The puzzling interpretation of NIR indices: The case of NaI2.21

We present a detailed study of the Na I line strength index centred in the K band at 22 100 A (NaI2.21 hereafter) relying on different samples of early-type galaxies. Consistent with previous studies, we find that the observed line strength indices cannot be fit by state-of-the-art scaled-solar stellar population models, even using our newly developed models in the near infrared (NIR). The models clearly underestimate the large NaI2.21 values measured for most early-type galaxies. However, we develop an Na-enhanced version of our newly developed models in the NIR, which - together with the effect of a bottom-heavy initial mass function - yield NaI2.21 indices in the range of the observations. Therefore, we suggest a scenario in which the combined effect of [Na/Fe] enhancement and a bottom-heavy initial mass function are mainly responsible for the large NaI2.21 indices observed for most early-type galaxies. To a smaller extent, also [C/Fe] enhancement might contribute to the large observed NaI2.21 values.

[1]  R. Peletier,et al.  MILES extended: Stellar population synthesis models from the optical to the infrared , 2016, 1609.05478.

[2]  M. Cappellari,et al.  Radial constraints on the initial mass function from TiO features and Wing–Ford band in early-type galaxies , 2015, 1509.08250.

[3]  David Carter,et al.  The IMF-sensitive 1.14-μm Na i doublet in early-type galaxies , 2015, 1509.01253.

[4]  R. Peletier,et al.  Single stellar populations in the near-infrared - II. Synthesis models , 2015, 1506.07183.

[5]  Tenerife,et al.  Stellar population synthesis models between 2.5 and 5 μm based on the empirical IRTF stellar library , 2015, 1505.01837.

[6]  J. Falcón-Barroso,et al.  Evolutionary stellar population synthesis with MILES – II. Scaled-solar and α-enhanced models , 2015, 1504.08032.

[7]  J. Lucey,et al.  The SINFONI Nearby Elliptical Lens Locator Survey: discovery of two new low-redshift strong lenses and implications for the initial mass function in giant early-type galaxies , 2015, 1503.02661.

[8]  W. Kausch,et al.  Flux calibration of medium-resolution spectra from 300 nm to 2500 nm: Model reference spectra and telluric correction , 2014, 1408.1797.

[9]  P. Prugniel,et al.  The X-shooter Spectral Library (XSL) - I. DR1: Near-ultraviolet through optical spectra from the first year of the survey , 2014, 1403.7009.

[10]  L. Koopmans,et al.  THE NON-UNIVERSALITY OF THE LOW-MASS END OF THE IMF IS ROBUST AGAINST THE CHOICE OF SSP MODEL , 2014, 1401.1145.

[11]  M. Sarzi,et al.  ON THE NATURE OF SODIUM EXCESS OBJECTS. I. DATA AND OBSERVED TRENDS , 2013, 1307.1472.

[12]  L. Koopmans,et al.  The stellar IMF in early-type galaxies from a non-degenerate set of optical line indices , 2013, 1305.2873.

[13]  R. D. Carvalho,et al.  SPIDER VIII - constraints on the stellar initial mass function of early-type galaxies from a variety of spectral features , 2013, 1305.2273.

[14]  G. Graves,et al.  EARLY-TYPE GALAXY ARCHEOLOGY: AGES, ABUNDANCE RATIOS, AND EFFECTIVE TEMPERATURES FROM FULL-SPECTRUM FITTING , 2013, 1303.6629.

[15]  R. D. Carvalho,et al.  Systematic variation of the stellar initial mass function with velocity dispersion in early-type galaxies , 2012, 1206.1594.

[16]  J. Reunanen,et al.  Near‐infrared spectroscopy of stellar populations in nearby spiral galaxies , 2012, 1205.6932.

[17]  J. Falc'on-Barroso,et al.  MIUSCAT: extended MILES spectral coverage – I. Stellar population synthesis models , 2012, 1205.5496.

[18]  W. Kausch,et al.  An atmospheric radiation model for Cerro Paranal - I. The optical spectral range , 2012, 1205.2003.

[19]  L. Koopmans,et al.  EVIDENCE FOR A MILD STEEPENING AND BOTTOM-HEAVY INITIAL MASS FUNCTION IN MASSIVE GALAXIES FROM SODIUM AND TITANIUM-OXIDE INDICATORS , 2012, 1204.3823.

[20]  R. Manuputy,et al.  X-shooter, the new wide band intermediate resolution spectrograph at the ESO Very Large Telescope , 2011, 1110.1944.

[21]  Pieter van Dokkum,et al.  COUNTING LOW-MASS STARS IN INTEGRATED LIGHT , 2011, 1109.0007.

[22]  Douglas P. Finkbeiner,et al.  MEASURING REDDENING WITH SLOAN DIGITAL SKY SURVEY STELLAR SPECTRA AND RECALIBRATING SFD , 2010, 1012.4804.

[23]  Pieter G. van Dokkum,et al.  A substantial population of low-mass stars in luminous elliptical galaxies , 2010, Nature.

[24]  A. J. Cenarro,et al.  Evolutionary stellar population synthesis with MILES – I. The base models and a new line index system , 2010, 1004.4439.

[25]  J. Brinchmann,et al.  ABSORPTION-LINE PROBES OF THE PREVALENCE AND PROPERTIES OF OUTFLOWS IN PRESENT-DAY STAR-FORMING GALAXIES , 2010, 1003.5425.

[26]  P. Lopes,et al.  Spheroid's Panchromatic Investigation in Different Environmental Regions (SPIDER) - I. Sample and galaxy parameters in the grizYJHK wavebands , 2009, 0912.4547.

[27]  A. Bolatto,et al.  The SAURON project: XV. Modes of star formation in early-type galaxies and the evolution of the red sequence , 2009, 0912.0274.

[28]  H. Kuntschner,et al.  EVIDENCE FOR INTERMEDIATE-AGE STELLAR POPULATIONS IN EARLY-TYPE GALAXIES FROM K-BAND SPECTROSCOPY , 2009, 0909.5385.

[29]  W. D. Vacca,et al.  THE INFRARED TELESCOPE FACILITY (IRTF) SPECTRAL LIBRARY: COOL STARS , 2009, 0909.0818.

[30]  R. Nichol,et al.  Modeling the color evolution of luminous red galaxies - improvements with empirical stellar spectra , 2008, 0809.1867.

[31]  M. Stiavelli,et al.  Early-type galaxies in the near-infrared: 1.5-2.4 μm spectroscopy , , 2008, 0808.3573.

[32]  L. Koesterke,et al.  Center-to-Limb Variation of Solar Three-dimensional Hydrodynamical Simulations , 2008, 0802.2177.

[33]  Robert Barkhouser,et al.  The Apache Point Observatory Galactic Evolution Experiment (APOGEE) , 2007, Astronomical Telescopes + Instrumentation.

[34]  B. Yanny,et al.  Submitted for publication in the Astronomical Journal The SEGUE Stellar Parameter Pipeline. III. Comparison with High-Resolution Spectroscopy of SDSS/SEGUE Field Stars 1 , 2022 .

[35]  H. Kuntschner,et al.  A New Approach to the Study of Stellar Populations in Early-Type Galaxies: K-Band Spectral Indices and an Application to the Fornax Cluster , 2007, 0710.5529.

[36]  Spain.,et al.  Stellar populations of massive elliptical galaxies in very rich clusters , 2006, astro-ph/0612119.

[37]  R. Schiavon.  Population Synthesis in the Blue. IV. Accurate Model Predictions for Lick Indices and UBV Colors in Single Stellar Populations , 2006, astro-ph/0611464.

[38]  J. González,et al.  Stellar populations of early-type galaxies in different environments - II. Ages and metallicities , 2006, astro-ph/0604568.

[39]  A. Pietrinferni,et al.  A Large Stellar Evolution Database for Population Synthesis Studies. II. Stellar Models and Isochrones for an α-enhanced Metal Distribution , 2006, astro-ph/0603721.

[40]  D. Schiminovich,et al.  Galaxy Evolution Explorer Ultraviolet Color-Magnitude Relations and Evidence of Recent Star Formation in Early-Type Galaxies , 2004, astro-ph/0411327.

[41]  C. Maraston.  Evolutionary population synthesis: models, analysis of the ingredients and application to high‐z galaxies , 2004, astro-ph/0410207.

[42]  P. Sánchez-Blázquez,et al.  On the Environmental Dependence of the Cluster Galaxy Assembly Timescale , 2004, astro-ph/0405512.

[43]  S. Cassisi,et al.  A Large Stellar Evolution Database for Population Synthesis Studies. I. Scaled Solar Models and Isochrones , 2004, astro-ph/0405193.

[44]  Spain.,et al.  Empirical calibration of the near-infrared Ca II triplet — IV. The stellar population synthesis models , 2003, astro-ph/0303297.

[45]  D. L. DePoy,et al.  An Accurate, Easy-To-Use Abundance Scale for Globular Clusters Based on 2.2 Micron Spectra of Giant Stars , 2001, astro-ph/0101275.

[46]  J. Lucey,et al.  On the dependence of spectroscopic indices of early-type galaxies on age, metallicity and velocity dispersion , 2000, astro-ph/0011234.

[47]  H. Kuntschner.  The Stellar Populations of Early-Type Galaxies in the Fornax Cluster , 2000, astro-ph/0001210.

[48]  S. Faber,et al.  The Stellar Population Histories of Local Early-Type Galaxies. I. Population Parameters , 2000, astro-ph/0001072.

[49]  L. Girardi,et al.  Evolutionary tracks and isochrones for low- and intermediate-mass stars: From 0.15 to 7 , and from to 0.03 , 1999, astro-ph/9910164.

[50]  R. Saglia,et al.  THE PECULIAR MOTIONS OF EARLY-TYPE GALAXIES IN TWO DISTANT REGIONS - V. THE MG-SIGMA RELATION, AGE AND METALLICITY , 1998, astro-ph/9811089.

[51]  G. Worthey.  Abundance Ratio Trends and Nucleosynthesis in Elliptical Galaxies and Spheroids , 1998 .

[52]  C. Maraston.  Evolutionary synthesis of stellar populations: a modular tool , 1998, astro-ph/9807338.

[53]  H. Kuntschner,et al.  The Ages and Metallicities of Early Type Galaxies in the Fornax Cluster , 1997, astro-ph/9710176.

[54]  K. Sellgren,et al.  Luminosity and Temperature from Near-Infrared Spectra of Late-Type Giant Stars , 1997 .

[55]  D. Depoy,et al.  Medium-Resolution Near-Infrared (2.15-2.35 micron) Spectroscopy of Late-Type Main-Sequence Stars , 1995, astro-ph/9509039.

[56]  Molefe Mokoene,et al.  The Messenger , 1995, Outrageous Fortune.

[57]  G. Worthey.  Comprehensive stellar population models and the disentanglement of age and metallicity effects , 1994 .

[58]  A. Pickles,et al.  THE DWARF STAR CONTENT OF ELLIPTIC AND LENTICULAR GALAXIES , 1986 .

[59]  D. Hall,et al.  Spectra of Late-Type Standard Stars in the Region 2.0--2.5 Microns , 1986 .

[60]  R. Peterson.  Overabundances of sodium and the heavy isotopes of magnesium in strong-lined elliptical galaxies , 1976 .

[61]  R. O’Connell,et al.  Galaxy spectral synthesis. I. Stellar populations in the nuclei of giant ellipticals , 1976 .

[62]  B. Freytag,et al.  in Modelling of Stellar Atmospheres , 2004 .

[63]  R. Peletier,et al.  Empirical Calibration of the near-IR Ca II triplet – IV . The stellar population synthesis models , 2003 .

[64]  S. Blazquez.  Differences in carbon and nitrogen abundances between field and cluster early-type galaxies , 2003 .

[65]  H. Spinrad,et al.  The Stellar Content of the Nuclei of Nearby Galaxies. I. M31, M32, and M81 , 1971 .