Active Galactic Nuclei at Parsec Scales

In this thesis we have performed a comprehensive investigation of the central few parsecs of nearby active galactic nuclei (AGNs), focused on two complementary aspects: (1) characterization of the nuclear emission of low-luminosity AGNs (LLAGNs, L_bol ≲ 10⁴² ergs s^-1) and comparison with the high-luminosity AGNs and (2) characterization of spatially resolved star-forming regions located very close (≲500 pc) to the active nucleus. We start from a multiwavelength and high spatial resolution data set of a sample of six active galaxies: NGC 253, NGC 1097, NGC 1386, NGC 1052, NGC 7582, and NGC 7469 (sorted by increasing distance). This data set includes the highest spatial resolution data available nowadays, covering a very wide range in wavelength: near-infrared adaptive optics images (VLT/NaCo); diffraction-limited imaging in the MIR (VLT/VISIR) and optical/UV (HST); radio maps from VLA, mostly in A-configuration plus VLBI and Very Long Baseline Array interferometry; and X-ray observations from Chandra. The result is a very consistent spatial resolution of a few tens of parsecs for the whole electromagnetic spectrum, which is precisely one of the biggest advantages when compared with previous studies.

LLAGNs are, by far, the largest class of AGN and are found in ≳50–70% of all galaxies (Ho 2008). However, they represent a challenging scenario for AGN unification theories. First, observations by Ho (1999) and Eracleous et al. (2010) suggest the absence in these objects of a big blue bump: i.e., the footprint of the accretion disk in the optical/UV range. LLAGNs are therefore postulated as radiatively inefficient sources lacking the standard accretion disk. Second, the obscuring torus, a key element in unification models, is expected to vanish below ≲10⁴² ergs s^-1 : i.e., in the luminosity range inhabited by LLAGNs (Elitzur & Shlosman 2006; Hönig et al. 2006). Furthermore, the spatial resolution has a critical impact for AGNs below ∼10⁴⁴ ergs s^-1. In this case, nuclear spectral energy distributions (SEDs) based on photometry from apertures larger than ∼1'' are, in general, dominated by the host-galaxy contribution and not by the AGN, in contrast with those based on apertures of ≲0.1'' (∼8 pc in our sample), which are AGN-dominated (Prieto et al. 2010).

Within the sample of six galaxies, we distinguish four LLAGNs: NGC 253, NGC 1097, NGC 1386, and NGC 1052. NGC 253 is the faintest one, the prototypical LINERs NGC 1097 and NGC 1052 are slightly more powerful, and NGC 1386 is a faint Seyfert 2. For these nuclei we built high spatial resolution SEDs and compared them with average SEDs for bright Seyfert galaxies (Prieto et al. 2010) and LINERs (Ho 1999; Eracleous et al. 2010). The main conclusions from this comparison are:

• 1.  
  The IR-to-optical SED of AGNs with luminosities in the range 10⁴²–10⁴⁴ ergs s^-1 reflects the presence of a central obscuring structure, less than a few parsecs in size, in both type 1 and type 2 Seyfert galaxies, in agreement with the torus considered in unified schemes.
• 2.  
  SEDs of LLAGNs with ≲10⁴² ergs s^-1 do not show signs for the presence of this structure. In contrast, they exhibit a gentle falloff in the MIR-to-UV range. This fall is not as drastic as the characteristic one in bright Seyfert 2s (Sy2s) and cannot be reproduced by moderate reddening of a high-luminosity Seyfert 1 (Sy1).
• 3.  
  At radio wavelengths, LLAGNs appear slightly different from the high-luminosity radio-quiet AGNs studied by us, but resemble what is seen in radio-loud quasars. These, such as Sy1s, show the characteristic blue bump in their spectra. In contrast, LLAGNs do not, and this absence is not caused by dust extinction (this work). Thus, we favor the scenario in which LLAGNs sustain a tenuous, radiatively inefficient, disk or even lack this structure.
• 4.  
  We found a very bright X-ray source in the center of NGC 253 (called X-1), which lacks optical, IR, and radio counterparts (Müller-Sánchez et al. 2010). This suggests the presence of a strongly obscured AGN, whereas the rest of the spectrum indicates a starburst-dominated nuclei. The case of this galaxy is quite similar to that of NGC 4945, a well-known starburst galaxy with an AGN core detected only in the hard X-ray band (L_2–10 keV ∼ 10⁴⁵ ergs s^-1, Marconi et al. 2000). We believe that X-1 in NGC 253 is a low-luminosity (L_2–10 keV ∼ 10⁴⁰ ergs s^-1) member of the same class as NGC 4945. These "embedded AGNs" are reminiscent of ultraluminous IR galaxies in the sense that the starburst component seems to be hiding or heavily obscuring the AGN.

We have been able to resolve star-forming regions in the central parsecs of all the objects in the sample at different distances from the active nucleus. These take the form of starburst rings with diameters of about 1 kpc (NGC 1097, NGC 1386, and NGC 7469), compact starburst disks within the inner ≲500 pc (NGC 253 and NGC 7582), or a random distribution within the central few kiloparsecs (NGC 1052). In each galaxy, and for each of the star-forming regions found, we obtained a multiwavelength SED and provided estimates for the age, mass, and size (Fernández-Ontiveros et al. 2011, in preparation). A comparative analysis of the properties of these regions is produced:

• 1.  
  Most of the star-forming regions appear to be very homogeneous, regardless of the galaxy type or AGN luminosity, with similar SEDs, very compact sizes (3–20 pc, in most cases still limited by the achieved spatial resolution), and ages in the 3–20 Myr range. These properties are similar to those of young stellar clusters (YSCs) found in the Milky Way (10³–10⁴ M_⊙, as well as in NGC 1386 and NGC 1052), starbursts, and interacting galaxies (≳10⁴ M_⊙, in the remaining objects; Mengel et al. 2005; Portegies Zwart et al. 2010).
• 2.  
  NGC 253 is a special case because of its proximity (∼4 Mpc), which allows us to compile the most complete SED for most of its nuclear star-forming regions from the UV to the radio down to scales of ∼3 pc (Fernández-Ontiveros et al. 2009). A characteristic IR bump (1–2 μm), seen in most of the SEDs for the YSCs in the nucleus of this galaxy, is identified as the signature of very young protostellar objects being currently produced in these clusters.
• 3.  
  The cluster mass function (CMF) for NGC 253—the youngest system with 4.3 Myr, on average—follows a power law with an index of γ ≈ 1.8, close to the common behavior of many diverse star-forming systems (γ ≈ 2, Portegies Zwart et al. 2010). NGC 7469 shows a steeper CMF, with γ ≈ 3.5, which is more similar to very massive star-forming clusters (≳10⁶ M_⊙). However, in this case we suspect that the clusters are not individually resolved.
• 4.  
  A truncated CMF is found in NGC 1097, which is very well fitted by a Schechter profile with a characteristic mass of M_∗ ∼ 2.1 × 10⁵ M_⊙. Since this is the oldest system with 10 Myr, on average, a possible explanation for this truncation is the internal dynamical evolution favoring the disruption of the less massive clusters (Fall & Zhang 2001).
• 5.  
  We found 25 compact clusters (≲11 pc) in the central region (2 × 2 kpc²) of the elliptical NGC 1052 (Fernández-Ontiveros et al. 2010). Their locations around the nucleus are sparse and do not correlate with the distribution of the radio lobes, H i, ionized gas, or dust. Of these clusters, at least 15 present strong Hα emission (L_Hα ∼ 3.8 × 10³⁶ ergs s^-1). The possible contribution of an old population to the UV continuum, via extreme horizontal branch stars, cannot account for the Hα luminosity. In contrast, cluster SEDs and L_Hα values can be explained in terms of a very young population (∼6 Myr, or ∼12 Myr for the lowest-metallicity case) with masses of about 10⁴ M_⊙ and extinction values of A_V ∼ 3.4 mag. Thus, we propose that these are YSCs formed in a very recent star-formation episode, probably related with a merger event suffered by this galaxy ∼1 Gyr ago (van Gorkom et al. 1986).