urtzi Jauregi Astro debata,

IR astronomy From 24 μm

TP-AGBs (II)

Are the blue lines planetary nebula? How else can you jump from log T= 4 to 6 at constant L?

Thermally Pulsating AGB Stars ● Late stage of the AGB branch and the fate of all low-to- medium mass stars (0.6 – 10 M O ). ● Energy comes from H burning w. periodical He shell flashes → periodic pulsations and mass loss → Large near- and mid-IR emission. ● Strong convection during flashes → dredge-up from lower layers. If M > 2Mo → C star; if less → M star. ● If C star, major dust productor.

Step 1: What IR flux to expect? ● IS medium is transparent to IR, so strong IR sources – like TP-AGPs– are essential to map stellar density and formation rate → we want to know expected near and mid- IR luminosities. ● If, for a sample of TP-AGS we know: their K-band luminosity with time (model) and the near-to-mid IR colors (observations), we can get their integrated mid-IR luminosity (how? like a IR- restricted variant of the bolometric correction?).

Step 1: Is IR flux mainly due to TP-AGP? Question: from what population do they derive the blue and red lines?

Step 1: Conclusions ● The mid-IR luminosity from TP-AGBs describes observed galactic luminosity well → TP-AGBs are indeed the dominant galactic IR source. ● Bonus result: The simple model of fig.1(a), based on just circumstellar material (for stellar populations aged up to 1.5 Gy, back to z = 2), explains observed IR luminosities very well!

Step 2: Building galaxies with different SFHs and calculating IR colors. ● They simulate galaxies with different formation rates (SFH), make them evolve with time, and derive their IR colors. ● For that, they build several Maraston model galaxies (see Sec. 4) with the parameters of Table 1 (question: do they use the color-color data of fig. 2 as input? Isn't the point getting them?).

Formation Histories

Step 2: Results Question: UGRIZ fotometry bands, redshifted to IR? ● Successfully reproduces blue stellar formation sequence and red quiescent stage (fig. a – c). The rest is observed data. ● Good correlation between the 24 μm luminosity and the optical (fig. b, c). ● Good agreement with previous observed data (fig. a – c). ● The key result of the paper: Successfully reproduces observed correlation of 12μm luminosity and star formation rate, but only when averaged over the TP-AGB formation age. (fig. d). ● The time-scale here is much longer than previously accepted.

Step 2: Implications ● The mid-IR is sensitive to the specific SFH over the timescale of TP-AGB star formation (1.5 Gy). You have to know the stellar formation history very well to be able to derive observed IR fluxes; the reverse is also true. ● Mis-estimating these can bring huge errors in the estimated stellar formation rate (between 2 and 6). (question: are the SFR averaged over many galaxies reliable?) ● This affects everything we know about star formation and galactic evolution.

Step 3: Modelling M81 Question: how do they derive extinction across the galactic disk from fig. 3d? And what is exactly the black line?

Step 3: M81 (2) ● The agreement is good (ignoring the many unknown variables and uncertainties in the “goodness” of the coordinates). ● Conclusion: The mid-IR flux from M81 is indeed largely due to TP-AGBs.

Conclusions! ● By assuming galactic flux in key IR bands (K, 24 μm) comes largely from TP-AGBs (with a very simple flux vs time law), they reproduced almost from first principles: ● much of the observed galactic IR colors; ● the correlation between galactic 24 μm flux and star formation rate in the TP-AGB formation era; ● the poorly-understood correlation between galactic optical (UGRIZ?) and mid-IR fluxes; ● And, as a nice extra, the observed 24μm flux of M81.

More conclusions! ● However, they found very different timescales of stellar formation from others. As a consequence, SFR in the TP- AGB production era might be up to 6 higher than previously thought. This would have deep consequences. ● There is still a lot of work to be done to find out SFR for z<2, but this is a step in the right direction.

Hvala!