Probing Accretion Disk Winds in AGN I. Asymmetric broad Balmer emission lines

55 Pages·2014·7.33 MB·English

Probing Accretion Disk Winds in AGN
I. Asymmetric broad Balmer emission
lines
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1
Cosmos DUMBA
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Department of Physics, Faculty of Science

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e Mbarara University of Science & Technology
S

9 P. O. Box 1410, Mbarara, Uganda
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] September 23, 2014
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The Broad Line Region of Active Galactic Nuclei is characterized by broad
-
o
Balmer emission lines in their optical spectra. The broad Balmer emission lines
r
t are found to be asymmetric, some blue sided and others red sided in their asym-
s
a metry. One of the components behind the asymmetry is thought to be an accretion
[
disk wind. We probe the accretion disk wind using the broad balmer emission line

1
profiles.
v
4
7
1 This asymmetry of the broad balma emission line profiles is measured in velocity
6 space after a measurement of the line shift at percentiles from 0, in increaments of
.
9 10, up to 90. In addition, the Kurtosis Index is obtained at appropriate points of the
0
emission lines’ profiles. This study is based on many hundreds of SDSS spectra,
4
1 starting with low redshift high signal to noise ratio spectra. We also consider a def-
:
v inite number in each bin of their FWHM, in bins of 1000 km/s (atleast 40 per bin),
i
X startingfrom1000km/stotheverybroademissionlines.
r
a
We present how strong the asymmetry (by plotting Asymmetry Index as a function
of percentile) of the broad/narrow lines (in percent) is, what the Kurtosis (R20,80)
is. WealsopresentwhattheAsymmetryIndexasafunctionoflinewidth(FWHM),
luminosity(V-band),core-radiofluxandIonizationDegree.
AGN: Accretion Disk, Accretion Disk Wind – Line: Asymmetry, Balmer, Emission,
Profiles
1
1 Introduction
the nucleus). This limit defines a charac-
teristic luminosity, the Eddington luminosity
Active Galaxies have been widely studied (L )throughtherelations;
Edd
by many authors revealing many fascinating
F = F (5)
properties which among all include; compact grav rad
nuclear emission (Clavel et al. 1990), non-
GM(m +m ) GMm σ L
p e p T Edd
thermalcontinuumemission ≈ = (6)
r2 r2 4πcr2
4πGm c
F ∼ ν−α (1) L = p M (7)
ν Edd
σ
T
(Bregman1990). M
L = 1.26×1031 [W] (8)
Edd
In addition, authors notice that; the con- M
sun
tinuum stretches from the radio to X- TheimportantparameterinAGNis L .
ray/Gamma rays (Mehdipour et al. 2011), LEdd
One of the consequences of high L are
the luminosity of the nucleus exceeds that LEdd
winds,AccretionDiskWinds. JustlikeinSo-
of the host (Osterbrock 1989), they have
larWinds....onceaparticleexceedstheescape
strongemissionlines(DeBreucketal.2000),
velocity, we see this as a wind from the ac-
are highly variable (Ulrich et al. 1997), and
cretion disk. Accretion disk winds have been
also have X-ray emission (Turner & Pounds
confirmed by some authors by studying BAL
1989).
Quasars(Hamann1998). Hamann(1998)ob-
The energy source in the AGN is believed
served strong absorption trouphs in the rest-
to be accretion (Blandford & Znajek 1977).
frame UV spectrum and estimated outflows
Thiscanbedemonstratedthroughtherelation
with10,000km/s.
:
In this work, we study the accretion disk
E = ηmc2 (2)
whereη istheefficiency.
Theluminositycanthenbere-writtenas:
dE dM
L = = η c2 = ηM˙ c2 (3)
dt dt
whereM˙ = dM istheaccretionrate.
dt
ForatypicalAGN;
L L
M˙ = ≈ 1.8×10−3 37M yr−1 (4)
ηc2 η sun
Figure1:The rest frame of the UV spectrum
where L37 is the luminosity in units of 1037 ofaBALQuasarshowingasignifi-
W. cantoutflowintheabsorptionlines
This energy, at one point, through the princi- ofupto10,000km/s
pleofhydrostaticequilibrium,reachesapoint Hamann(1998)
at which gravitational forces (causing the ac-
cretion) balance with radiative forces (from winds by analysing broad Balmer line profile
2
asymmetryandsteepnessvalues. of obtaining quality spectra in all ranges of
We define the two parameters in line shape FWHM from 1000 km/s to above 10,000
analysisas: Km/s. This is so because as we are selecting
thespectrawiththehighestsignaltonoisera-
S
A.I = (9) tio, there is a tendency to obtain only spectra
IPV
thatarebothfromsourcesnear(lowredshift),
IPV thusneglectingthosewithhigherredshift,and
U
K.I = (10)
IPV also biasing the data primarily on signal to
L
noise ratio. This is avoided when we split the
where S is the line shift, IPV is the inter-
sampleintwosamples,oneforthosebetween
percentile velocity, IPV and IPV are the
U L
1000 km/s FWHM and 3000 km/s FWHM,
interpercentile velocities for the upper and
and another for those having FWHM greater
lower parts of the emission line profile. The
than 3000 km/s. We end up having spectra
following sections will outline what we did
with very high signal to noise ratio across
inmoredetailandexplainthemostimportant
the whole spectrum of broad Balmer emis-
stepswecarriedoutduringthestudy.
sionlines. RememberbroadBalmeremission
linesarethosewithFWHMgreaterthan1000
2 Data km/s.
In this section, we explain how we download
2.1 Introduction to Data the data, how we treat the data, describing
the software we use and all the tasks used.
Analysis
We also explain all the steps we carry out,
In order to study any sample of galaxies and giving details of the output in each step and
quasars,itisimportantnottoforgetafewuse- why it is carried out. In addition we show
ful conditions that make the sample produce samplesofthevaluesextractedfromtheplots
reliableinformation: (thewholelistbeingfoundintheappendices).
Lastlywedescribethesecondarytreatmentof
• thesizeofthesampleand
the extracted data from the plots, explaining
how and why we carried out the treatment in
• the quality of the spectra, both in terms
such ways and show samples of the results
of resolution and Signal to Noise ra-
in tables (the actual results being shown in
tio.(Whittle1985a)
the next chapter). But then, it is necessary to
Our data meets both criteria thanks to dedi- first briefly describe the database (SDSS) and
cated surveys like the SDSS that have made thereafter the telescope and the surveys it has
such homogeneous data sets available to the carried out so that one gets a feeling of the
public. whole process from observing, collection of
Wehaveobtained,ineachsample,thetop600 data,treatmentofdataandanalysis.
highsignaltonoisegalaxiesandquasarsfrom
the seventh data release (DR7) (York et al.
2.1.1 SDSS Database
2000). The samples have the highest spectro-
scopic quality in the release (Abazajian et al. The Sloan Digital Sky Survey consists of
2009),andareuniformintermsofcalibration three major surveys that together provide sci-
letalonebeingcomplete. entists with immense volumes of data ob-
We have two samples for the same reasons tained from a dedicated 2.5m telescope lo-
3
cated at Apache Point Observatory in South- Encodedwithinthespectraldataarethecom-
ern New Mexico. This survey has been col- position and temperature of these stars, vi-
lecting data since the year 2000 (Abazajian tal clues for determining the age and origin
et al. 2009; York et al. 2000). The three sur- of different populations of stars within the
veysare; Galaxy(Yannetal.2009).
• Legacy
2.1.4 The SDSS Supernova Survey
• SEGUE
The SDSS Supernova Survey was one of
threecomponents(alongwiththeLegacyand
• Supernova
SEGUE surveys) of SDSS-II, a 3-year ex-
tension of the original SDSS that operated
2.1.2 SDSS Legacy Survey
from July 2005 to July 2008. The Super-
The SDSS Legacy Survey provided a uni- nova Survey was a time-domain survey, in-
form, well-calibrated map in ugriz of more volving repeat imaging of the same region
than7,500squaredegreesoftheNorthGalac- of sky every other night, weather permitting.
tic Cap, and three stripes in the South Galac- The primary scientific motivation was to de-
tic Cap totaling 740 square degrees. The cen- tectandmeasurelightcurvesforseveralhun-
tral stripe in the South Galactic Gap, Stripe dred supernovae through repeat scans of the
82, was scanned multiple times to enable a SDSS Southern equatorial stripe 82 (about
deep co-addition of the data and to enable 2.5◦ wide by 120◦ long) (Frieman et al.
discovery of variable objects. Legacy data 2008). Theabovethreesurveyshaveprovided
supported studies ranging from asteroids and scientists with a catalog derived from the im-
nearby stars to the large-scale structure of the ages obtained by the 2.5m telescope. These
universe. Almost all of these data were ob- images include more than 350 million celes-
tainedinSDSS-I,butasmallpartofthefoot- tial objects, and spectra of 930,000 galaxies,
printwasfinishedinSDSS-II. 120,000quasars,and460,000stars. Thisdata
is not only fully calibrated and reduced, care-
fully checked for quality, and publicly acces-
2.1.3 SEGUE - Sloan Extension for
sible through efficient databases, but has also
Galactic Understanding and
been been publicly released in a series of an-
Exploration
nual data releases. It is through this effort
SEGUE was designed to explore the struc- that we are able to carry out this study on the
ture; formation history; kinematics; dynam- asymmetry of the broad emission lines of ac-
ical evolution; chemical evolution; dark mat- tivegalacticnuclei.
terdistributionoftheMilkyWay. Theimages In this Data release, this study exploits the
and spectra obtained by SEGUE allowed as- immense volume of spectra of galaxies and
tronomers to map the positions and velocities quasars (Abazajian et al. 2009; York et al.
of hundreds of thousands of stars, from faint, 2000). The sample obtained from the release
relativelynear-by(withinabout100parsecor includes around 600 spectra of both galaxies
roughly 300 light-years) ancient stellar em- and quasars in the redshift range of 0 and 1,
bers known as white dwarfs to bright stellar withbroadHβ linesfrom 1500km/senabling
giants located in the outer reaches of the stel- us contain all groups of broad line emission
lar halo, more than 100,000 light-years away. objects.SDSS(2013)
4
Figure2:The2.5mtelescopelocatedatApachePointObservatoryinSouthernNewMexico
BBC(2013)
2.1.5 Obtaining the Data Inaddition,thisdataconsistsofaround283
spectra with measured Hβ profiles around
In order to obtain data from the SDSS
165 measured Hα profiles. This is because
database with your own constraints on the
not all spectra could have both the Hβ emis-
sample,oneneedstowriteanSQLQuerythat
sion line and Hα emission line due to red-
generates a list of the sources that meets your
shifting at both ends of the optical window.
requests.
However, most of the spectra with Hβ lines
In this study, we focus on the parameters
also have Hα lines but not vice versa. Of
of the Hα & Hβ emission line, looking for
course spectra that are at the extreme end of
asymmetryintheselines.
our redshift selection are the ones affected
with not having the Hα line visible, but this
2.1.6 Data Properties and Constraints is not an issue for us to worry about since we
Invoked weremoreinterestedintheHβ lineprofiles.
Figures 3 show the distribution of the sig-
The data in our samples combined consists
nal to noise ratio of our sample and the red-
of around 300 objects, galaxies and quasars,
shift. It is seen here in the signal to noise
restricted to a redshift range between 0 and
distribution that we have a fairly good sig-
1. The samples consist of reduced spectra of
nal to noise ratio, the minimum being 37 and
these objects in fits files that we renamed in
maximum 79. The distribution is binned in 5
ascending order from those with the highest
starting from 35. The figure therefore clearly
signal to noise ratio. This helps us analyze
shows that most of our sample spectra have
those with the highest signal to noise ratio
a signal to noise ratio between 40 and 45, to
first. Thisisimportantbecausetheresultsob-
bespecificaround120outof300(40%),with
tained from high signal to noise ratio sources
a few having more than that. This is not a
are more reliable for the derivation empiri-
problem since, given the nature of our study,
cal relations. The redshift limit was chosen
asignaltonoiseratioofeven30wouldbesuf-
so that we have a good coverage of the Hβ
ficient for good measurements (Thorne et al.
line and its adjacent spectral regions. Figure
1999).
presents the general properties of the sample
in terms of redshift, signal to noise ratio and In the same way, the redshift distribution
thebroadeningoftheHβline. is shown. Because we chose spectra with the
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Figure3:SignaltoNoiseratio&RedshiftdistributionofSample
highest signal to noise ratio, it seemed obvi- 9000 km/s, there isn’t any increase in counts
ous that most of the sample spectra will be as there are very few AGN with such ex-
in the near end of the redshift window we tremely broad lines. Our study will obtain in-
chose with those below redshift 0.2 domi- formationaboutthebroadlinesbasingalmost
nating. However, since the distribution does entirelyonthefirstsixorsevenbinswithsuf-
not fall rapidly as we move to higher red- ficientnumbers.
shift, the data will provide a sufficient study The distribution of the FWHM in our sam-
of AGN within this redshift bin chosen. In ple is in agreement with the AGN statistics
a later study, it would be good to also study obtainedbyHaoetal.(2005)whentheyplot-
other redshift ranges, possibly using Ultravi- ted the distribution of the FWHM values of
olet emission lines which can be obtained in the Hα emission line for over 40,000 emis-
the optical spectra after they have been red sion line galaxies. Their distribution was bi-
shifted. modal since they included all AGN, narrow
Figures 4 show the distribution of the and broad line AGN. Their boarder line of
FWHM of all the Hβ emission lines mea- broadlineAGNwasnaturallyplacedatthose
sured. The distribution shows that there is with a FWHM of over 1200 km/s. To com-
a peak between 3000 km/s and 4000 km/s. pare with our distribution, we only consider
But still it is not a big range from the bins thoseover1000km/s,anditgivesusthesame
at both ends which are at 50 each. Our inten- distribution. Intheirstudy,definingbroadline
tion is to have a good distribution across the AGN as objects with a FWHM greater than
whole range from 1000 km/s to over 10,000 1200 km/s, they obtained 1,317 objects out
km/s, and since we obtain enough counts for of 42,435 emission line galaxies. This makes
the first five bins, then the sample will pro- our sample of 300 objects not bad since it is
vide a statistically sufficient analysis for our a quarter of this value, let alone having been
findings. restrictedtoaredshiftbetween0and1.
The other figure here is a cumulative dis- Following this simple look at the gen-
tribution, which shows that from FWHM of eral properties of the downloaded data, it is
6
Figure4:FullWidthatHalfMaximumdistributionofSample
now necessary to explain the treatment of the information i needed from the broad Balmer
data in order to obtain measurements that are lines in my sample. This was principally the
needed for our later analysis. The detailed partofthethesisthatoccupiedmemostsince
spectral properties of the sample will follow itinvolvedcarefulvisualanalysisofthespec-
laterintheforthcomingchapters. trum first and accurate execution of the re-
quired commands for which a mistake with
oneofthemmeansredoingthewholeprocess
2.2 Image Plotting and
from the beginning. Explained below are the
Analysis with IRAF
stepsitookduringmydataextraction.
IRAF (an acronym for Image Reduction and
Analysis Facility) is a collection of software 2.2.1 Spectrum Visual Analysis
written at the National Optical Astronomy
After downloading the data, (the fits files),
Observatory (NOAO) geared towards the re-
and having renamed them, starting from the
duction of astronomical images in pixel array
one having the highest signal to noise ra-
form. In this thesis, i used IRAF to plot spec-
tio, i plotted each spectrum and analyzed the
trafromoursampleAGNandanalyzedafew
Balmer emission lines. The reason why i
spectral lines needed to derive more infor-
needed to do this is because each spectrum is
mation for analysis later (Tody 1986; Valdes
unique and the tasks to perform on each var-
1986). To be specific, we plotted the Balmer
ied from one spectrum to another. Some of
lines, Hα and Hβ emission lines, although
thethingsilookedoutforwere;
not all spectra contained both emission lines.
Most of them had the Hβ emission line, a
• The availability of both Hβ and Hα
good number had both Hβ and Hα emission
emissionlines.
line, while very few(∼2%) had only the Hα
emissionline. • Iron emission around the Hβ emission
In the following subsections, i will explain line,theFeEmission.
brieflythestepsandtasksiusedtoextractthe This was important to note because a lot
7
of Fe emission could provide a poor es- 1000 km/s and 3000 km/s, and sample 2 be-
timate for the continuum measurements. ingthatofAGNwithFWHMfrom3000km/s
Iftheemissionwasonlyoneithersideof and above, this is what we observed from the
theBalmerline,iwouldusetheendwith visualanalysis;
lessemissionasmyproxyforthecontin-
uum. Incaseswheretherewastoomuch • TheAGNinsample1havebothHαand
Fe emission, i excluded such a spectrum Hβ,meaningtheyarefoundinthelower
from my sample since it meant an extra part of the redshift range from 0 to 1,
process of using a template to subtract preferably less than z = 0.6. There are
theFeemissionfirst. less than 10 out of 81 that had only the
Hβ emissionline.
• Availability of neighboring narrow lines
([OIII]forHβ &[SII]forHα). • Sample 2 has at least 40% of the AGN
having both Hα and Hβ. This means
Thiswasforuseinestimatinganequiva-
60% ofthem are foundin the higherend
lent FWHM of the narrow component to
of the redshift bin, preferably above z =
subtract from the Balmer line. However,
for the case of the Hα emission line, 0.7. Recallthatmyredshiftrangeisfrom
z=0toz=1.
the [SII] emission doublet was not accu-
rate enough because the two lines were
blended together in most cases. It meant
2.2.2 Continuum Subtraction
usingthe[NII]linesthatwerewithinthe
broadened Hα emission. This meant a This is the task done after visual analysis of
visualestimationonceboth[SII]&[NII] the spectrum. Spectra with both the Hα and
lines were unavailable. On a positive Hβ emission lines look similar to the ones in
note, there were very few cases where i figure 5 although some with significant red-
couldnotuseanyofthetwo. shiftwillonlyhavetheHβ emissionlinestill
available. The subtraction of the continuum
• Thefunctiontousewhilesubtractingthe isdoneinsplot,withthekeys”t”followedby
continuum. a ”-”, followed with the appropriate function
This was important because the unique- necessaryfortheselectedregion. Thecontin-
ness of each spectrum, and the quest to uum is subtracted for each emission line sep-
apply a good estimate to a flat spectrum, arately and separate images are saved out of
entails using different functions for each theoriginalimagehavingbothemissionlines.
spectrum in order to obtain a flat spec- Thiscanbeseeninthesampleplotsinfigure6
trum from the power law spectra down- on page 9. Continuum subtraction is the sec-
loaded. ond step to spectrum analysis for the Balmer
emission line features and it sets a vantage
All the above mentioned reasons vary from point for line normalization, as we shall dis-
spectrumtospectrumandeachofthemisim- cussinthenextsubsection.
portant in order for me to obtain the best re-
sults from the spectrum, and for minimizing
2.2.3 Line Normalization
errorsasmuchasican.
In our data, which has two samples, sample To normalize the emission line simply means
1 being that of AGN with FWHM between to have its base at a zero point and its peak
8
Figure5:Seenabovearetwosamplesofthespectraduringvisualanalysis. Itshouldbenoted
thatnotallspectracontaintheHαandHβ emissionlinesasseenhere,reasonbeing
that those red-shifted significantly will eliminate the Hα since it will be Doppler
shifted to wavelengths outside the optical range. In addition, not all spectra are
relativelyflatliketheonesshownduetothevaryingamountsofcontinuumemission
atdifferentwavelengthsformanyAGN
Figure6:The images show an example of extracted Balmer emission lines after continuum
subtraction on each of them. One should note here that all the extracted emission
lines have their continuum at zero, making it the starting point of the percentile
rangesweneedexaminelater.
9
at unity. It should be noted that we are nor- threat to any measurements needed from the
malizing the broad Balmer lines. This means Hα emission line. This can be seen in the
thatweeithersubtractthenarrowcomponents examplesbelow.
or simply leave them visible, but ignore their
additional height as we simply place a unity However, for the Hβ emission line, there
valueatthepeakofthebroadline. is an undeniable problem. The neighboring
Inthefigure7,examplesonthediversityof emission lines are significant enough to pose
broad and narrow components in our sample athreattomeasurementstakenfromit. These
with some not having narrow components at emission lines are; the Oxygen III lines and
all and others dominated by the narrow com- the Iron emission at both sides of the Hβ
ponentareshown. emission line. The Iron emission will in-
To perform the normalization of the broad crease on the error in the continuum subtrac-
Balmer emission lines, we use the arithmetic tion since it creates a pseudo continuum and
task,”imarith”,whichcanbeusedanytimein thus making it difficult to estimate a contin-
IRAF. However, after continuum subtraction, uumlevel. Toreduceontheerrorinthismea-
thecountsatthepeakofthebroadBalmerline surement, one has to subtract the iron emis-
arerecordedinatable. Alsointhesametable sion, preferably using some already devel-
isrecordedthevelocityatwhichtheemission oped templates before the actual continuum
line is centered. This velocity will be of use levelcanbeestimated.
while transforming to velocity space, center- In our case, since it was a small fraction
ingtheemissionlinepeakatthatvelocity. of spectra that had significant iron emission
Table 1 shows part of the table. The whole on both sides, we neglected this since it af-
tablecanbeviewedintheappendicessection. fects less than 5% of the data. The Oxy-
It shows the Spectrum Number, the Balmer gen III emission lines also increase on the
line heights used to normalize the profile to uncertainty of the FWHM values measured
unity and the corresponding wavelengths at since they are embedded in the broad Balmer
which they are identified. These wavelengths Hβ emission lines. The most reliable way to
are the wavelengths at which we center the deal with this uncertainty is to subtract them
correspondinglineswhiletransformingtove- before any values are measured. However,
locityspace. since they are also broadened at their bases,
their subtraction, using a Gaussian model, in-
creases absorption features in the residue Hβ
2.2.4 Line Correction : Filtering out
emission line left. Thus, since the emission
other lines and Smoothening
lines are already broad, we decided to simply
out the noise
cutthemoutinordertoleaveasmootherpro-
Inanyspectrum,thereisalwaysaprobability file for the residue for better measurements
that the emission line you are interested in is without introducing a smoothing factor. It is
surrounded by other emission or absorption noticed that subtracting the OIII lines intro-
lines. For some emission lines, the neigh- duces absorption features which in turn adds
boring emission lines or absorption lines are anuncertaintytothemeasurements,sosimply
not a threat to any measurements pertaining cutting them off and preventing this uncer-
the emission line under study, for example, tainty seemed better because the uncertainty
for the Hα emission line, the neighboring we deal with by not doing the subtraction it-
Sulphur II emission lines do not pose a selfismuchless.
10