Fake Star Formation Bursts: Blue Horizontal Branch Stars Masquerade as Young Massive Stars in Optical Integrated Light Spectroscopy

Abstract

Model color-magnitude diagrams of low-metallicity globular clusters (GCs) usually show a deficit of hot evolved stars with respect to observations. We investigate quantitatively the impact of such modeling inaccuracies on the significance of star formation history reconstructions obtained from optical integrated spectra. To do so, we analyze the sample of spectra of galactic globular clusters of Schiavon et al. with STECKMAP (Ocvirk et al.), and the stellar population models of Vazdekis et al. and Bruzual & Charlot, and focus on the reconstructed stellar age distributions. First, we show that background/foreground contamination correlates with E(B − V ), which allows us to define a clean subsample of uncontaminated GCs, on the basis of an E(B − V ) filtering. We then identify a "confusion zone" where fake young bursts of star formation pop up in the star formation history although the observed population is genuinely old. These artifacts appear for 70%-100% of cases depending on the population model used, and contribute up to 12% of the light in the optical. Their correlation with the horizontal branch (HB) ratio indicates that the confusion is driven by HB morphology: red HB clusters are well fitted by old stellar population models while those with a blue HB require an additional hot component. The confusion zone extends over [Fe/H] = [−2, −1.2], although we lack the data to probe extreme high and low metallicity regimes. As a consequence, any young starburst superimposed on an old stellar population in this metallicity range could be regarded as a modeling artifact, if it weighs less than 12% of the optical light, and if no emission lines typical of an H ii region are present. This work also provides a practical method for constraining HB morphology from high signal to noise integrated light spectroscopy in the optical. This will allow post-asymptotic giant branch evolution studies in a range of environments and at distances where resolving stellar populations is impossible with current and planned telescopes.

The ionized gas in the CALIFA early-type galaxies

“…The STARLIGHT fits do indicate a small contribution (0.1% by mass) of young stars in the nucleus, but these are almost certainly old blue stars that are not well accounted for by current evolutionary synthesis models (Koleva et al 2008;Cid Fernandes & González Delgado 2010). The weakness of Hα reinforces the conclusion that these are "fake bursts" as described by Ocvirk (2010), otherwise EW(Hα) should be much stronger. As discussed above, our energetic balance approach indicates that an additional excitation mechanism other than pABG stars might operate at large radii (Fig.…”

Resolving galaxies in time and space

“…The Salpeter IMF is adopted. Like the other bases discussed next, GM does not adequately cover the blue horizontal branch, both because it does not reach very low Z and because, as thoroughly discussed by Ocvirk (2010), these models perform poorly in this phase anyway.…”

“…Rotation in massive stars (Levesque et al 2012), rapid phases during the post-main-sequence evolution of intermediate-mass stars (Maraston 2005), and binarity (Li et al 2013) are examples of phenomena that affect the radiative output of stellar populations, but are still at the forefront of research, and thus prone to uncertainties. A practical example of how incomplete treatment of evolutionary phases affects spectral synthesis analysis is discussed by Koleva et al (2008), Cid Fernandes &Ocvirk (2010). Using different codes, these studies found that spectral synthesis of old Milky Way and LMC globular clusters suggests the presence of very young stellar populations accounting for some 10-20% of the optical light, an astrophysically absurd result that also happens in some old, "red and dead" galaxies (Cid Fernandes et al 2011).…”

“…Using different codes, these studies found that spectral synthesis of old Milky Way and LMC globular clusters suggests the presence of very young stellar populations accounting for some 10-20% of the optical light, an astrophysically absurd result that also happens in some old, "red and dead" galaxies (Cid Fernandes et al 2011). These "fake young bursts" (Ocvirk 2010) are in fact by-products of inadequate modeling of the horizontal branch (and/or blue straglers, Cenarro et al 2008). Seeking for a best match, the spectral synthesis uses young hot stars to replace the old hot ones missing in the models.…”

What Distinguishes the Host Galaxies of Radio-loud and Radio-quiet AGNs?

“…Significant differences (p < 0.05 in the AD and MW tests) occur only at log t ≤ 7.3 for the 15.8 ≤ log R < 16.8 bin, and at 9.7 ≤ log t ≤ 9.8 for the log R ≥ 16.8 bin. However, the upturn for RL at ages log t < 7.5 is probably artificial and should be disregarded (see the discussion on blue horizontal branches stars in stellar population models in Ocvirk 2010 andStasińska et al 2015). The difference at large ages in the bin log R ≥ 16.8 is noticable only in the last two age bins.…”