Free Access
Issue
A&A
Volume 558, October 2013
Article Number A68
Number of page(s) 8
Section Catalogs and data
DOI https://doi.org/10.1051/0004-6361/201322016
Published online 04 October 2013

© ESO, 2013

1. Introduction

The advent of the Sloan Digital Sky Survey (SDSS, York et al. 2000) revolutionized the course of astronomy at the turn of the millennium. However some residual incompleteness remains in the SDSS spectroscopic database, especially at the bright luminosity end, due to shredding of the large galaxies and fiber conflict (Blanton et al. 2005a,b,c). Mitigating this problem is a task that even 1.5 m class telescopes can contribute to. With this idea, we decided to continue the spectroscopic project that began in 2005 at the Loiano Observatory (Gavazzi et al. 2011) with an aim at searching for previously unknown optically selected active galactic nuclei (AGNs) in the local Universe1. In 2012−2013, we obtained 127 new nuclear spectra, bringing the total number of galaxies independently observed to 376. These are spectra taken with the red-channel of the spectrograph, namely between 6000 and 8000 Å. Since nuclear spectra of about half these galaxies were neither obtained by SDSS nor distributed by NED, we provide our new observations to NED in order to make them publicly available in FITS (Flexible Image Transport System) format.

The classification of AGNs based on optical nuclear spectra is routinely (e.g., Decarli et al. 2007; Reines et al. 2013) performed using the BPT (Baldwin, Phillips & Terlevich) diagnostic diagram (Baldwin et al. 1981), which requires the measurement of four spectral lines: Hβ, [OIII], Hα, and [NII]. General AGNs are disentangled from nuclear starbursts using the ratio [NII]/Hα (where Hα must be corrected for any underlying stellar absorption, as stressed by Ho et al. 1997), while strong AGNs (sAGN) can be separated from the weaker (weak AGNs or wAGN) low-ionization nuclear emission-line region (LINERs) using the ratio [OIII]/Hβ. However a recent two-line diagnostic diagram named WHAN, which is based on the [NII] /Hα ratio combined with the strength of the Hα line was introduced by Cid Fernandes et al. (2010, 2011) to disentangle strong and weak AGNs, believed to be triggered by supermassive black holes from “fake AGNs”, dubbed as retired galaxies, whose ionization mechanism is probably provided by their old stellar population (Trinchieri & di Serego Alighieri 1991; Binette et al. 1994; Macchetto et al. 1996; Stasińska et al. 2008; Sarzi et al. 2010; Capetti & Baldi 2011). For a nucleus to be considered ionized by a central black hole, it is necessary that the equivalent width (EW) of Hα exceeds 3 Å (Cid Fernandes et al. 2010, 2011). This quantitative threshold has been, however, questioned by Gavazzi et al. (2011), who adopted EW Hα ≥ 1.5 Å.

Given the two-line WHAN diagnostic diagram, the red-channel spectra presented in this work can contribute to increasing the number of known AGNs, especially those associated with nearby bright galaxies, most affected by the residual incompleteness of the SDSS spectral database. Nevertheless we reiterate that the present sample is not complete by any means, thus inadequate to perform any demographic study.

The outline of this paper is as follows. In Sect. 2 we describe the galaxy sample. In Sect. 3 we illustrate the observations taken at the Loiano observatory and the data reduction procedures. The nuclear spectra are given and classified in Sect. 4. In Sect. 5 we outline the fitting method applied to broad-line systems.

2. The sample

This paper is based on a miscellaneous set of 376 northern galaxies visible in spring, which is not complete by any means, and therefore not useful for performing any statistical study. They comprise of well-known bright local galaxies within zc = 10 000 km s-1, most of which have a nuclear spectrum already available from the literature, mixed with fainter previously unobserved objects. The latter were mainly selected from the Coma and Local superclusters (RA ~ 180 deg, see Fig. 1) for not having a nuclear spectrum available from SDSS or for the being unavailable, in either ASCII or FITS form, in the nuclear optical spectra from NED2. Figure 2 (top) illustrates the r-band absolute magnitude distribution of the selected galaxies, showing a dramatic lack of objects fainter than −20 mag with respect to those predicted by the r-band luminosity function (e.g. Blanton et al. 2003). Figure 2 (bottom) highlights the preferential membership of the selected targets to the Coma supercluster (V ~ 7000 km s-1) and the Virgo cluster and its surroundings (V ~ 1000 km s-1).

thumbnail Fig. 1

Distribution in celestial coordinates of the 376 galaxies for which we obtained a nuclear spectrum. Color coding is as follows: available fiber spectra from SDSS (#86, green); available nuclear spectra from NED (#162, blue); available spectra from both SDSS and NED (#36, cyan); and newly obtained spectra from this work (#164, red).

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thumbnail Fig. 2

Histograms of the absolute magnitudes (r-band and other bands mixed together; top panel) and of the heliocentric recessional velocities (bottom panel) of the 376 studied galaxies.

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3. Observations and data reduction

We used the Bologna faint object spectrograph and camera (BFOSC, Gualandi & Merighi 2001) attached to the 152 cm F/8 Cassini Telescope located in Loiano, which belongs to the Observatory of Bologna, to obtain optical spectra of the nuclei of 376 galaxies. The observations took place from 2005 to 2013 (see Table 1). The long-slit spectra were taken through a slit of 2 or 2.5 arcsec width (depending on the seeing conditions), with a intermediate-resolution red-channel grism (R ~ 2200) that covers the 6100−8200 Å portion of the spectrum, which contains Hα, [NII], and [SII] lines (three galaxies were observed also using a blue-channel grism covering Hβ and [OIII], as shown in Fig. 5). The detector used by BFOSC is an EEV LN/1300-EB/1 CCD of 1300 × 1340 pixels, with 90% quantum efficicency near 5500 Å. Its spatial scale of 0.58 arcsec/pixel results in a field of view of 12.6′ × 13′. The dispersion of the red-channel grism is 8.8 nm/mm and results in spectra with 1.6 Å/pix. The instrumental broadening is typically ~6 Å full-width-half-maximum (FWHM), as checked on the 6300.3 Å sky line. We obtained exposures of 3−5 min, repeated typically three-six times per run (to remove the cosmic rays), but several galaxies were re-observed in more than one run (see Table 4). The seeing at Loiano is typically 1.5−2.5 arcsec. The slit was mostly set in the E-W direction, except when it was positioned along the galaxy major axis or along the direction connecting two adjacent objects to accomodate both in one exposure. The wavelength was calibrated using frequent exposures of a He-Ar hollow-cathode lamp. We used several sky lines to check a posteriori the wavelength calibration. The spectrograph response was obtained by daily exposures of the star Feige-34.

Table 1

Log of the observations at Loiano.

thumbnail Fig. 3

WHAN diagnostic diagram for nuclei whose Hαcorr line is detected in emission. The equivalent width of Hα is corrected by 1.3 Å for underlying absorption. Passive nuclei never appear in the figure, because they have either Hαcorr in absorption or undetected [NII]. The 4 SEY1 objects identified by their broad-line systems are not plotted.

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The spectra were reduced using the IRAF-STSDAS3 packages, following reduction procedures identical to Gavazzi et al. (2011) that are not repeated here. In summary, the flux and wavelength-calibrated 1D spectra were extracted in apertures of 5.8 arcsec, normalized to the intensity of the continuum under Hα, transformed to rest-frame velocity using the redshift provided by NED, and measured to obtain the intensity and EW of the Hα and [NII] lines.

The presence of a strong telluric absorption feature near 6866 Å hampers the detection of [SII] lines at redshift near zc = 6000−7000 km s-1. Therefore, these lines are not analyzed4. In the presence of broad permitted lines, the measurements of the EW and FWHM of the broad and narrow emission lines were obtained using the fitting procedure discussed in Sect. 5.

4. Spectral classification

The classification of the nuclear activity based on optical spectra was performed following the methods of Gavazzi et al. (2011) based on the WHAN diagram (Cid Fernandes et al. 2010, 2011). In short, the WHAN classification scheme is based on the strength of the Hαcorr line (Hα, corrected for underlying stellar absorption, Ho et al. 1997) and the ratio between the flux of [NII] and Hαcorr. As in Gavazzi et al. (2011), we adopt a mean underlying stellar absorption at Hα of 1.3 Å, irrespective of luminosity5.

thumbnail Fig. 4

Red restframe-spectra (from 6100 Å to 7300 Å) obtained with the red-channel grism in this work, normalized to the continuum near 6400 Å. The vertical dashed lines give the position of [NII] 6548, Hα (λ6562.8), [NII] 6583, [SII] 6717, [SII] 6731. The first three digits of the label give the year of observation. L00 marks those spectra that are a median of two or more spectra obtained in different runs. Galaxies are listed in order of catalog names. A representative sample of 12 spectra is shown. The full set of 376 spectra is available in electronic form at the CDS.

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thumbnail Fig. 5

Spectra obtained with the blue- and red-channel grisms normalized to the continuum near 6400 Å.

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Figure 3 shows the WHAN diagram with the six adopted spectral classification thresholds. These thresholds (whose detailed description can be obtained from Gavazzi et al. 2011) allow to separate HII region-like nuclei (marked HII in Table 4) from strong AGNs (sAGN), weak AGNs or LINERs (wAGN), and fake AGNs (dubbed as “retired nuclei” or RET whose ionization mechanism is provided by their old stellar population). Passive nuclei (PAS) display no star formation activity. Their spectra show either Hαcorr in absorption or undetected [NII]. Four Seyfert 1 galaxies are classified by visual inspection of the individual spectra for the presence of broad permitted lines (see Sect. 5).

Table 3 gives the parameters of the 376 independently observed galaxies as follows: Column 1: serial number; Column 2: Galaxy name. The first three digits of the label give the year of observation. L00 marks those spectra that are a median of two or more spectra obtained in different runs. Then the catalog name and serial number: Arecibo galaxy catalog (AGC, Haynes et al. 2011), SDSS (DR7, Abazajian et al. 2009); catalog of galaxies and clusters of galaxies (CGCG, Zwicky et al. 1961), Uppsala galaxy catalog (UGC, Nilson 1973), new galaxy catalog (NGC, Dreyer 1888) and index catalog (IC, Dreyer 1908), Virgo cluster catalog (VCC, Binggeli et al. 1985), Markarian (MRK, Markarian 1967), Herschel Reference Survey (HRS, Boselli et al. 2010) designations; Columns 3 and 4: J2000 celestial coordinates; Column 5: heliocentric redshift, as listed by NED; Column 6: redshift independent distance in Mpc, as listed by NED. When this is unavailable, the galactocentric Hubble-flow distance is listed; Column 7: r-band (AB) magnitude from SDSS when available. Otherwise, the total (Vega) magnitude in the Johnson R, V or B band (as specified in parenthesis) from NED is used; Column 8: a cross indicates the availability of a nuclear spectrum in NED; Column 9: a cross indicates the availability of a fiber nuclear spectrum in SDSS.

Table 4 summarizes the classification of spectra as follows: Column 1: serial number; Column 2: Galaxy name; Column 3: number of individual exposures; Column 4: duration of the individual exposures; Column 5: slit aperture in arcsec; Column 6: slit orientation (counterclockwise from N) in the various runs (yy); Column 7: measured EW of λ 6548.1 [NII] line (dubbed [NII]1) (negative EW values represent emission); Column 8: measured EW of the (narrow) Hα line; Column 9: measured EW of λ 6583.6 [NII] line (dubbed [NII]2); Column 10: rms noise of the individual spectra determined between 6400 and 6500 Å; Column 11: six bins nuclear activity classification.

We note that the EW and FWHM of the narrow Hα and of the two [NII] lines in Seyfert 1 galaxies have been computed applying the procedure described in Sect. 5 to consider the underlying broad Hα component. Adding all 160 spectra of HII-like nuclei, we obtain a template spectrum with a high signal-to-noise ratio (2370 at Hα). A ratio of [NII] 6583/ [NII] 6548 = 3.11 is obtained, which is consistent with the theoretical value of 3.0 obtained by Osterbrock & Ferland (2006).

5. Broad line measurements

Four (L09 UGC-1935, L12 MRK-0079, L11 NGC-3758E, and L13 MRK-0841) of the 376 galaxies show the presence of a broad Hα line. To properly estimate the EW and FWHM of these broad lines and of the narrow Hα and [NII] lines, both the broad and narrow components have to be fit at the same time. We perform a very simple minimum χ-square fit, assuming that the underlying continuum follows a power-law profile and that every single broad and narrow line is well described by a single Gaussian profile. As a disclaimer, we note that the FWHM of the broad Hα lines can be slightly affected by a poor treatment of the underlying continuum, which does not include any specific galactic feature and where the very low flux tails of the lines are not fitted as well as possible by assuming two broad Gaussian, a Lorentzian, or a Voigt profile. The EW is almost unaffected by these small approximations. A high precision measurement of the FWHM of the broad Hα is beyond the scope of this analysis.

Table 2

Broad Hα line measurements.

Table 3

General parameters for a representative sample of 20 galaxies.

Table 4

Observed line-parameters for a representative sample of 20 galaxies.

thumbnail Fig. 6

Spectra of the four Seyfert 1 galaxies in the sample. The black lines refer to the original spectra. The red lines show the full fit (continuum + broad + narrow lines), while the blue lines highlight the contribution of the broad Hα component.

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The fitting procedure has a total of nine free parameters: two describe the power-law continuum, four are related to the narrow [NII] and Hα lines, and three describe the broad Hα line. More specifically, these include

  • the normalization and exponent of the power law continuum;

  • the three normalizations of the narrow Hα and [NII] lines and their FWHM, which are assumed to be the same for every narrow line;

  • the normalization, FWHM, and peak frequency of the broad Hα line. We let the peak frequency vary to best fit the line profile, although no significant shifts have been found.

The numerical values of the FWHM and EW obtained for the broad lines are reported in Table 2. For these four Seyfert 1 galaxies, the full fits with the fits of only the broad component are superimposed to the original spectra in Fig. 6.


1

The first year master students of G.G. are invited annualy to participate in some observing runs at the 1.5 m Loiano telescope, which are kindly provided by the Observatory of Bologna (It).

2

Except for 162, all 376 galaxies in our sample have a redshift from NED. This does not necessarily mean that an optical nuclear spectrum is distributed by NED for all of them.

3

IRAF is the Image Analysis and Reduction Facility made available to the astronomical community by the National Optical Astronomy Observatories, which are operated by AURA, Inc., under contract with the US National Science Foundation. STSDAS is distributed by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under NASA contract NAS 5–26555.

4

The same telluric feature would overlap with Hα near zc = 13 900 km s-1, which is larger than the largest redshift in our sample.

5

Because Hα and [NII] are close one another and since we use fluxes normalized to the continuum under Hα, the same correction for stellar absorption is applied to the fluxes as to the EWs.

Acknowledgments

G. Gavazzi thanks his first year master students for their assistance during many observing nights at Loiano. We thank Giorgio Calderone for discussions on the measurements of the broad-line systems. We are grateful to Paolo Franzetti and Alessandro Donati for their contribution to GoldMine, the Galaxy On Line Database (Gavazzi et al. 2003) extensively used in this work (http://goldmine.mib.infn.it). We thank Barry Madore for useful comments on the manuscript. We also thank the coordinator of the TAC, Valentina Zitelli for the generous time allocation. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. The present study made extensive use of the DR7 of SDSS. Funding for the Sloan Digital Sky Survey (SDSS) and SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, the US Department of Energy, the National Aeronautics and Space Administration, the Japanese Monbukagakusho, and the Max Planck Society, and the Higher Education Funding Council for England. The SDSS Web site is http://www.sdss.org/. The SDSS is managed by the Astrophysical Research Consortium (ARC) for the Participating Institutions. The Participating Institutions are the American Museum of Natural History, Astrophysical Institute Potsdam, University of Basel, University of Cambridge, Case Western Reserve University, The University of Chicago, Drexel University, Fermilab, the Institute for Advanced Study, the Japan Participation Group, The Johns Hopkins University, the Joint Institute for Nuclear Astrophysics, the Kavli Institute for Particle Astrophysics and Cosmology, the Korean Scientist Group, the Chinese Academy of Sciences (LAMOST), Los Alamos National Laboratory, the Max-Planck-Institute for Astronomy (MPIA), the Max-Planck-Institute for Astrophysics (MPA), New Mexico State University, Ohio State University, University of Pittsburgh, University of Portsmouth, Princeton University, the United States Naval Observatory, and the University of Washington.

References

All Tables

Table 1

Log of the observations at Loiano.

Table 2

Broad Hα line measurements.

Table 3

General parameters for a representative sample of 20 galaxies.

Table 4

Observed line-parameters for a representative sample of 20 galaxies.

All Figures

thumbnail Fig. 1

Distribution in celestial coordinates of the 376 galaxies for which we obtained a nuclear spectrum. Color coding is as follows: available fiber spectra from SDSS (#86, green); available nuclear spectra from NED (#162, blue); available spectra from both SDSS and NED (#36, cyan); and newly obtained spectra from this work (#164, red).

Open with DEXTER
In the text
thumbnail Fig. 2

Histograms of the absolute magnitudes (r-band and other bands mixed together; top panel) and of the heliocentric recessional velocities (bottom panel) of the 376 studied galaxies.

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In the text
thumbnail Fig. 3

WHAN diagnostic diagram for nuclei whose Hαcorr line is detected in emission. The equivalent width of Hα is corrected by 1.3 Å for underlying absorption. Passive nuclei never appear in the figure, because they have either Hαcorr in absorption or undetected [NII]. The 4 SEY1 objects identified by their broad-line systems are not plotted.

Open with DEXTER
In the text
thumbnail Fig. 4

Red restframe-spectra (from 6100 Å to 7300 Å) obtained with the red-channel grism in this work, normalized to the continuum near 6400 Å. The vertical dashed lines give the position of [NII] 6548, Hα (λ6562.8), [NII] 6583, [SII] 6717, [SII] 6731. The first three digits of the label give the year of observation. L00 marks those spectra that are a median of two or more spectra obtained in different runs. Galaxies are listed in order of catalog names. A representative sample of 12 spectra is shown. The full set of 376 spectra is available in electronic form at the CDS.

Open with DEXTER
In the text
thumbnail Fig. 5

Spectra obtained with the blue- and red-channel grisms normalized to the continuum near 6400 Å.

Open with DEXTER
In the text
thumbnail Fig. 6

Spectra of the four Seyfert 1 galaxies in the sample. The black lines refer to the original spectra. The red lines show the full fit (continuum + broad + narrow lines), while the blue lines highlight the contribution of the broad Hα component.

Open with DEXTER
In the text

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