Optical/NIR stellar absorption and emission-line indices from luminous infrared galaxies

We analyze a set of optical-to-near-infrared long-slit nuclear spectra of 16 infrared-luminous spiral galaxies. All of the studied sources present H$_2$ emission, which reflects the star-forming nature of our sample, and they clearly display H I emission lines in the optical. Their continua contain many strong stellar absorption lines, with the most common features due to Ca I, Ca II, Fe I, Na I, Mg I, in addition to prominent absorption bands of TiO, VO, ZrO, CN and CO. We report a homogeneous set of equivalent width (EW) measurements for 45 indices, from optical to NIR species for the 16 star-forming galaxies as well as for 19 early type galaxies where we collected the data from the literature. This selected set of emission and absorption-feature measurements can be used to test predictions of the forthcoming generations of stellar population models. We find correlations among the different absorption features and propose here correlations between optical and NIR indices, as well as among different NIR indices, and compare them with model predictions. While for the optical absorption features the models consistently agree with the observations,the NIR indices are much harder to interpret. For early-type spirals the measurements agree roughly with the models, while for star-forming objects they fail to predict the strengths of these indices.Comment: accepted for publication in MNRA

A close look at the dwarf AGN of NGC 4395 : optical and near-IR integral field spectroscopy

Intermediate-mass black holes (103–105 M⊙) in the centre of dwarf galaxies are believed to be analogous to growing active galactic nuclei (AGNs) in the early Universe. Their characterization can provide insight about the early galaxies. We present optical and near-infrared integral field spectroscopy of the inner ∼50 pc of the dwarf galaxy NGC 4395, known to harbour an AGN. NGC 4395 is an ideal candidate to investigate the nature of dwarf AGN, as it is nearby (d ≈ 4.4 Mpc) enough to allow a close look at its nucleus. The optical data were obtained with the Gemini GMOS-IFU covering the 4500 –7300 Å spectral range at a spatial resolution of 10 pc. The J and K-band spectra were obtained with the Gemini NIFS at spatial resolutions of ∼5 pc. The gas kinematics show a compact, rotation disc component with a projected velocity amplitude of 25 kms−1. We estimate a mass of 7.7 × 105 M⊙ inside a radius of 10 pc. From the Hα broad-line component, we estimate the AGN bolometric luminosity as Lbol=(9.9±1.4)×1040 erg s−1 and a mass MBH=(2.5+1.0−0.8)×105 M⊙ for the central black hole. The mean surface mass densities for the ionized and molecular gas are in the ranges (1–2) M⊙ pc−2 and (1–4) × 10−3 M⊙ pc−2 and the average ratio between ionized and hot molecular gas masses is ∼500. The emission-line flux distributions reveal an elongated structure at 24 pc west of the nucleus, which is blueshifted relative to the systemic velocity of the galaxy by ≈30 kms−1. We speculate that this structure is originated by the accretion of a gas-rich small satellite or by a low-metallicity cosmic clou

The X-shooter Spectral Library (XSL): Data Release 3

We present the third data release (DR3) of the X-shooter Spectral Library (XSL). This moderate-to-high resolution, near-ultraviolet-to-near-infrared ($350-2480$ nm, R $\sim$ 10 000) spectral library is composed of 830 stellar spectra of 683 stars. DR3 improves upon the previous data release by providing the combined de-reddened spectra of the three X-shooter segments over the full $350-2480$ nm wavelength range. It also includes additional 20 M-dwarf spectra from the ESO archive. We provide detailed comparisons between this library and Gaia EDR3, MILES, NGSL, CaT library, and (E-)IRTF. The normalised rms deviation is better than $D=0.05$ or 5$\%$ for the majority of spectra in common between MILES (144 spectra of 180), NGSL (112$/$116), and (E-)IRTF (55$/$77) libraries. Comparing synthetic colours of those spectra reveals only negligible offsets and small rms scatter, such as the median offset(rms) 0.001$\pm$0.040 mag in the (box1-box2) colour of the UVB arm,-0.004$\pm$0.028 mag in (box3-box4) of the VIS arm, and -0.001$\pm$0.045 mag in (box2-box3) colour between the UVB and VIS arms, when comparing stars in common with MILES. We also find an excellent agreement between the Gaia published (BP-RP) colours and those measured from the XSL DR3 spectra, with a zero median offset and an rms scatter of 0.037 mag for 449 non-variable stars. The unmatched characteristics of this library, which combine a relatively high resolution, a large number of stars, and an extended wavelength coverage, will help us to bridge the gap between the optical and the near-IR studies of intermediate and old stellar populations, and to probe low-mass stellar systems.Comment: 26 pages, 25 figures, accepted to Astronomy & Astrophysics. The data are available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/ or on the XSL web-page http://xsl.astro.unistra.f

Observational constraints on the stellar recycled gas in active galactic nuclei feeding

Near-infrared long-slit spectroscopy has been used to study the stellar population (SP) of the low luminosity active galactic nuclei (AGNs) and matched analogues (LLAMA) sample. To perform the SP fits we have employed the X-shooter simple stellar population models together with the STARLIGHT code. Our main conclusions are: The star formation history of the AGNs is very complex, presenting many episodes of star formation during their lifetimes. In general, AGN hosts have higher fractions of intermediate-age SP (light-weighted mean ages, L ≲ 4.5 Gyr) when compared with their analogues ( L ≲ 8.0 Gyr). AGNs are more affected by reddening and require significant fractions of featureless continuum and hot dust components. The ratio between the AGN radiated energy and the gravitational potential energy of the molecular gas (ERad/EPG) for the AGN is compared with the L and a possible anticorrelation is observed. This suggests that the AGN is affecting the star formation in these galaxies, in the sense that more energetic AGN [log(ERad/EPG) ≳ 3] tend to host nuclear younger SP ( L ≲4 Gyr). We found that the recent (t <2 Gyr) returned (recycled) stellar mass is higher in AGN than in the controls. We also provide evidence that the mass-loss of stars would be enough to feed the AGN, thus providing observational constraints for models that predict that AGN feeding is partially due to the recycled gas from dying stars

The stellar populations of low-luminosity active galactic nuclei. II. Space telescope imaging spectrograph observations

We present a study of the stellar populations of low-luminosity active galactic nuclei (LLAGNs). Our goal is to search for spectroscopic signatures of young and intermediate-age stars and to investigate their relationship with the ionization mechanism in LLAGNs. The method used is based on the stellar population synthesis of the optical continuum of the innermost (20–100 pc) regions in these galaxies. For this purpose, we have collected high spatial resolution optical (2900–5700 Å) STIS spectra of 28 nearby LLAGNs that are available in the Hubble Space Telescope archive. The analysis of these data is compared with a similar analysis also presented here for 51 ground-based spectra of LLAGNs. Our main findings are as follows: (1) No features due to Wolf- Rayet stars were convincingly detected in the STIS spectra. (2) Young stars contribute very little to the optical continuum in the ground-based aperture. However, the fraction of light provided by these stars is higher than 10% in most of the weak–[O I] ([O I]/Hα ≤ 0.25) LLAGN STIS spectra. (3) Intermediate-age stars contribute significantly to the optical continuum of these nuclei. This population is more frequent in objects with weak than with strong [O I]. Weak–[O I] LLAGNs that have young stars stand out for their intermediate-age population. (4) Most of the strong–[O I] LLAGNs have predominantly old stellar population. A few of these objects also show a featureless continuum that contributes significantly to the optical continuum. These results suggest that young and intermediate-age stars do not play a significant role in the ionization of LLAGNs with strong [O I]. However, the ionization in weak–[O I] LLAGNs with young and/or intermediate-age populations could be due to stellar processes. A comparison of the properties of these objects with Seyfert 2 galaxies that harbor a nuclear starburst suggests that weak–[O I] LLAGNs are the lower luminosity counterparts of the Seyfert 2 composite nuclei

Probing evolutionary population synthesis models in the near infrared with early-type galaxies

We performed a near-infrared (NIR; ∼1.0 –2.4 μm) stellar population study in a sample of early-type galaxies. The synthesis was performed using five different evolutionary population synthesis libraries of models. Our main results can be summarized as follows: low-spectral-resolution libraries are not able to produce reliable results when applied to the NIR alone, with each library finding a different dominant population. The two newest higher resolution models, on the other hand, perform considerably better, finding consistent results to each other and to literature values. We also found that optical results are consistent with each other even for lower resolution models. We also compared optical and NIR results and found out that lower resolution models tend to disagree in the optical and in the NIR, with higher fraction of young populations in the NIR and dust extinction ∼1 mag higher than optical values. For higher resolution models, optical and NIR results tend to agree much better, suggesting that a higher spectral resolution is fundamental to improve the quality of the results

The stellar populations of low-luminosity active galactic nuclei – III. Spatially resolved spectral properties

In a recently completed survey of the stellar population properties of low-ionization nuclear emission-line regions (LINERs) and LINER/H II transition objects (TOs), we have identified a numerous class of galactic nuclei which stand out because of their conspicuous 10 8−9 yr populations, traced by high-order Balmer absorption lines and other stellar indices. These objects are called ‘young-TOs’, because they all have TO-like emission-line ratios. In this paper we extend this previous work, which concentrated on the nuclear properties, by investigating the radial variations of spectral properties in low-luminosity active galactic nuclei (LLAGNs). Our analysis is based on high signal-to-noise ratio (S/N) long-slit spectra in the 3500–5500 Å interval for a sample of 47 galaxies. The data probe distances of typically up to 850 pc from the nucleus with a resolution of ~100 pc (~1 arcsec) and S/N ~ 30. Stellar population gradients are mapped by the radial profiles of absorption-line equivalent widths and continuum colours along the slit. These variations are further analysed by means of a decomposition of each spectrum in terms of template galaxies representative of very young (10 7 yr), intermediate age (10 8–9 yr) and old (≤10 10 yr) stellar populations. This study reveals that young-TOs also differ from old-TOs and old-LINERs in terms of the spatial distributions of their stellar populations and dust. Specifically, our main findings are as follows. (i) Significant stellar population gradients are found almost exclusively in young- TOs. (ii) The intermediate age population of young-TOs, although heavily concentrated in the nucleus, reaches distances of up to a few hundred pc from the nucleus. Nevertheless, the half width at half-maximum of its brightness profile is more typically 100 pc or less. (iii) Objects with predominantly old stellar populations present spatially homogeneous spectra, be they LINERs or TOs. (iv) Young-TOs have much more dust in their central regions than other LLAGNs. (v) The B-band luminosities of the central < ~ 1 Gyr population in young-TOs are within an order of magnitude of MB =−15, implying masses of the order of ~ 10 7–10 8 Mʘ. This population was 10–100 times more luminous in its formation epoch, at which time young massive stars would have completely outshone any active nucleus, unless the AGN too was brighter in the past

The stellar populations of low-luminosity active galactic nuclei. I. Ground-based observations

We present a spectroscopic study of the stellar populations of low-luminosity active galactic nuclei (LLAGNs). Our main goal is to determine whether the stars that live in the innermost (100 pc scale) regions of these galaxies are in some way related to the emission-line properties, which would imply a link between the stellar population and the ionization mechanism. High signal-to-noise ratio, ground-based long-slit spectra in the 3500– 5500 Å interval were collected for 60 galaxies: 51 LINERs and LINER/H II transition objects, two starburst galaxies, and seven nonactive galaxies. In this paper, the first of a series, we (1) describe the sample; (2) present the nuclear spectra; (3) characterize the stellar populations of LLAGNs by means of an empirical comparison with normal galaxies; (4) measure a set of spectral indices, including several absorption-line equivalent widths and colors indicative of stellar populations; and (5) correlate the stellar indices with emission-line ratios that may distinguish between possible excitation sources for the gas. Our main findings are as follows: (1) Few LLAGNs have a detectable young (~<10 7 yr) starburst component, indicating that very massive stars do not contribute significantly to the optical continuum. In particular, no features due to Wolf-Rayet stars were convincingly detected. (2) High-order Balmer absorption lines of HI (HOBLs), on the other hand, are detected in ~40% of LLAGNs. These features, which are strongest in 10 8–10 9 yr intermediate-age stellar populations, are accompanied by diluted metal absorption lines and bluer colors than other objects in the sample. (3) These intermediateage populations are very common (~50%) in LLAGNs with relatively weak [O I] emission ([O I/Hα ≤ 0.25) but rare (~10%) in LLAGNs with stronger [O I]. This is intriguing since LLAGNs with weak [O I] have been previously hypothesized to be ‘‘transition objects’’ in which both an AGN and young stars contribute to the emission-line excitation. Massive stars, if present, are completely outshone by intermediate-age and old stars in the optical. This happens in at least a couple of objects where independent UV spectroscopy detects young starbursts not seen in the optical. (4) Objects with predominantly old stars span the whole range of [O I]/Hα values, but (5) sources with significant young and/or intermediate-age populations are nearly all (~90%) weak– [O I] emitters. These new findings suggest a link between the stellar populations and the gas ionization mechanism. The strong–[O I] objects are most likely true LLAGNs, with stellar processes being insignificant. However, the weak–[O I] objects may comprise two populations, one where the ionization is dominated by stellar processes and another where it is governed by either an AGN or a more even mixture of stellar and AGN processes. Possible stellar sources for the ionization include weak starbursts, supernova remnants, and evolved poststarburst populations. These scenarios are examined and constrained by means of complementary observations and detailed modeling of the stellar populations in forthcoming communications

Probing evolutionary population synthesis models in the near infrared with early-type galaxies

We performed a near-infrared (NIR; ∼1.0 –2.4 μm) stellar population study in a sample of early-type galaxies. The synthesis was performed using five different evolutionary population synthesis libraries of models. Our main results can be summarized as follows: low-spectral-resolution libraries are not able to produce reliable results when applied to the NIR alone, with each library finding a different dominant population. The two newest higher resolution models, on the other hand, perform considerably better, finding consistent results to each other and to literature values. We also found that optical results are consistent with each other even for lower resolution models. We also compared optical and NIR results and found out that lower resolution models tend to disagree in the optical and in the NIR, with higher fraction of young populations in the NIR and dust extinction ∼1 mag higher than optical values. For higher resolution models, optical and NIR results tend to agree much better, suggesting that a higher spectral resolution is fundamental to improve the quality of the results