Ignacio Martín-Navarro

Universidad de La Laguna, San Cristóbal de La Laguna, Canary Islands, Spain

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Publications (10)41.5 Total impact

  • Ignacio Martín-Navarro
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    ABSTRACT: In the local Universe, massive early-type galaxies exhibit enhanced [Mg/Fe] ratios, which has been traditionally interpreted as the result of a rapid ($\tau \lesssim 1$ Gyr) collapse. However, recent claims of a non-universal, steep initial mass function call for a revision of this standard interpretation. In the present work we show how the simultaneous consideration of a high [Mg/Fe] and a steep IMF slope would imply unreasonably short ($\tau \sim 7$ Myr) and intense (SFR $\sim 10^{5}$ Msun yr$^{-1}$) formation events for massive early-type galaxies. We discuss possible caveats and explanations to this apparent inconsistency, and we suggest that further IMF determinations, both in the local Universe and at high redshift, are necessary to better understand the problem.
    No preview · Article · Nov 2015 · Monthly Notices of the Royal Astronomical Society Letters
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    ABSTRACT: We investigate the compact, early-type galaxy NGC 1281 with integral field unit observations to map the stellar line-of-sight velocity distribution (LOSVD) out to five effective radii and construct orbit-based dynamical models to constrain its dark and luminous matter content. Under the assumption of mass-follows-light, the H-band stellar mass-to-light ratio (M/L) is ϒ⋆ = 2.7 ± 0.1 ϒ⊙, and higher than expected from our stellar population synthesis fits with either a canonical Kroupa (ϒ⋆ = 1.3 ϒ⊙) or Salpeter (ϒ⋆ = 1.7 ϒ⊙) stellar initial mass function. Such models also cannot reproduce the details of the LOSVD. Models with a dark halo recover the kinematics well and indicate that NGC 1281 is dark matter dominated, making up ∼ 90 per cent of the total enclosed mass within the kinematic bounds. Parametrized as a spherical NFW profile, the dark halo mass is 11.5 ≤ log(MDM/M⊙) ≤ 11.8 and the stellar M/L is 0.6 ≤ ϒ⋆/ϒ⊙ ≤ 1.1. However, this M/L is lower than predicted by its old stellar population. Moreover, the halo mass within the kinematic extent is 10 times larger than expected based on Λ-cold-dark-matter predictions, and an extrapolation yields cluster-sized dark halo masses. Adopting ϒ⋆ = 1.7 ϒ⊙ yields more moderate dark halo virial masses, but these models fit the kinematics worse. A non-NFW model might solve the discrepancy between the unphysical consequences of the best-fitting dynamical models and models based on more reasonable assumptions for the dark halo and stellar M/L, which are disfavoured according to our parameter estimation.
    Preview · Article · Nov 2015 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: At present, the main challenge to the interpretation of variations in gravity-sensitive line strengths as driven by a non-universal initial mass function (IMF) lies in understanding the effect of the other population parameters. Most notably, [α/Fe]-enhanced populations or even departures in the individual element abundances with respect to the solar-scaled ratio may lead to similar observational results. We combine various TiO-based, IMF-sensitive indicators in the optical and NIR spectral windows, along with the FeH-based Wing–Ford band to break this degeneracy. We obtain a significant radial trend of the IMF slope in XSG1, a massive early-type galaxy (ETG), with velocity dispersion σ ∼ 300 km s−1, observed with the Very Large Telescope/X-shooter instrument. In addition, we constrain – for the first time – both the shape and normalization of the IMF, using only a stellar population analysis. We robustly rule out a single power law to describe the IMF, whereas a power law tapered off to a constant value at low masses (defined as a bimodal IMF) is consistent with all the observational spectroscopic data and with the stellar M/L constraints based on the Jeans anisotropic modelling method. The IMF in XSG1 is bottom-heavy in the central regions (corresponding to a bimodal IMF slope Γb ∼ 3, or a mass normalization mismatch parameter α ∼ 2), changing towards a standard Milky Way-like IMF (Γb ∼ 1.3; α ∼ 1) at around one half of the effective radius. This result, combined with previous observations of local IMF variations in massive ETGs, reflects the varying processes underlying the formation of the central core and the outer regions in this type of galaxies.
    Preview · Article · Sep 2015 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: Variations in the stellar initial mass function (IMF) have been invoked to explain the spectroscopic and dynamical properties of early-type galaxies. However, no observations have yet been able to disentangle the physical driver. We analyse here a sample of 24 early-type galaxies drawn from the CALIFA survey, deriving in a homogeneous way their stellar population and kinematic properties. We find that the local IMF is tightly related to the local metallicity, becoming more bottom-heavy towards metal-rich populations. Our result, combined with the galaxy mass-metallicity relation, naturally explains previous claims of a galaxy mass-IMF relation, derived from non-IFU spectra. If we assume that - within the star formation environment of early-type galaxies - metallicity is the main driver of IMF variations, a significant revision of the interpretation of galaxy evolution observables is necessary.
    Preview · Article · Jun 2015
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    ABSTRACT: Massive relic galaxies formed the bulk of their stellar component before z ∼ 2 and have remained unaltered since then. Therefore, they represent a unique opportunity to study in great detail the frozen stellar population properties of those galaxies that populated the primitive Universe. We have combined optical to near-infrared line-strength indices in order to infer, out to 1.5 Re, the initial mass function (IMF) of the nearby relic massive galaxy NGC 1277. The IMF of this galaxy is bottom-heavy at all radii, with the fraction of low-mass stars being at least a factor of two larger than that found in the Milky Way. The excess of low-mass stars is present throughout the galaxy, while the velocity dispersion profile shows a strong decrease with radius. This behaviour suggests that local velocity dispersion is not the only driver of the observed IMF variations seen among nearby early-type galaxies. In addition, the excess of low-mass stars shown in NGC 1277 could reflect the effect on the IMF of dramatically different and intense star formation processes at z ∼ 2, compared to the less extreme conditions observed in the local Universe.
    Full-text · Article · May 2015 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: Several authors have reported that the dynamical masses of massive compact galaxies (M⋆ ≳ 1011 M⊙, re ∼ 1 kpc), computed as $M_\mathrm{dyn}= 5.0 \ \sigma _\mathrm{e}^2 r_\mathrm{e}/ G$, are lower than their stellar masses M⋆. In a previous study from our group, the discrepancy is interpreted as a breakdown of the assumption of homology that underlie the Mdyn determinations. Here, we present new spectroscopy of six redshift z ≈ 1.0 massive compact ellipticals from the Extended Groth Strip, obtained with the 10.4 m Gran Telescopio Canarias. We obtain velocity dispersions in the range 161–340 km s−1. As found by previous studies of massive compact galaxies, our velocity dispersions are lower than the virial expectation, and all of our galaxies show Mdyn < M⋆ (assuming a Salpeter initial mass function). Adding data from the literature, we build a sample covering a range of stellar masses and compactness in a narrow redshift range z ≈ 1.0. This allows us to exclude systematic effects on the data and evolutionary effects on the galaxy population, which could have affected previous studies. We confirm that mass discrepancy scales with galaxy compactness. We use the stellar mass plane (M⋆, σe, re) populated by our sample to constrain a generic evolution mechanism. We find that the simulations of the growth of massive ellipticals due to mergers agree with our constraints and discard the assumption of homology.
    Preview · Article · Apr 2015 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We explore the stellar initial mass function (IMF) of a sample of 49 massive quiescent galaxies (MQGs) at 0.9?< z <?1.5. We base our analysis on intermediate resolution spectro-photometric data in the GOODS-N field taken in the near-infrared and optical with the Hubble Space Telescope Wide Field Camera 3 G141 grism and the Survey for High-z Absorption Red and Dead Sources. To constrain the slope of the IMF, we have measured the TiO2 spectral feature, whose strength depends strongly on the content of low-mass stars, as well as on stellar age. Using ultraviolet to near-infrared individual and stacked spectral energy distributions, we have independently estimated the stellar ages of our galaxies. Knowing the age of the stellar population, we interpret the strong differences in the TiO2 feature as an IMF variation. In particular, for the heaviest z ~?1 MQGs (M >?1011M ?), we find an average age of 1.7???0.3?Gyr and a bottom-heavy IMF (?b = 3.2???0.2). Lighter MQGs (2?×?1010 < M <?1011M ?) at the same redshift are younger on average (1.0???0.2?Gyr) and present a shallower IMF slope (). Our results are in good agreement with the findings about the IMF slope in early-type galaxies of similar mass in the present-day universe. This suggests that the IMF, a key characteristic of the stellar populations in galaxies, is bottom-heavier for more massive galaxies and has remained unchanged in the last ~8?Gyr.
    Full-text · Article · Dec 2014 · The Astrophysical Journal Letters
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    ABSTRACT: The hypothesis of a universal initial mass function (IMF) – motivated by observations in nearby stellar systems – has been recently challenged by the discovery of a systematic variation of the IMF with the central velocity dispersion, σ, of early-type galaxies (ETGs), towards an excess of low-mass stars in high-σ galaxies. This trend has been derived so far from integrated spectra, and remains unexplained at present. To test whether such trend depends on the local properties within a galaxy, we have obtained new, extremely deep, spectroscopic data, for three nearby ETGs, two galaxies with high σ (∼300 km s− 1), and one lower mass system, with σ ∼ 100 km s− 1. From the analysis of IMF-sensitive spectral features, we find that the IMF depends significantly on galactocentric distance in the massive ETGs, with the enhanced fraction of low-mass stars mostly confined to their central regions. In contrast, the low-σ galaxy does not show any significant radial gradient in the IMF, well described by a shallower distribution, relative to the innermost regions of massive galaxies, at all radii. Such a result indicates that the IMF should be regarded as a local (rather than global) property, and suggests a significant difference between the formation process of the core and the outer regions of massive ETGs.
    Preview · Article · Apr 2014 · Monthly Notices of the Royal Astronomical Society
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    Ignacio Martin-Navarro · Ignacio Trujillo · Johan H. Knapen · Judit Bakos · Juergen Fliri
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    ABSTRACT: The absence of stellar disc truncations in low-inclined spiral galaxies has been a matter of debate in the last decade. Disc truncations are often observed in highly inclined galaxies but no obvious detection of this feature has so far been made in face-on spirals. Here we show, using a simple exponential disc plus stellar halo model based on current observational constraints, that truncations in face-on projections occur at surface brightness levels comparable to the brightness of stellar haloes at the same radial distance. In this sense, stellar haloes outshine the galaxy disc at the expected position of the truncations, forcing their studies only in highly inclined (edge-on) orientations.
    Full-text · Article · Jan 2014 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We have developed an innovative `index scanning technique" to map the stellar kinematics of early-type galaxies by measuring for the first time the absorption line strength of the near-IR CaII triplet with the Red Tunable Filters of OSIRIS at GTC. Unlike classical spectroscopy, these filters allow us to perform a two-dimensional study, taking advantage of a 10.4 meter class telescope with a unvignetted field of view of 7.8 × 7.8 arcmin. We show the velocity fields obtained for two Virgo elliptical galaxies of very different masses and their globular cluster systems reaching galactocentric distances beyond 2 effective radii.
    No preview · Article · May 2013