Featureless Continuum Component Research Articles

Disentangling the near-infrared continuum spectral components of the inner 500 pc of Mrk 573: two-dimensional maps

We present a near infrared study of the spectral components of the continuum in the inner 500$\times$500 pc$^2$ of the nearby Seyfert galaxy Mrk573 using adaptive optics near-infrared integral field spectroscopy with the instrument NIFS of the Gemini North Telescope at a spatial resolution of $\sim$50 pc. We performed spectral synthesis using the {\sc starlight} code and constructed maps for the contributions of different age components of the stellar population: young ($age\leq100$ Myr), young-intermediate ($100<age\leq700$ Myr), intermediate-old ($700$ Myr $<age\leq2$ Gyr) and old ($age>2$ Gyr) to the near-IR K-band continuum, as well as their contribution to the total stellar mass. We found that the old stellar population is dominant within the inner 250 pc, while the intermediate age components dominate the continuum at larger distances. A young stellar component contributes up to $\sim$20% within the inner $\sim$70 pc, while hot dust emission and featureless continuum components are also necessary to fit the nuclear spectrum, contributing up to 20% of the K-band flux there. The radial distribution of the different age components in the inner kiloparsec of Mrk573 is similar to those obtained by our group for the Seyfert galaxies Mrk1066, Mrk1157 and NGC1068 in previous works using a similar methodology. Young stellar populations ($\leq$100 Myr) are seen in the inner 200-300 pc for all galaxies contributing with $\ge$20% of the K-band flux, while the near-IR continuum is dominated by the contribution of intermediate-age stars ($t=$100 Myr-2 Gyr) at larger distances. Older stellar populations dominate in the inner 250 pc.

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.

Probing the stellar population of seyfert galaxies: a near infrared perspective

AbstractWe employ IRTF SpeX NIR (0.8–2.4μm) spectra to investigate the stellar population (SP), active galactic nuclei (AGN), featureless continuum (FC) and hot dust properties in 9 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. Our synthesis shows significant differences between Sy 1 and Sy 2 galaxies: the hot dust component is required to fit the K-band spectra of ~90% of the Sy 1 galaxies, and only of ~25% of the Sy 2; about 50% of the Sy 2 galaxies require an FC component contribution ≳20%; this fraction increases to about 60% in the Sy 1. In about 50% of the Sy2, the combined FC and young components contribute with more than 20%, while this occurs in 90% of the Sy1, suggesting recent star formation in the central region. The central few hundred parsecs of our galaxy sample contain a substantial fraction of intermediate-age SPs with a mean metallicity near solar. Our SP synthesis confirms that the 1.1μm CN band can be used as a tracer of intermediate-age stellar populations.

The star formation history of Seyfert 2 nuclei

We present a study of the stellar populations in the central ∼200 pc of a large and homogeneous sample comprising 79 nearby galaxies, most of which are Seyfert 2s. The star formation history of these nuclei is reconstructed by means of state‐of‐the‐art population synthesis modelling of their spectra in the 3500–5200 Å interval. A quasar‐like featureless continuum (FC) is added to the models to account for possible scattered light from a hidden active galactic nucleus (AGN). We find the following. (1) The star formation history of Seyfert 2 nuclei is remarkably heterogeneous: young starbursts, intermediate‐age and old stellar populations all appear in significant and widely varying proportions. (2) A significant fraction of the nuclei show a strong FC component, but this FC is not always an indication of a hidden AGN: it can also betray the presence of a young, dusty starburst. (3) We detect weak broad Hβ emission in several Seyfert 2s after cleaning the observed spectrum by subtracting the synthesis model. These are most likely the weak scattered lines from the hidden broad‐line region envisaged in the unified model, given that in most of these cases independent spectropolarimetry data find a hidden Seyfert 1. (4) The FC strengths obtained by the spectral decomposition are substantially larger for the Seyfert 2s which present evidence of broad lines, implying that the scattered non‐stellar continuum is also detected. (5) There is no correlation between the star formation in the nucleus and either the central or overall morphology of the parent galaxies.

What is FC2?

The unified model has it that the Featureless Continuum (FC) observed in most type 2 Seyferts is a mirror image of a hidden Seyfert 1 nucleus. The observations, however, tell us that this is not the whole story. There is now abundant evidence that the optical-UV spectrum of Seyfert 2s contains a substantial contribution from a second FC component (“FC2”) besides the scattered one. What is it?

An Alignment between Optical Continuum and Emission-Line Structures in the Circumnuclear Regions of Seyfert Galaxies

We compare the emission-line and optical continuum axes in a sample of 29 Seyfert galaxies. We find a strong trend for the [O III] λ5007-emitting gas to align with the continuum on a scale of several arcseconds (hundreds of parsecs). This trend is observed for both Seyfert types. There is no alignment, however, between the small-scale emission-line axis and the overall orientation of the host galaxy. We investigate several explanations for the alignment of the [O III] λ5007 emission with the continuum on small scales, including the possibility that the featureless continuum is extended in Seyfert galaxies. An extended featureless continuum component in Seyfert galaxies might solve several outstanding problems in currently popular unified models.