J/ApJ/794/75 HST-COS UV spectra observations of AGNs (Stevans+, 2014)
HST-COS observations of AGNs.
II. Extended survey of ultraviolet composite spectra from 159 active galactic
nuclei.
Stevans M.L., Shull J.M., Danforth C.W., Tilton E.M.
<Astrophys. J., 794, 75 (2014)>
=2014ApJ...794...75S 2014ApJ...794...75S (SIMBAD/NED BibCode)
ADC_Keywords: Active gal. nuclei ; Galaxies, spectra ; Photometry, ultraviolet ;
Redshifts ; Photometry, HST ; Photometry, hydrogen-line
Keywords: galaxies: active - line: profiles - quasars: emission lines -
ultraviolet: galaxies
Abstract:
The ionizing fluxes from quasars and other active galactic nuclei
(AGNs) are critical for interpreting their emission-line spectra and
for photoionizing and heating the intergalactic medium. Using
far-ultraviolet (FUV) spectra from the Cosmic Origins Spectrograph
(COS) on the Hubble Space Telescope (HST), we directly measure the
rest-frame ionizing continua and emission lines for 159 AGNs at
redshifts 0.001<zAGN<1.476 and construct a composite spectrum
from 475 to 1875 Å. We identify the underlying AGN continuum and
strong extreme ultraviolet (EUV) emission lines from ions of oxygen,
neon, and nitrogen after masking out absorption lines from the
H I Lyα forest, 7 Lyman-limit systems (NHI≥1017.2/cm2)
and 214 partial Lyman-limit systems (14.5<logNHI<17.2). The 159 AGNs
exhibit a wide range of FUV/EUV spectral shapes,
Fv∝ναν_^, typically with -2≤αν≤0
and no discernible continuum edges at 912 Å (H I) or 504 Å (He I).
The composite rest-frame continuum shows a gradual break at
λbr~ 1000 Å, with mean spectral index
αν=-0.83±0.09 in the FUV (1200-2000 Å) steepening to
αν=-1.41±0.15 in the EUV (500-1000 Å). We discuss the
implications of the UV flux turnovers and lack of continuum edges for
the structure of accretion disks, AGN mass inflow rates, and
luminosities relative to Eddington values.
Description:
Here in Paper II, we enlarge our composite spectrum from 22 to
159 AGNs, confirm the validity of our early results, and explore
possible variations of the EUV spectral index with AGN type and
luminosity. Both studies were enabled by high-quality,
moderate-resolution spectra taken with the Cosmic Origins Spectrograph
installed on the HST during the 2009 May servicing mission. The COS
instrument (Green et al. 2012ApJ...744...60G 2012ApJ...744...60G) was designed explicitly for
point-source spectroscopy of faint targets, particularly quasars and
other AGNs used as background sources for absorption-line studies of
the intergalactic medium (IGM), circumgalactic medium (CGM), and
interstellar medium (ISM). Our survey is based on high-quality spectra
of the numerous AGNs used in these projects. Our new survey contains
159 AGNs out to z=1.476 with 16 targets at z>0.90, sufficient to
probe below 600 Å by observing with G130M down to 1135 Å.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 80 159 COS Observations (159 AGN Targets Ordered
by Redshift)
table2.dat 47 221 Lyman-limit Systems and Partial Lyman-limit
Systems
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See also:
J/ApJ/817/111 : HST/COS survey of z<0.9 AGNs. I. (Danforth+, 2016)
Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 24 A24 --- AGN AGN target's name
26- 31 A6 --- Type AGN type
33- 40 F8.6 --- z Redshift
42- 46 F5.2 --- alphal Fitted spectral index (1)
48- 53 F6.4 --- e_alphal Error in alphal
55- 61 F7.4 10-16W/m2/nm F0 Rest-frame flux normalization at 1100Å
(in 10-14 erg/cm2/s/Å)
63- 67 F5.2 10-16W/m2/nm Fl Observed flux at 1300Å
(in 10-14 erg/cm2/s/Å) (2)
69- 73 F5.2 [10-7W] loglLl Band luminosity at 1100Å (in erg/s)
75- 76 I2 --- S/N1 Signal-to-noise ratio at 1250Å and
1550Å for data with
G130M (1132-1460Å) grating
77 A1 --- --- [,]
79- 80 I2 --- S/N2 Signal-to-noise ratio at 1250Å and
1550Å for data with
G160M (1394-1798Å) grating
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Note (1): Corresponds to frequency index αν=-[2+αλ].
Finn et al. (2014MNRAS.440.3317F 2014MNRAS.440.3317F) fitted a harder spectral index,
αλ=-0.64, for J0209-0438 at z=1.131, using additional COS/G230L
spectra extending to longer wavelengths.
Note (2): Flux at 1300Å for SBS 1108+560 is low, owing to LyC absorption
(λ<1334Å) from a LLS at z=0.46335.
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Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 24 A24 --- AGN AGN target's name
26- 32 F7.5 --- z Redshift
34- 38 F5.2 [cm-2] logNHI Log of H I column density
40- 44 F5.3 [cm-2] e_logNHI Error in logNHI
46- 47 I2 km/s b Doppler parameter (1)
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Note (1): Derived by fitting Lyα and higher Lyman series absorbers.
When the Doppler parameter is unconstrained, we used mean b=25 km/s to
propagate uncertainties in the continuum slope of individual spectra.
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History:
From electronic version of the journal
References:
Shull et al., Paper I, 2012ApJ...752..162S 2012ApJ...752..162S
(End) Prepared by Tiphaine Pouvreau [CDS] 27-Apr-2017