The Stellar Initial Mass Function and Population Properties of M89 from Optical and NIR Spectroscopy: Addressing Biases in Spectral Index Analysis

Abstract

The complexity of constraining the stellar initial mass function (IMF) in early-type galaxies cannot be overstated, given the necessity of both very high signal-to-noise (S/N) data and the difficulty of breaking the strong degeneracies that occur among several stellar population parameters including age, metallicity and elemental abundances. With this paper, the second in a series, we present a detailed analysis of the biases that can occur when retrieving the IMF shape by exploiting both optical and NIR IMF sensitive spectral indices. As a test case, here we analyze data for the nearby galaxy M89, for which we have high S/N spectroscopic data that cover the 3500 − 9000 Å spectral region and allow us to study the radial variation of the stellar population properties out to 1R e . Carrying out parallel simulations that mimic the retrieval of all the explored stellar parameters from a known input model, we quantify the amount of bias at each step of our analysis. From more general simulations we conclude that to accurately retrieve the IMF, it is necessary not only to retrieve accurate estimates of the age and metallicity, but also of all the elemental abundances that the spectral index fits are sensitive to. With our analysis technique applied to M89, we find consistency with a bottom-heavy IMF with a negative gradient from the center to half R e when using the Conroy et al. (2018) as well as Vazdekis et al. ( 2016) EMILES stellar population models. We find agreement both with a parallel full spectral fitting of the same data and with literature results.

The Fornax3D project: Intrinsic Correlations between Orbital Properties and the Stellar Initial Mass Function

“…Miller & Scalo 1979;Chabrier 2003), while many studies utilise generic power-laws with varying levels of flexibility in the slopes (e.g. Kroupa 2002;Conroy et al 2017Conroy et al , 2018van Dokkum et al 2017;Vaughan et al 2018;Lonoce et al 2021). Moreover, for a given functional form, the IMF may be parametrised in any number of ways, with dwarf-to-giant ratios being frequently used (e.g.…”

Stellar Population and Elemental Abundance Gradients of Early-type Galaxies

“…For the CaT spectra (grating 600 /∼17 • ), we had an additional step in order to correct for fringes. Following Lonoce et al (2021), we divided the science frames by spectroscopic flat field frames, which were taken close in time to the observations. This step was performed before distortion correction.…”

“…We also ran fits with two IMF slope parameters instead of one, for the two mass ranges 0.08-0.5 M and 0.5-1.0 M . However, the two parameters x 1 and x 2 are anti-correlated (see also Feldmeier-Krause et al 2020;Lonoce et al 2021), which led us to use only one slope for the full mass range 0.08-1.0 M instead. The differences for the other parameters when using two instead of one IMF slope are within the 1σ uncertainties.…”

“…NGC 4552 is in the Virgo cluster. It has a high, radially constant age and a steeply decreasing [Z/H] ( Lonoce et al 2021). The IMF slope is slightly decreasing with radius.…”

Modelling simple stellar populations in the near-ultraviolet to near-infrared with the X-shooter Spectral Library (XSL)

“…The comparison with the E-MILES is relevant because these models are widely used in the study of intermediate and old stellar populations (Neumann et al 2021;Rodriguez Beltran et al 2021;Barbosa et al 2021;Lonoce et al 2021, to name a few recent works). Here, we use the E-MILES models calculated using the Salpeter IMF and both the Padova00 and BaSTI (Pietrinferni et al 2004(Pietrinferni et al , 2006Cordier et al 2007;Percival et al 2009) isochrones.…”