We employ the NASA Infrared Telescope Facilitys near-infrared spectrograph SpeX at 0.8-2.4$mu$m to investigate the spatial distribution of the stellar populations (SPs) in four well known Starburst galaxies: NGC34, NGC1614, NGC3310 and NGC7714. We us
e the STARLIGHT code updated with the synthetic simple stellar populations models computed by Maraston (2005, M05). Our main results are that the NIR light in the nuclear surroundings of the galaxies is dominated by young/intermediate age SPs ($t leq 2times10^9$yr), summing from $sim$40% up to 100% of the light contribution. In the nuclear aperture of two sources (NGC1614 and NGC3310) we detected a predominant old SP component ($t > 2times10^9$yr), while for NGC34 and NGC7714 the younger component prevails. Furthermore, we found evidence of a circumnuclear star formation ring-like structure and a secondary nucleus in NGC1614, in agreement with previous studies. We also suggest that the merger/interaction experienced by three of the galaxies studied, NGC1614, NGC3310 and NGC7714 can explain the lower metallicity values derived for the young SP component of these sources. In this scenario the fresh unprocessed metal poorer gas from the destroyed/interacting companion galaxy is driven to the centre of the galaxies and mixed with the central region gas, before star formation takes place. In order to deepen our analysis, we performed the same procedure of SP synthesis using Maraston (2011, M11) EPS models. Our results show that the newer and higher resolution M11 models tend to enhance the old/intermediate age SP contribution over the younger ages.
We present mid infrared (MIR) spectra of the Seyfert 2 (Sy 2) galaxy NGC 1808, obtained with the Geminis Thermal-Region Camera Spectrograph (T-ReCS) at a spatial resolution of 26 pc. The high spatial resolution allowed us to detect bright polycyclic
aromatic hydrocarbons (PAHs) emissions at 8.6micron and 11.3micron in the galaxy centre (26 pc) up to a radius of 70 pc from the nucleus. The spectra also present [Ne ii]12.8micron ionic lines, and H2 S(2)12.27micron molecular gas line. We found that the PAHs profiles are similar to Peeterss A class, with the line peak shifted towards the blue. The differences in the PAH line profiles also suggests that the molecules in the region located 26 pc NE of the nucleus are more in the neutral than in the ionised state, while at 26 pc SW of the nucleus, the molecules are mainly in ionised state. After removal of the underlying galaxy contribution, the nuclear spectrum can be represented by a Nenkovas clumpy torus model, indicating that the nucleus of NGC 1808 hosts a dusty toroidal structure with an angular cloud distribution of sigma = 70degree, observers view angle i = 90degree, and an outer radius of R0 = 0.55 pc. The derived column density along the line of sight is NH = 1.5 x 10^24 cm-2, which is sufficient to block the hard radiation from the active nucleus, and would explain the presence of PAH molecules near to the NGC 1808s active nucleus.
We present mid infrared (Mid-IR) spectra of the Compton-thick Seyfert 2 galaxy NGC,3281, obtained with the Thermal-Region Camera Spectrograph (T-ReCS) at the Gemini South telescope. The spectra present a very deep silicate absorption at 9.7,$mu$m, an
d [S{sc,iv]},10.5,$mu$m and [Ne{sc,ii]},12.7,$mu$m ionic lines, but no evidence of PAH emission. We find that the nuclear optical extinction is in the range 24 $leq$ A$_{V}$ $leq$ 83,mag. A temperature T = 300,K was found for the black-body dust continuum component of the unresolved 65,pc nucleus and at 130,pc SE, while the region at 130,pc 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 $R_0/R_d$ = 20, with {bf 14} clouds in the equatorial radius with optical depth of $tau_{V}$ = 40,mag. We would be looking in the direction of the torus equatorial radius ($i$ = {bf 60$^{circ}$}), which has outer radius of R$_{0},sim$ 11,pc. The column density is N$_{H}approx$,{bf 1.2},$times,10^{24},cm^{-2}$ and iron K$alpha$ equivalent width ($approx$ 0.5 - 1.2,keV) are 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,$mu$m providing strong evidence that the silicate dust responsible for this absorption can be located in the AGN torus.
We study the Polycyclic Aromatic Hydrocarbons (PAH) bands, ionic emission lines, and Mid-infrared continuum properties, in a sample of 171 emission line galaxies taken from literature plus 15 new active galactic nuclei (AGN) Spitzer spectra. The cont
inuum shape steeply rises for longer wavelengths and can be fitted with a warm blackbody distribution of T=150-300K. The brightest PAH spectral bands (6.2, 7.7, 8.6, 11.3, and 12.7$mu$m) and the forbidden emission lines of [Si II] 34.8$mu$m, [Ar II] 6.9, [S III] 18.7 and 33.4 were detected in all the Starbursts and in ~80% of the Seyfert~2. Taking under consideration only the PAH bands at 7.7$mu$m, 11.3$mu$m, and 12.7$mu$m we find they are present in ~80% of the Seyfert 1, while only half of this type of activity show the 6.2$mu$m and 8.6 PAH bands. The observed intensities ratios for neutral and ionized PAHs (6.2/7.7 x 11.3/7.7) were compared to theoretical intensity ratios, showing that AGNs have higher ionization fraction and larger PAH (> 180 carbon atoms) than SB galaxies. The ratio between the ionized (7.7) and the neutral PAH bands (8.6 and 11.3) are distributed over different ranges for AGNs and SB galaxies, suggesting that these ratios could depend on the ionization fraction, as well as on the hardness of the radiation field. The ratio between the 7.7 and 11.3 bands is nearly constant with the increase of [Ne III]15.5/[Ne II], indicating that the fraction of ionized to neutral PAH bands does not depend on the hardness of the radiation field. The equivalent width of both PAH features show the same dependence with [Ne III]/[Ne II], suggesting that the PAH, emitting either ionized (7.7) or neutral (11.3) bands, may be destroyed with the increase of the hardness of the radiation field.
We present SOAR/OSIRIS cross-dispersed NIR integrated spectra of 12 Galactic globular clusters that are employed to test Maraston (2005, M05) NIR EPS models, and to provide spectral observational constraints to calibrate future models. We measured Ew
of the most prominent NIR absorption features. Optical Ew were also measured. The globular clusters Ew were compared with model predictions with ages within 4-15 Gyr, and metallicities between 1/200 and 2 Zsun. Observed integrated colours were also compared with models. The NIR integrated spectra among our sample appear qualitatively similar in most the absorption features. The M05 models can properly predict the optical Ew observed in globular clusters. Regarding the NIR, they do underestimate the strength of Mg I 1.49mum, but they can reproduce the observed Ew of Fe I 1.58mum, Si I 1.59mum, and CO 2.29mum, in about half of our sample. The remaining objects require the inclusion of intermediate-age populations. Thus, we suggest that the presence of C- and O-rich stars in models is important to reproduce the observed strengths of metallic lines. Another possibility is the lack of alpha-enhancement in the models. In the case of the optical and NIR Fe I lines, standard models and those that include blue horizontal branch stars, produce similar results. A similar trend is observed for Na I 5895A, while in the case of the G-band, the models with blue horizontal branch do describe better the observations. For most of the sample the optical to NIR colours are well described by the M05 models. In general, M05 models can provide reliable information on the NIR stellar population of galaxies, but only when Ew and colours are taken together, in other words, Ew and continuum fluxes should be simultaneously fitted. However, the results should be taken with caution, since the models tend to predict results biased towards young ages.
