Rogério Riffel

Probing the near-infrared stellar population of Seyfert galaxies

We employ Infrared Telescope Facility SpeX near-infrared (NIR; 0.8-2.4 μm) spectra to investigate the stellar population (SP), active galactic nuclei (AGN) featureless continuum (FC) and hot dust properties in nine Sy 1 and 15 Sy 2 galaxies. Both the STARLIGHT code and the hot dust as an additional base element were used for the first time in this spectral range. We found evidence of correlation among the equivalent widths (W λ ) Si I 1.59 μm x Mg I 1.58 μm, equally for both kinds of activity. Part of the W NaI2.21 μm and W CO 2.3 μm strengths may be related to galaxy inclination. Our synthesis shows significant differences between Sy I and Sy 2 galaxies: the hot dust component is required to fit the K-band spectra of ∼90 per cent of the Sy 1 galaxies, and only of ∼25 per cent of the Sy 2; about 50 per cent of the Sy 2 galaxies require an FC component contribution ≳20 per cent, while this fraction increases to about 60 per cent in the Sy 1; also, in about 50 per cent of the Sy2, the combined FC and young components contribute with more than 20 per cent, while this occurs in 90 per cent of the Syl, suggesting recent star formation in the central region. The central few hundred parsec of our galaxy sample contain a substantial fraction of intermediate-age SPs with a mean metallicity near solar. Our SP synthesis confirms that the 1.1 μm CN band can be used as a tracer of intermediate-age SPs. The simultaneous fitting of SP, FC and hot dust components increased in ∼150 per cent the number of AGNs with hot dust detected and the mass estimated. The NIR emerges as an excellent window to study the SP of Sy I galaxies, as opposed to the usually heavily attenuated optical range. Our approach opens a new way to investigate and quantify the individual contribution of the three most important NIR continuum components observed in AGNs.

The stellar populations of starburst galaxies through near-infrared spectroscopy

We study the central (inner few hundred parsecs) stellar populations of four starburst galaxies (NGC 34, 1614, 3310 and 7714) in the near-infrared (NIR), from 0.8 to 2.4 μm, by fitting combinations of stellar population models of various ages and metallicities. The NIR spectra of these galaxies feature many absorption lines. For the first time, we fit simultaneously as much as 15 absorption features in the NIR. The observed spectra are best explained by stellar populations containing a sizable amount (20–56 per cent by mass) of ∼1-Gyr-old stellar population with thermally pulsing asymptotic giant branch stars. We found that the metallicity of the stars which dominates the light is solar. Metallicities substantially different from solar give a worse fit. Though the ages and metallicities we estimate using the NIR spectroscopy are in agreement with values from the literature based on the ultraviolet/optical, we find older ages and a larger age spread. This may be due to the fact that the optical is mostly sensitive to the last episode of star formation, while the NIR better maintains the record of previous stellar generations. Another interesting result is that the reddening estimated from the whole NIR spectrum is considerably lower than that based on emission lines. Finally, we find a good agreement of the free emission-line spectrum with photoionization models, using as input spectral energy distribution the synthetic composite template we derived as best fit.

SDSS-IV MaNGA : spatially resolved star formation histories in galaxies as a function of galaxy mass and type

We study the internal gradients of stellar population properties within 1.5 Re for a representative sample of 721 galaxies, with stellar masses ranging between 109 M⊙ and 1011.5 M⊙ from the SDSS-IV MaNGA Integral-Field-Unit survey. Through the use of our full spectral fitting code FIREFLY, we derive light- and mass-weighted stellar population properties and their radial gradients, as well as full star formation and metal enrichment histories. We also quantify the impact that different stellar population models and full spectral fitting routines have on the derived stellar population properties and the radial gradient measurements. In our analysis, we find that age gradients tend to be shallow for both early-type and late-type galaxies. Mass-weighted age gradients of early-types arepositive [sic] (∼0.09 dex/Re) pointing to ‘outside–in’ progression of star formation, while late-type galaxies have negative light-weighted age gradients (∼−0.11 dex/Re), suggesting an ‘inside–out’ formation of discs.We detect negative metallicity gradients in both early- and late-type galaxies, but these are significantly steeper in late-types, suggesting that the radial dependence of chemical enrichment processes and the effect of gas inflow and metal transport are far more pronounced in discs. Metallicity gradients of both morphological classes correlate with galaxy mass, with negative metallicity gradients becoming steeper with increasing galaxy mass. The correlation with mass is stronger for late-type galaxies, with a slope of d(∇[Z/H])/d(log M) ∼ −0.2 ± 0.05 , compared to d(∇[Z/H])/d(log M) ∼ −0.05 ± 0.05 for early-types. This result suggests that the merger history plays a relatively small role in shaping metallicity gradients of galaxies.

Insights on the Dusty Torus and Neutral Torus from Optical and X-Ray Obscuration in a Complete Volume Limited Hard X-Ray AGN Sample

We describe a complete volume limited sample of nearby active galaxies selected by their 14?195 keV luminosity, and outline its rationale for studying the mechanisms regulating gas inflow and outflow. We also describe a complementary sample of inactive galaxies, selected to match the host galaxy properties. The active sample appears to have no bias in terms of active galactic nucleus (AGN) type, the only difference being the neutral absorbing column, which is two orders of magnitude greater for the Seyfert 2s. In the luminosity range spanned by the sample, ?43.7, the optically obscured and X-ray absorbed fractions are 50%?65%. The similarity of these fractions to more distant spectroscopic AGN samples, although over a limited luminosity range, suggests that the torus does not strongly evolve with redshift. Our sample confirms that X-ray unabsorbed Seyfert 2s are rare, comprising not more than a few percent of the Seyfert 2 population. At higher luminosities, the optically obscured fraction decreases (as expected for the increasing dust sublimation radius), but the X-ray absorbed fraction changes little. We argue that the cold X-ray absorption in these Seyfert 1s can be accounted for by neutral gas in clouds that also contribute to the broad-line region (BLR) emission, and suggest that a geometrically thick neutral gas torus co-exists with the BLR and bridges the gap to the dusty torus.

Molecular hydrogen and [Fe II] in active galactic nuclei – III. Low-ionization nuclear emission-line region and star-forming galaxies

We study the kinematics and excitation mechanisms of H 2 and [Fe ii] lines in a sample of 67 emission-line galaxies with Infrared Telescope Facility SpeX near-infrared (0.8–2.4 μm) spectroscopy together with new photoionization models. H 2 emission lines are systematically narrower than narrow-line region lines, suggesting that the two are, very likely, kinematically disconnected. The new models and emission-line ratios show that the thermal excitation plays an important role not only in active galactic nuclei but also in star-forming galaxies. The importance of the thermal excitation in star-forming galaxies may be associated with the presence of supernova remnants close to the region emitting H 2 lines. This hypothesis is further supported by the similarity between vibrational and rotational temperatures of H 2 . We confirm that the diagram involving the line ratios H 2 2.121 μm/Brγ and [Fe ii] 1.257 μm/Paβ is an efficient tool for separating emission-line objects according to their dominant types of activities. We suggest new limits to the line ratios in order to discriminate between the different types of nuclear activities.

Intermediate-age stars as origin of the low-velocity dispersion nuclear ring in Mrk 1066

We report the first two-dimensional stellar population synthesis in the near-infrared of the nuclear region of an active galaxy, namely, Mrk 1066. We have used integral field spectroscopy with adaptative optics at the Gemini North Telescope to map the age distribution of the stellar population in the inner 300 pc at a spatial resolution of 35 pc. An old stellar population component (age 5 Gyr) is dominant within the inner ≈160 pc, which we attribute to the galaxy bulge. Beyond this region, up to the borders of the observation field (∼300 pc), intermediate-age components (0.3–0.7 Gyr) dominate. We find a spatial correlation between this intermediate-age component and a partial ring of low stellar velocity dispersions (σ∗). Low-σ∗ nuclear rings have been observed in other active galaxies and our result for Mrk 1066 suggests that they are formed by intermediate-age stars. This age is consistent with an origin for the low-σ∗ rings in a past event which triggered an inflow of gas and formed stars which still keep the colder kinematics (as compared to that of the bulge) of the gas from which they have formed. At the nucleus proper we detect, in addition, two unresolved components: a compact infrared source, consistent with an origin in hot dust with mass ≈1.9 × 10 −2 M� , and a blue featureless power-law continuum, which contributes with only ≈15% of the flux at 2.12 μm.

SDSS-IV MaNGA: stellar population gradients as a function of galaxy environment

We study the internal radial gradients of stellar population properties within 1.5 Re and analyse the impact of galaxy environment. We use a representative sample of 721 galaxies with masses ranging between 109 M⊙ and 1011.5 M⊙ from the SDSS-IV survey MaNGA. We split this sample by morphology into early-type and late-type galaxies. Using the full spectral fitting code firefly, we derive the light and mass-weighted stellar population properties, age and metallicity, and calculate the gradients of these properties. We use three independent methods to quantify galaxy environment, namely the Nth nearest neighbour, the tidal strength parameter Q and distinguish between central and satellite galaxies. In our analysis, we find that early-type galaxies generally exhibit shallow light-weighted age gradients in agreement with the literature and mass-weighted median age gradients tend to be slightly positive. Late-type galaxies, instead, have negative light-weighted age gradients. We detect negative metallicity gradients in both early- and late-type galaxies that correlate with galaxy mass, with the gradients being steeper and the correlation with mass being stronger in late-types. We find, however, that stellar population gradients, for both morphological classifications, have no significant correlation with galaxy environment for all three characterizations of environment. Our results suggest that galaxy mass is the main driver of stellar population gradients in both early and late-type galaxies, and any environmental dependence, if present at all, must be very subtle.

SDSS-IV MaNGA : environmental dependence of stellar age and metallicity gradients in nearby galaxies

We present a study on the stellar age and metallicity distributions for 1105 galaxies using the STARLIGHT software on MaNGA integral field spectra. We derive age and metallicity gradients by fitting straight lines to the radial profiles, and explore their correlations with total stellar mass M*, NUV-r colour and environments, as identified by both the large scale structure (LSS) type and the local density. We find that the mean age and metallicity gradients are close to zero but slightly negative, which is consistent with the inside-out formation scenario. Within our sample, we find that both the age and metallicity gradients show weak or no correlation with either the LSS type or local density environment. In addition, we also study the environmental dependence of age and metallicity values at the effective radii. The age and metallicity values are highly correlated with M* and NUV-r and are also dependent on LSS type as well as local density. Low-mass galaxies tend to be younger and have lower metallicity in low-density environments while high-mass galaxies are less affected by environment.

A Mid-IR comparative analysis of the Seyfert galaxies NGC 7213 and NGC 1386

New Gemini mid-infrared spectroscopic observations together with Spitzer Space telescope archival data, are used to study the properties of the dusty torus and circumnuclear star formation in the active galaxies NGC 7213 and NGC 1386. Our main conclusions can be summarised as follows. Polycyclic aromatic hydrocarbon (PAH) emission is absent in the T-ReCS nuclear spectra but is ubiquitous in the data from Spitzer at distances above 100 pc. Star formation rates surface densities are estimated from the 12.8µm [Neii] line strengths leading to values close to 0.1M⊙ yr −1 kpc −2 . Analogous estimates based on photometric fluxes of IRAC’s 8µm images are higher by a factor of almost 15, which could be linked to excitation of PAH molecules by older stellar populations. T-ReCS high spatial resolution data reveal silicate absorption at λ 9.7µm in the central tens of parsecs of the Seyfert 2 NGC 1386, and silicate emission in the Seyfert 1 galaxy NGC 7213. In the case of NGC 1386 this feature is confined to the inner 20 pc, implying that the silicate might be linked to the putative dusty torus. Finally, by fitting CLUMPY models to the T-ReCS nuclear spectra we estimate the torus physical properties for both galaxies, finding line of sight inclinations consistent with the AGN unified model.

THE COMPTON-THICK SEYFERT 2 NUCLEUS OF NGC 3281: TORUS CONSTRAINTS FROM THE 9.7 μm SILICATE ABSORPTION

We present mid-infrared (mid-IR) spectra of the Compton-thick Seyfert 2 galaxy NGC?3281, obtained with the Thermal-Region Camera Spectrograph at the Gemini-South telescope. The spectra present a very deep silicate absorption at 9.7 ?m, and [S IV] 10.5 ?m and [Ne II] 12.7 ?m ionic lines, but no evidence of polycyclic aromatic hydrocarbon emission. We find that the nuclear optical extinction is in the range 24?mag ? AV ? 83?mag. A temperature T = 300?K was found for the blackbody dust continuum component of the unresolved 65?pc nucleus and the region at 130?pc SE, while the region at 130?pc NW reveals a colder temperature (200?K). We describe the nuclear spectrum of NGC?3281 using a clumpy torus model that suggests that the nucleus of this galaxy hosts a dusty toroidal structure. According to this model, the ratio between the inner and outer radius of the torus in NGC?3281 is R 0/Rd = 20, with 14 clouds in the equatorial radius with optical depth of ? V = 40?mag. We would be looking in the direction of the torus equatorial radius (i = 60?), which has outer radius of R 0 ~ 11?pc. The column density is N H 1.2 ? 1024 cm?2 and the iron K? equivalent width (0.5-1.2?keV) is used to check the torus geometry. Our findings indicate that the X-ray absorbing column density, which classifies NGC?3281 as a Compton-thick source, may also be responsible for the absorption at 9.7 ?m providing strong evidence that the silicate dust responsible for this absorption can be located in the active galactic nucleus torus.