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ABSTRACT: Optical studies of starbursts, AGN and their connections usually leave
out galaxies whose emission lines are too weak to warrant reliable
measurement and classification. Yet, weak line galaxies abound, and
deserve a closer look. We show that these galaxies are either massive,
metal rich star-forming systems, or, more often, LINERs. From our
detailed stellar population analysis, we find that these LINERs have
stopped forming stars long ago. Moreover, their ionizing radiation field
is amazingly consistent with that expected from their old stellar
populations alone. The black-hole in the centers of these massive,
early-type galaxies is not active enough to overwhelm stellar
ionization, and thus, despite their looks, they should not be called
AGN.
09/2009; 408:122.
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ABSTRACT: In a former paper, we have presented spectra of 64 active, nine normal and five starburst galaxies in the region around the near-infrared calcium triplet (CaT) absorption lines and the [S iii]λ9069 line. In the present paper, we analyse the CaT strength (WCaT) and kinematical products derived in that study, namely stellar (σ★) and ionized gas (σgas) velocity dispersions. Our main results may be summarized as follows. (1) Type 2 Seyfert galaxies show no sign of dilution in WCaT with respect to the values spanned by normal galaxies, even when optical absorption lines such as the Ca iiK band at 3933 Å are much weaker than in old, bulge-like stellar populations. (2) The location of type 2 Seyfert galaxies in the WCaT–WCaK plane is consistent with evolutionary synthesis models. The implication is that the source responsible for the dilution of optical lines in these active galactic nuclei (AGN) is a young stellar population, rather than an AGN featureless continuum, confirming the conclusion of the pioneer study of Terlevich, Díaz & Terlevich. (3) In type 1 Seyfert galaxies, both W[S iii] and WCaT tend to be diluted due to the presence of a non-stellar component, in agreement with the unification paradigm. (4) A comparison of σ★ with σgas (obtained from the core of the [S iii] emitting line) confirms the existence of a correlation between the typical velocities of stars and clouds of the narrow line region. The strength and scatter around this correlation are similar to those previously obtained from the [O iii]λ5007 linewidth.
Monthly Notices of the Royal Astronomical Society 02/2009; 393(3):846 - 857. · 4.90 Impact Factor
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ABSTRACT: In a former paper (Garcia-Rissmann et al. 2005; hereafter Paper I), we have presented spectra of 64 active, 9 normal and 5 Starburst galaxies in the region around the near-IR Calcium triplet absorption lines and the [SIII]9069 line. In the present paper we analyze the CaT strength (WCaT), and kinematical products derived in that study, namely stellar and ionized gas velocity dispersions. Our main results may be summarized as follows: (1) Seyfert 2s show no sign of dilution in WCaT with respect to the values spanned by normal galaxies, even when optical absorption lines such as the CaII K band at 3933 A are much weaker than in old, bulge-like stellar populations. (2) The location of Seyfert 2s in the WCaT-WCaK plane is consistent with evolutionary synthesis models. The implication is that the source responsible for the dilution of optical lines in these AGN is a young stellar population, rather than an AGN featureless continuum, confirming the conclusion of the pioneer study of Terlevich, Diaz & Terlevich. (3) In Seyfert 1s, both W[SIII] and WCaT tend to be diluted due to the presence of a non-stellar component, in agreement with the unification paradigm. (4) A comparison of stellar and gas velocity dispersions confirms the existence of a correlation between the typical velocities of stars and clouds of the Narrow Line Region. The strength and scatter around this correlation are similar to those previously obtained from the [OIII]5007 line width.
10/2008;
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ABSTRACT: Retrieving the Star Formation History (SFH) of a galaxy out of its integrated spectrum is the central goal of stellar population synthesis. Recent advances in evolutionary synthesis models have given new breath to this old field of research. Modern spectral synthesis techniques incorporating these advances now allow the fitting of galaxy spectra on an angstrom-by-angstrom basis. These detailed fits are useful for a number of studies, like emission line, stellar kinematics, and specially galaxy evolution. Applications of this semi-empirical approach to mega data sets are teaching us a lot about the lives of galaxies. The STARLIGHT spectral synthesis code is one of the tools which allows one to harness this favorable combination of plentifulness of data and models. To illustrate this, we show how SFHs vary across classical emission line diagnostic diagrams. Systematic trends are present along both the star-forming and active-galaxy sequences. We also briefly describe experiments with new versions of evolutionary synthesis models. Last but not least, we announce the public availability of both STARLIGHT and a database of detailed spectral fits and related products for over half a million galaxies from the SDSS. This facility allows more physically inspired explorations of the parameter space than is possible in terms of raw observed properties, offering new ways to navigate through the realm of galaxies.
03/2008;
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ABSTRACT: This paper, the sixth in the Semi-Empirical Analysis of Galaxies series, studies the evolution of 82 302 star-forming (SF) galaxies from the Sloan Digital Sky Survey. Star formation histories (SFHs) are derived from detailed spectral fits obtained with our publicly available spectral synthesis code starlight. Our main goals are to explore new ways to derive SFHs from the synthesis results and apply them to investigate how SFHs vary as a function of nebular metallicity (Zneb). A number of refinements over our previous work are introduced, including (1) an improved selection criterion; (2) a careful examination of systematic residuals around Hβ; (3) self-consistent determination of nebular extinctions and metallicities; (4) tests with several Zneb estimators; (5) a study of the effects of the reddening law adopted and of the relation between nebular and stellar extinctions and the interstellar component of the Na i D doublet.Our main achievements may be summarized as follows. (1) A conventional correlation analysis is performed to study how global properties relate to Zneb, leading to the confirmation of previously known relations, such as those between Zneb and galaxy luminosity, mass, dust content, mean stellar metallicity and mean stellar age. (2) A simple formalism which compresses the results of the synthesis while at the same time yielding time-dependent star formation rates (SFR) and mass assembly histories is presented. (3) A comparison of the current SFR derived from the population synthesis with that obtained from Hα shows that these independent estimators agree very well, with a scatter of a factor of 2. An important corollary of this finding is that we now have a way to estimate SFR in galaxies hosting active galactic nuclei, where the Hα method cannot be applied. (4) Fully time-dependent SFHs were derived for all galaxies, and then averaged over six Zneb bins spanning the entire SF wing in the diagram. (5) We find that SFHs vary systematically along the SF sequence. Though all SF galaxies formed the bulk of their stellar mass over 1 Gyr ago, low-Zneb systems evolve at a slower pace and are currently forming stars at a much higher relative rate. Galaxies at the tip of the SF wing have current specific SFRs about two orders of magnitude larger than the metal-rich galaxies at its bottom. (6) At any given time, the distribution of specific SFRs for galaxies within a Zneb bin is broad and approximately lognormal. (7) The whole study was repeated grouping galaxies within bins of stellar mass and surface mass density, both of which are more fundamental drivers of SFH. Given the existence of strong Zneb−M★−Σ★ relations, the overall picture described above remains valid. Thus, low-M★ (low-Σ★) systems are the ones which evolve slower, with current specific SFRs much larger than more massive (dense) galaxies. (8) This overall pattern of SFHs as a function of Zneb, M★ or Σ★ is robust against changes in selection criteria, choice of evolutionary synthesis models for the spectral fits, and differential extinction effects.
Monthly Notices of the Royal Astronomical Society 10/2007; 381(1):263 - 279. · 4.90 Impact Factor
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ABSTRACT: We study the evolution of 82302 star-forming (SF) galaxies from the SDSS. Our main goals are to explore new ways of handling star formation histories (SFH) obtained with our publicly available spectral synthesis code STARLIGHT, and apply them to investigate how SFHs vary as a function of nebular metallicity (Zneb). Our main results are: (1) A conventional correlation analysis shows how global properties such as luminosity, mass, dust content, mean stellar metallicity and mean stellar age relate to Zneb. (2) We present a simple formalism which compresses the results of the synthesis into time-dependent star formation rates (SFR) and mass assembly histories. (3) The current SFR derived from the population synthesis and that from H-alpha are shown to agree within a factor of two. Thus we now have a way to estimate SFR in AGN hosts, where the H-alpha method cannot be applied. (4) Fully time-dependent SFHs are derived for all galaxies and averaged over six Zneb bins spanning the entire SF wing in the [OIII]/H-beta X [NII]/H-alpha diagram. (5) We find that SFHs vary systematically along the SF sequence, such that low-Zneb systems evolve slower and are currently forming stars at a higher relative rate. (6) At any given time, the distribution of specific SFRs for galaxies within a Zneb-bin is broad and roughly log-normal. (7) The same results are found grouping galaxies in stellar mass (M*) or surface mass density (S*) bins. (8) The overall pattern of SFHs as a function of Zneb, M* or S* is robust against changes in selection criteria, choice of evolutionary synthesis models for the spectral fits, and differential extinction effects. (Abridged) Comment: Accepted for publication in MNRAS
07/2007;
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ABSTRACT: We explore the mass-assembly and chemical enrichment histories of star-forming galaxies by applying a population synthesis method to a sample of 84 828 galaxies from the Sloan Digital Sky Survey Data Release 5. Our method decomposes the entire observed spectrum in terms of a sum of simple stellar populations spanning a wide range of ages and metallicities, thus allowing the reconstruction of galaxy histories. A comparative study of galaxy evolution is presented, where galaxies are grouped on to bins of nebular abundances or mass. We find that galaxies whose warm interstellar medium is poor in heavy elements are slow in forming stars. Their stellar metallicities also rise slowly with time, reaching their current values (Z★∼ 1/3 Z⊙) in the last ∼100 Myr of evolution. Systems with metal-rich nebulae, on the other hand, assembled most of their mass and completed their chemical evolution long ago, reaching Z★∼ Z⊙ at lookback times of several Gyr. These same trends, which are ultimately a consequence of galaxy downsizing, appear when galaxies are grouped according to their stellar mass. The reconstruction of galaxy histories to this level of detail out of integrated spectra offers promising prospects in the field of galaxy evolution theories.
Monthly Notices of the Royal Astronomical Society Letters 02/2007; 375(1):L16 - L20.
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ABSTRACT: We consider the techniques to distinguish normal star forming (NSF) galaxies and active galactic nuclei (AGN) hosts using optical spectra. The observational data base is a set of 20000 galaxies extracted from the Sloan Digital Sky Survey, for which we have determined the emission line intensities after subtracting the stellar continuum obtained from spectral synthesis. Our analysis is based on photoionization models computed using the stellar ionizing radiation predicted by Starburst 99 and, for the AGNs, a broken power-law spectrum. We explain why, among the four classical emission line diagnostic diagrams, the [OIII]/Hb vs [NII]/Ha one works best. We show however, that none of these diagrams is efficient in detecting AGNs in metal poor galaxies, should such cases exist. We propose a new divisory line between ``pure'' NSF galaxies and AGN hosts. We also show that a classification into NSF and AGN galaxies using only [NII]/Ha is feasible and useful. Finally, we propose a new classification diagram, the DEW diagram, plotting D_n(4000) vs max(EW[OII],EW[NeIII]). This diagram can be used with optical spectra for galaxies with redshifts up to z = 1.3, meaning an important progress over classifications proposed up to now. Since the DEW diagram requires only a small range in wavelength, it can also be used at even larger redshifts in suitable atmospheric windows. It also has the advantage of not requiring stellar synthesis analysis to subtract the stars and of allowing one to see ALL the galaxies in the same diagram, including passive galaxies. Comment: 14 pages, 9 figures, accepted for publication in MNRAS (replaced on august 3, 2006, eqs 6 and 7 corrected)
06/2006;
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Monthly Notices of the Royal Astronomical Society 05/2005; · 4.90 Impact Factor
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ABSTRACT: We present a spectroscopic atlas of active galactic nuclei covering the region around the λλ8498, 8542, 8662 calcium triplet (CaT). The sample comprises 78 objects, divided into 43 Seyfert 2s, 26 Seyfert 1s, three starburst and six normal galaxies. The spectra pertain to the inner ∼300 pc in radius, and thus sample the central kinematics and stellar populations of active galaxies. The data are used to measure stellar velocity dispersions (σ★) with both cross-correlation and direct fitting methods. These measurements are found to be in good agreement with each other and with those in previous studies for objects in common. The CaT equivalent width is also measured. We find average values and sample dispersions of WCaT of 4.6 ± 2.0, 7.0 ± 1.0 and 7.7 ± 1.0 Å for Seyfert 1s, Seyfert 2s and normal galaxies, respectively. We further present an atlas of [S iii]λ9069 emission-line profiles for a subset of 40 galaxies. These data are analysed in a companion paper which addresses the connection between stellar and narrow-line region kinematics, the behaviour of the CaT equivalent width as a function of σ★, activity type and stellar population properties.
Monthly Notices of the Royal Astronomical Society 04/2005; 359(2):765 - 780. · 4.90 Impact Factor
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ABSTRACT: We present a spectroscopic atlas of active galactic nuclei covering the region around the 8498, 8542, 8662 Calcium triplet (CaT) lines. The sample comprises 78 objects, divided into 43 Seyfert 2s, 26 Seyfert 1s, 3 Starburst and 6 normal galaxies. The spectra pertain to the inner ~300 pc in radius, and thus sample the central kinematics and stellar populations of active galaxies. The data are used to measure stellar velocity dispersions (sigma_star) both with cross-correlation and direct fitting methods. These measurements are found to be in good agreement with each-other and with those in previous studies for objects in common. The CaT equivalent width is also measured. We find average values and sample dispersions of W_CaT of 4.6+/-2.0, 7.0 and 7.7+/-1.0 angstrons for Seyfert 1s, Seyfert 2s and normal galaxies, respectively. We further present an atlas of [SIII]\lambda 9069 emission line profiles for a subset of 40 galaxies. These data are analyzed in a companion paper which addresses the connection between stellar and Narrow Line Region kinematics, the behaviour of the CaT equivalent width as a function of sigma_star, activity type and stellar population properties. Comment: 18 pages, 10 figures, accepted for publication in MNRAS
02/2005;
