136 research outputs found
Semi-empirical analysis of Sloan Digital Sky Survey galaxies III. How to distinguish AGN hosts
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
Retired galaxies: not to be forgotten in the quest of the star formation -- AGN connection
We propose a fresh look at the Main Galaxy Sample of the Sloan Digital Sky
Survey by packing the galaxies in stellar mass and redshift bins. We show how
important it is to consider the emission-line equivalent widths, in addition to
the commonly used emission-line ratios, to properly identify retired galaxies
(i.e. galaxies that have stopped forming stars and are ionized by their old
stellar populations) and not mistake them for galaxies with low-level nuclear
activity. We find that the proportion of star-forming galaxies decreases with
decreasing redshift in each mass bin, while that of retired galaxies increases.
Galaxies with have formed all their stars at
redshift larger than 0.4. The population of AGN hosts is never dominant for
galaxy masses larger than . We warn about the effects of
stacking galaxy spectra to discuss galaxy properties. We estimate the lifetimes
of active galactic nuclei (AGN) relying entirely on demographic arguments ---
i.e. without any assumption on the AGN radiative properties. We find
upper-limit lifetimes of about 1--5 Gyr for detectable AGN in galaxies with
masses between --. The lifetimes of the AGN-dominated
phases are a few yr. Finally, we compare the star-formation histories of
star-forming, AGN and retired galaxies as obtained by the spectral synthesis
code STARLIGHT. Once the AGN is turned on it inhibits star formation for the
next 0.1 Gyr in galaxies with masses around , 1
Gyr in galaxies with masses around .Comment: accepted for MNRAS figure resolution has been degraded with respect
to what will be published in MNRA
An atlas of Calcium triplet spectra of active galaxies
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 MNRA
Uncovering the chemical enrichment and mass-assembly histories of star-forming galaxies
We explore the mass-assembly and chemical enrichment histories of star
forming galaxies by applying a population synthesis method to a sample of 84828
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 onto 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 () in the last
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 already 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.Comment: Accepted for publication in MNRA
The CaT strength in Seyfert nuclei revisited: analyzing young stars and non-stellar light contributions to the spectra
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.Comment: 14 pages, 15 figures. Paper accepted for publication in MNRA
Gas Emission Spectrum in the Irr Galaxy IC 10
Spectroscopic long-slit observations of the dwarf Irr galaxy IC 10 were
conducted at the 6-m Special Astrophysical Observatory telescope with the
SCORPIO focal reducer. The ionized-gas emission spectra in the regions of
intense current star formation were obtained for a large number of regions in
IC 10. The relative abundances of oxygen, N+, and S+ in about twenty HII
regions and in the synchrotron superbubble were estimated. We found that the
galaxy-averaged oxygen abundance is 12 + log(O/H) = 8.17 +- 0.35 and the
metallicity is Z = 0.18 +- 0.14 Z_sun. Our abundances estimated from the strong
emission lines are found to be more reliable than those obtained by comparing
diagnostic diagrams with photoionization models.Comment: Abridged; accepted in Astronomy Letter
Less than the sum of its parts : the dust-corrected Hα luminosity of star-forming galaxies explored at different spatial resolutions with MaNGA and MUSE
Funding: NVA would like to thank the University of St Andrews for providing support during her visit. NVA acknowledges support of the Royal Society and the Newton Fund via the award of a Royal SocietyâNewton Advanced Fellowship (grant NAF\R1\180403), and of Fundação de Amparo Ă Pesquisa e Inovação de Santa Catarina (FAPESC) and Conselho Nacional de Desenvolvimento CientĂfico e TecnolĂłgico (CNPq). AW acknowledges financial support from Fundação de Amparo Ă Pesquisa do Estado de SĂŁo Paulo (FAPESP) process number 2019/01768-6. MG receives funding from the European Research Council (ERC) under the European Union Horizon 2020 research and innovative programme (MagneticYSOs programme, grant agreement Nber 679937). EWP, RSK, SR, SCOG and DR acknowledge funding from the Deutsche Forschungsgemeinschaft (DFG) via the Collaborative Research Center (SFB 881) âThe Milky Way System (subprojects A1, B1, and B2) and from the Heidelberg Cluster of Excellence STRUCTURES in the framework of Germanyâs Excellence Strategy (grant EXC-2181/1- 390900948).The Hα and HÎČ emission line luminosities measured in a single integrated spectrum are affected in non-trivial ways by point-to-point variations in dust attenuation in a galaxy. This work investigates the impact of this variation when estimating global Hα luminosities corrected for the presence of dust by a global Balmer decrement. Analytical arguments show that the dust-corrected Hα luminosity is always underestimated when using the global Hα/HÎČ flux ratio to correct for dust attenuation. We measure this effect on 156 face-on star-forming galaxies from the Mapping Nearby Galaxies at APO (MaNGA) survey. At 1â2 kpc spatial resolution, the effect is small but systematic, with the integrated dust-corrected Hα luminosity underestimated by 2â4 per cent (and typically not more than by 10 per cent), and depends on the specific star formation rate of the galaxy. Given the spatial resolution of MaNGA, these are lower limits for the effect. From Multi Unit Spectroscopic Explorer (MUSE) observations of NGC 628 with a resolution of 36 pc we find the discrepancy between the globally and the point-by-point dust-corrected Hα luminosity to be 14 ± 1 per cent, which may still underestimate the true effect. We use toy models and simulations to show that the true difference depends strongly on the spatial variance of the Hα/HÎČ flux ratio, and on the slope of the relation between Hα luminosity and dust attenuation within a galaxy. Larger samples of higher spatial resolution observations are required to quantify the dependence of this effect as a function of galaxy properties.PostprintPeer reviewe
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