# In extragalactic spectroscopy, what is D4000?

In relation to astronomy what is D4000? In my work I understand it is a good indicator of a stars age and I believe it is a spectral region, but I am having a hard time even finding resources to help me better understand it. My specific questions are this: what is D4000? what units is it measured in? how is it used to determine stellar population age? Alternatively, what are some resources or recommended literature for getting a better understanding of it?

For some additional context, I am reading Van Der Wel, et al. (2016).

• This is unanswerable (except possibly for a narrow cross-section of people who are doing exactly the same thing as you?) without additional context, which you should provide. – Emilio Pisanty Jul 19 '18 at 14:35
• @EmilioPisanty any extragalactic astronomer should know this. – Kyle Oman Jul 19 '18 at 14:41
• Sure, as for context: 'Balmer absorption lines and the spectral region around D4000 are the most sensitive diagnostics of stellar population age' I wish to understand the technical reasons for why this is. – QuantumPanda Jul 19 '18 at 14:45
• @KyleOman That may be, but this is still the kind of niche term where the site standards generally require at least a reference to a place where the asker saw the term being used. (Note that googling D4000 on an astronomy-agnostic machine does not return any relevant links.) Extragalactic astronomy should be no different. – Emilio Pisanty Jul 19 '18 at 14:49

A good technical overview is available here, and includes references for further reading.

I'll quote one passage:

Hereafter the 4000 angstrom break is defined as the ratio between the average flux density in ${\rm erg}\,{\rm s}^{−1}\,{\rm cm}^{−2}\,{\rm Hz}^{−1}$ between 4050 and 4250 angstrom and that between 3750 and 3950 angstrom (Bruzual 1983).

As it is a ratio, it has no units.

The drop in intensity at the blue end of the spectrum basically comes from (i) a relative lack of young, bright, blue stars in old stellar populations and (ii) increased metal absorption in stellar atmospheres in an older, more metal rich population. The particularly relevant spectral features for (ii) are Calcium H and K (3934 angstrom and 3969 angstrom), G-band (4304 angstrom), Magnesium (5175 angstrom) and Sodium (5894 angstrom).

• Thank you, i am currently an undergrad working for Dr. Muzzin he told me to say hi! :) – QuantumPanda Jul 20 '18 at 16:53
• Ha, say hi to Adam for me! – Kyle Oman Jul 21 '18 at 14:38

A Google search turned up the following information:

D4000 is a term used by Gallazzi e. al. [1] for the 4000-Å break index of Balogh et al. [2] which they define as the ratio of the average flux densities in the spectral regions 4000–4100 Å and 3850–3950 Å.

Balogh et al. [2] found that galaxies with older stellar populations have weaker OII and Hδ lines and stronger D4000 indexes. Conversely, weaker D4000 indexes and stronger Hδ and OII lines indicate younger populations.

As it is a ratio, there are no units.

1. Anna Gallazzi Stéphane Charlot Jarle Brinchmann Simon D. M. White Christy A. Tremonti, MNRAS, 362(1), 41

2. Balogh M. L. Morris S. L. Yee H. K. C. Carlberg R. G. Ellingson E., 1999, ApJ , 527, 54

• I apologize, I should have made it more clear in my question, you did answer the questions I have asked but I wish to know how D4000 determines stellar population age. As per my question I understand it is indicative of age, but how? – QuantumPanda Jul 19 '18 at 14:49