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    • Curent research
involves various combinations of multi-wavelength observations (X-ray, UV, IR, optical and radio) of quasars and nearby low luminosity AGNs that allow us to study the variety of possible links between their physical properties and their small and large scale environment, which are expected to tell us about the distribution of matter fluctuations in the universe, and about the physics of black-hole formation and evolution.

    • so far (briefly)...
I searched for:

-- the observational constraints that their spectra posed on the cosmic chemical enhancement in their close environments and on the history of star formation in the universe (e.g., Constantin et al. 2002 ),

-- evidence -or the lack of evidence- for cosmological evolution between the distant (z > 4) quasars and the more nearby Seyfert galaxies, and iii) clues on the origin of their optical-UV emission and absorption properties (e.g., Constantin and Shields 2003 ).

I investigated the nature of the dominant energy source in the nearby weakly active nuclei by directly studying the properties of their nuclear nebular emission (Constantin et al. 2009), and by comparing spatial clustering properties that relate the nuclear emission-line activity to the characteristics of their host dark matter halos ( Constantin and Vogeley 2006 ).

I have conducted the most comprehensive analysis to date of the accretion activity within the most underdense regions of the universe, the voids (Constantin et al. 2008 ). This study shows for the first time that all types of narrow-line emission systems are present in these extreme galactic habitats, and provides pivotal evidence for the fact that the strength of accretion is correlated with the strength and age of associated star formation. This analysis also reveals the first observational evidence for a possible transitional sequence within various types (or degree) of accretion in galaxy centers, which fits well theoretical predictions for how the environment affects how quickly galaxies proceed through their evolutionary cycle.

I have also lead a study of serendipitously detected X-ray sources in Chandra fields ( ChaMP ) that allowed the first statistical analysis of the largest and most homogeneously selected sample of nearby galaxies for investigating the relation between X-ray nuclear emission, nebular line-emission, black hole masses, and properties of the associated stellar populations. ( Constantin et al. 2009 ). We have discovered in this study a double-sloped correlation between the shape of the X-ray spectral energy density (Gamma) and the accretion rate onto supermassive black holes in galaxy centers, which... DRUM ROLL ... is similar (if not identical) to what temporal variability of stellar mass black holes (i.e., X-ray binaries) appear to exhibit:

-----This remarkable similarity links the black hole accretion process for more than FIVE or SIX orders of magnitude in the black hole mass, and thus should provide novel ways of investigating: 1) the role of accretion in powering the ambiguous low luminosity AGN and 2) the proposed analogy between accretion onto supermassive and stellar size black holes.


    • My PhD work at a glance
My dissertation, conducted under Joe Shields' sage advise, has as aim searching for common grounds among the diversity of properties exhibited by the emission-line nuclei of galaxies, especially those powered by accreting black holes (the active galactic nuclei, or AGNs), from large look-back times to the local universe.

This work includes: (1) a program of high signal-to-noise spectroscopy for 44 z >∼ 4 quasars using the MMT and Keck observatories; (2) a detailed analysis of the ultraviolet and optical spectral behavior of 22 Narrow Line Seyfert 1 (NLS1) galaxies based on archival Hubble Space Telescope (HST) spectra; (3) an in-depth investigation of the proposed link between NLS1s and z >∼ 4 quasars, by means of comparison of composite spectra, and a Principal Component Analysis; (4) a simulation of Seyferts/quasars designed to explore the role of dust in modifying their observed spectral energy distribution; and (5) a sensitive search for accretion signatures in a large sample of nearby emission-line galaxy nuclei, employing a quantitative comparison of the nebular line flux ratios in small (HST) and large (ground-based) apertures.

We have found that:

-- the low and high redshift quasars are very similar in their emission characteristics, although differences exist. In particular, the data bolster indications of supersolar metallicities in the luminous, z >~ 4 sources, which support scenarios that assume substantial star formation concurrent or preceding the quasar phenomena.

- -the z >~4 quasars appear more metal enriched and more spectroscopically heterogeneous than the NLS1s, indicating that a close connection between these objects remains doubtful.

-- NLS1s reveal redder UV-blue continua than those measured in other quasars and Seyferts. The sources with UV line absorption are in general less powerful and show redder spectra, suggesting that luminosity-dependent dust absorption may be important in modifying their continua. A receding-torus -like geometry seems to explain these trends and other observed correlations between quasar luminosity and continuum slope. This relatively simple framework also predicts a number of absorbed AGNs that is consistent with recent modelling and observations of the X-ray background light and extragalactic point source population.

Finally,

-- in most of the nearby emission-line nuclei, the expected increased quasar-like behavior at smaller scales is not seen, although the nuclear emission is resolved. This suggests that these sources are not necessarily powered by accretion onto a compact object, and that the composite model proposed for the LINER/H II transition nuclei (that assumes a central accreting-type nucleus surrounded by star-forming regions) is not generally supported. Only about 10% of the narrow-lined objects reveal broad Halpha features, changing thus their spectral classification from type 2 to type 1.

    • Other works
The Cat's Eye... seen (and studied) through a 10 inch telescope HERE.





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