Active Galactic Nuclei from HeII: a more complete census of AGN in SDSS galaxies yields a new population of low-luminosity AGN in highly star-forming galaxies
Rudolf Bär, Anna K. Weigel, Lia F. Sartori, Kyuseok Oh, Michael Koss, Kevin Schawinski
MMNRAS , 1–10 (2016) Preprint 16 December 2016 Compiled using MNRAS L A TEX style file v3.0
Active Galactic Nuclei from He II : a more complete census of AGNin SDSS galaxies yields a new population of low-luminosity AGN inhighly star-forming galaxies Rudolf E. B¨ar (cid:63) , Anna K. Weigel , Lia F. Sartori , Kyuseok Oh , Michael Koss † and Kevin Schawinski Institute for Astronomy, Department of Physics, ETH Zurich, Wolfgang-Pauli-Strasse 27, CH-8093 Zurich, Switzerland
ABSTRACT
In order to perform a more complete census of active galactic nuclei (AGN) in the local Uni-verse, we investigate the use of the He II λ emission line diagnostic diagram by Shirazi &Brinchmann (2012) in addition to the standard methods based on other optical emission lines.The He II based diagnostics is more sensitive to AGN ionization in the presence of strongstar formation than conventional line diagnostics. We survey a magnitude-limited sample of63,915 galaxies from the Sloan Digital Sky Survey Data Release 7 at . < z < . anduse both the conventional BPT emission line diagnostic diagrams, as well as the He II diagramto identify AGN. In this sample, 1,075 galaxies are selected as AGN using the BPT diagram,while an additional 234 galaxies are identified as AGN using the He II diagnostic diagram,representing a 22% increase of AGN in the parent galaxy sample. We explore the host galaxyproperties of these new He II selected AGN candidates and find that they are most common instar-forming galaxies on the blue cloud and on the main sequence where ionization from star-formation is most likely to mask AGN emission in the BPT lines. We note in particular a highHe II AGN fraction in galaxies above the high-mass end of the main sequence where quench-ing is expected to occur. We use archival Chandra observations to confirm the AGN nature ofcandidates selected through He II based diagnostic.Finally, we discuss how this technique canhelp inform galaxy/black hole co-evolution scenarios. Key words: galaxies: active; galaxies: nuclei; galaxies: Seyfert; galaxies: evolution; galaxies:star formation
Black holes are recognized as an integral part of galaxy evolution(Sanders et al. 1988; Silk & Rees 1998; Springel et al. 2005; Hop-kins et al. 2008a,b; Mancini et al. 2015). The interaction betweenblack holes and their host galaxies is highly complex. Many dif-ferent models providing possible physical explanations have beendeveloped; several recent reviews describe in detail the different ap-proaches explaining the links between the star formation and evo-lution of galaxies with their black holes (Fabian 2012; Treister &Urry 2012; Kormendy & Ho 2013; Heckman & Best 2014; Brandt& Alexander 2015, and references therein). Over the last 10 yearsa large number of publications have addressed possible physicalcauses (Peng et al. 2010, 2012; Schawinski et al. 2014a), such asAGN feedback (Salim et al. 2007; Kauffmann et al. 2007; Geor-gakakis et al. 2008; Hickox et al. 2009; Mancini et al. 2015; Bon-giorno et al. 2016), major mergers (Sanders et al. 1988; Mihos &Hernquist 1996; Hopkins et al. 2006), environmental effects (Woo (cid:63)
E-mail: [email protected] † Ambizione fellow et al. 2015; Knobel et al. 2015; Peng et al. 2015) and secular pro-cesses (Kormendy & Kennicutt 2004; Masters et al. 2011; Cheunget al. 2013), for the quenching of star formation in galaxies. Allthese processes could explain the bi-modality in colour-mass andcolour-magnitude space (Bell et al. 2003; Baldry et al. 2004; Faberet al. 2007; Martin et al. 2007; Schawinski et al. 2014b; Taylor et al.2015).Observations of AGN in different wavebands lead to different,sometimes contradictory results. According to current models AGNoperate in different modes and the various components of AGN arethe source of different types of emissions (Elvis et al. 1994; Peter-son et al. 1997; Krolik 1999; Osterbrock & Ferland 2006; Netzer2013; Yuan & Narayan 2014; Heckman & Best 2014). Dependingon their luminosity and the star formation rate of the host galaxythe emissions from AGN can be contaminated by stellar emissionsfrom the host galaxies. As Stern et al. (2012) pointed out, mostsurveys for AGN are severely biased towards unobscured (type 1)AGN. The most promising techniques for a complete census of type2 AGN are radio, hard X-ray and infrared selection, but only 10%of AGN are radio loud (Stern et al. 2012). X-ray observations are c (cid:13) a r X i v : . [ a s t r o - ph . GA ] D ec Rudolf E. B¨ar et al. less affected by dust absorption and mid infra red observations al-lows the detection of both type 1 and type 2 AGN (Stern et al. 2005,2012). Nuclear dust and gas can obscure direct emission from AGNmaking the selection of obscured and heavily obscured AGN morechallenging (Urry & Padovani 1995; Draine 2003a,b; Koss et al.2011; Treister & Urry 2012). The most promising techniques for acomplete census of type 2 AGN are narrow emission lines, radio,hard X-ray and mid infrared selection, but only 10% of AGN are ra-dio loud (Stern et al. 2012). X-ray observations are less affected bydust absorption and mid infrared observations allow the detectionof both type 1 and type 2 AGN (Stern et al. 2005, 2012). Narrowemission line diagnostic diagrams are thus a widely used methodfor selecting Type 2 AGN (Kauffmann et al. 2003; Kewley et al.2006a; Schawinski et al. 2007; Juneau et al. 2014).The analysis of galaxy colours can be extremely difficult be-cause the light of the galaxies can be contaminated by the dust,gas, and star formation (Simmons & Urry 2008). At low redshift(z < . ) this can be overcome because the central sources in thein galaxies can be resolved with the Hubble Space Telescope orground based optical images (Schawinski et al. 2009). The correctcolour determination is critical as the present models are based ona galaxy evolution starting in the star forming blue cloud, progress-ing through the intermediate zone, the green valley, to the inactivegalaxies of red sequence. As discussed in Schawinski et al. (2009)the different models also constrain the possible lifetimes of AGNand the timing of the quenching of the star formation. In particular,if AGN phases occur solely in the green valley, then they cannotbe the cause of quenching as they appear at least several hundredMyr after the quenching event. Finding AGN populations in stillblue, star-forming galaxies is therefore vital for testing AGN-drivenquenching scenarios (Schawinski et al. 2009).In the present work, we concentrate on the standard emis-sion line methods which are based on the [N II ]/H α and [O III ]/H β ˙ratios (Baldwin et al. 1981; Veilleux & Osterbrock 1987; Kew-ley et al. 2001, 2006b, 2013; Kauffmann et al. 2003; Groveset al. 2004a,b; Schawinski et al. 2007; Stasi´nska et al. 2006, 2008;Juneau et al. 2014). We show that using the He II ˙/ H β ˙ratio (Shirazi& Brinchmann 2012) we can increase the number of AGN candi-dates identified and we analyze this group of additional AGN candi-dates in detail. We show that this may lead to a better understandingof the role of AGN in quenching the star formation.This paper is organized as follows: In section 2 we describeunder section 2.1 the sample selection and then introduce in sec-tion 2.2 the two emission line methods used for the analysis; wedescribe the standard BPT diagnostic diagram and the He II baseddiagnostic. We combine the two methods and discuss the charac-teristics of the He II selected AGN candidates. We discuss in sec-tion 2.3 the demographics of the classical standard BPT and theHe II selected AGN and the host galaxy properties in section 2.4.In section 2.5 we analyse the He II line luminosity. To further con-firm the nature of the He II selected AGN candidates we search forcounterparts in the X-rays (section 2.6). In section 3 we discussour findings and their possible implications for the star formationquenching scenarios. We present the summary in section 4.Throughout this paper, we use a standard Λ CDM Cosmologyconsistent with observational measurements (Komatsu et al. 2011).All magnitudes are in the AB system.
We base our sample on the seventh data release (DR7) of the SloanDigital Sky Survey (SDSS; York et al. 2000; Abazajian et al. 2009).We cross match the New York Value-Added Galaxy Catalog (NYUVAGC, Blanton et al. 2005; Padmanabhan et al. 2008) with the MaxPlanck Institute for Astrophysics John Hopkins University (MPAJHU, Kauffmann et al. 2003; Brinchmann et al. 2004) catalogue toobtain properties such as stellar masses and star formation rates. Weused nebular emission line strengths provided by the OSSY catalogOh et al. (2011). This yields a total of 605,019 objects. In orderto obtain a magnitude limited sample we limit the redshift intervalto 0.02 < z < < -19.0. The remaining sample con-sists of 63,915 galaxies. For the BPT diagram we further restrictour sample to the presence of the [OIII], [NII], H α , and H β lineswith S/N > α , and H β lines with S/N > In this section we use emission line diagnostic diagrams to clas-sify the dominant source of ionization in our galaxy sample, withthe goal of performing a more complete census of galaxies host-ing AGN. The classic emission-line diagnostic, also known as BPTdiagram (Baldwin et al. 1981; Veilleux & Osterbrock 1987; Kew-ley et al. 2001; Kauffmann et al. 2003) based on the [O
III ]/H β vs[N II ]/H α ratio (with [S II ]/H α and [O I ]/H α as variations) is com-monly used. The key point of this paper is to apply a newer di-agnostic diagram where [O III ] λ is replaced by He II , sinceHe II offers a cleaner AGN/star formation separation as proposedand developed by Shirazi & Brinchmann (2012). As a standard tool, the BPT diagram is widely used to classifygalaxies and in particular to identify AGN. It is however not with-out biases, as the total emission line ratio of a galaxy is driven bythe balance of star formation and AGN ionization. This can leadto bias against AGN in highly star-forming galaxies (e.g. Maiolinoet al. 2003; Schawinski et al. 2010; Sartori et al. 2015; Trump et al.2015; Jones et al. 2016).We use the standard BPT diagram ([O
III ]/H β vs. [N II ]/H α )diagnostic diagram. We identify the population of AGN selected bythe standard BPT diagram as shown in the top left panel of Figure 1;S/N > MNRAS000
III ]/H β vs. [N II ]/H α )diagnostic diagram. We identify the population of AGN selected bythe standard BPT diagram as shown in the top left panel of Figure 1;S/N > MNRAS000 , 1–10 (2016)
GN from HeII log[NII] λ / H α l o g [ O III ] λ / H β Ka03 Ke01SO7 log [NII] λ /H α l o g [ H e II ] λ / H β log [NII] λ /H α l o g [ H e II ] λ / H β log[NII] λ / H α l o g [ O III ] λ / H β Ka03 Ke01SO7
Figure 1.
The emission line diagnostic diagrams used in this paper. In the top row, we show the classic BPT diagram ([O
III ]/H β vs. [N II ]/H α ) ( left ) and theHe II diagram ( right ). AGN candidates selected on the BPT diagram are shown as blue diamonds, while AGN candidates selected on the He II diagram areshown as red points. The grey shading represents the total population of AGN candidates. In the bottom row, we show how the respective AGN selectionscorrespond to each other: in the bottom-left , we show the He II diagram with the BPT-selected AGN candidates shown as blue diamonds. 95 % of the BPT-selected AGN candidates are also selected as AGN candidates using the He II diagram. In the bottom-right , we show the BPT diagram with the He II -selectedAGN candidates as red points. While some He II AGN candidates lie in the Seyfert region of the BPT diagram, many do not and scatter across the transitionregion and into the purely star-forming locus. II λ emission line diagnostic diagram Schawinski et al. (2010) address the weakness of the BPT diagramin missing low luminosity AGN candidates in star forming galaxies.AGN photoionization can be overwhelmed by star formation, andthe ionizing radiation produced my metal poor stars can produceline ratios similar to AGN. The ionizing energy of 54.4 eV for He + is much higher than the ionizing energy of O ++ (35.2 eV) and N + (15.5 eV) used in the BPT diagram. He II is also less affected byany gas phase metallicity dependence. This makes it possible toidentify AGN candidates using the He II diagram which cannot beidentified using the standard BPT diagram. However, the use of theHeII line for diagnostic purposes can be challenging as the HeIIline is usually weak.We therefore use the He II / H α vs [N II ] / H β diagnostic di-agram following Shirazi & Brinchmann (2012). In the top rightpanel of Figure 1 we show the He II diagram for our galaxy sample.Shirazi & Brinchmann (2012) showed that above the black dottedline more than 10% of the He II flux and that above the solid blackline more than 50 % of He II flux originates from AGN candidates. This solid line is therefore used as the dividing line for the selectionof He II AGN candidates. Using the He II diagnostic we selected559 AGN shown as red points. In the following text will refer tothe AGN candidate selected with HeII diagnostic as HeII AGN. We combine the BPT and He II method results in the two lowerpanels of Figure 1. In the lower left panel we show the position ofthe BPT AGN candidates on the He II diagram as blue diamonds.95% of the BPT AGN are above the dividing line for AGN candi-dates selected also as AGN using the He II diagnostic.In Figure 1 we also plot in the bottom right panel the He II AGN candidates as red circles on the standard BPT [OIII] vs [NII]diagram. Their positions extend well into the area below the Ke01and Ka03 line (Kewley et al. 2001; Kauffmann et al. 2003) respec-tively, which puts them into the star forming region. If the He II AGN candidates are bona-fide AGN, then their location in the star-
MNRAS , 1–10 (2016)
Rudolf E. B¨ar et al.
SDSS ID RA DEC z MASS COLOUR BPT AGN HeII AGN HeII BPT AGN HeIIonly BAD588848899929014499 230.10231 -0.2252278 0.035 9.64 1.86 1. 0. 0. 0. 0.587722982835749016 217.36247 -0.20893236 0.028 10.12 1.69 0. 1. 0. 1. 0.
Table 1.
We provide this table of all the BPT selected AGN candidates and the additional AGN candidates selected by the He II method in electronic form.We give the SDSS ID, RA, DEC, z, Mass and Colour for all AGN candidates. The respective categories BPT selected AGN candidates, HeII selected AGNcandidates, the AGN candidates selected by both methods and HeII-only AGN candidates have a flag 1. We visually inspected all HeII spectra and set a flag 1in the ”bad” column for those which we considered unreliable. BPT only HeII onlyHeII & BPT1075 BPT selected AGN 559 HeII selected AGN234 HeII only325 HeII BPT
Figure 2.
With the Venn diagram we show the relationship between the BPTand He II AGN candidates selections. From our sample of 63’915 galaxiesthe BPT diagnostic identified 1’075 AGN candidates and the He II diagnos-tic, according to Shirazi & Brinchmann (2012), identified 559 AGN candi-dates. 325 AGN candidates are found by both methods. The He II methodidentifies 234 AGN candidates with are not detected by the BPT method,which represents an increase of 22% to the number of AGN candidatesidentified by the BPT diagram. forming region of the BPT diagram can be explained by strong starformation in the host galaxies overwhelming the AGN signal.We provide a complete list of the objects selected by bothmethods in Table 1. II -selected AGN In order to summarize the analysis performed in Figure 1 wepresent the results in a Venn diagram in Figure 2. We can distin-guish four different groups of AGN candidates:(i) 1,075 galaxies identified as AGN by the standard BPT diag-nostic diagram only (hereafter BPT AGN)(ii) 559 galaxies identified as AGN by the HeII diagnostic dia-gram (hereafter HeII AGN)(iii) 325 galaxies identified as AGN by both diagrams (hereafterHeII BPT AGN)(iv) 234 galaxies identified as AGN candidates by the He II di-agnostic diagram only (hereafter He II -only AGN)As we show here, there exists a group of AGN candidates inthe He II selected sample which the BPT method misses. Before wemade the detailed analysis we visually inspected the He II region of their SDSS spectra and eliminated 9 spectra where we deemed thefits to the He II line to be unreliable following Oh et al. (2015).From the total sample of 63,915 galaxies we identify with theBPT method 1,075 AGN candidates and with the He II method 559AGN candidates. Of these 559 AGN 325 overlap with BPT selectedAGN candidates; this means that they are selected by both methods.However the He II method selected 234 He II -only AGN candidatesnot selected by the the BPT method. This corresponds to an in-crease in the detection rate of 22 % compared to the 1,075 AGNcandidates identified by the BPT method (see Figure 2). To analyze the characteristics of the different groups of AGN candi-dates we plot the galaxy colour-mass diagram in Figure 3. We showboth the distribution of AGN host galaxies from the two selectionmethods and the implied AGN fraction as a function of colour andmass. We recover the well-known result that AGN host galaxiestend to lie in the optical green valley both in terms of distribution,and more strongly in terms of AGN fraction. (Nandra et al. 2007;Salim et al. 2007; Schawinski et al. 2007, 2009, 2010, 2014a; Sil-verman et al. 2008; Hickox et al. 2009).Both the standard BPT selection and the He II AGN exhibitthis behaviour. When we consider the He II -only AGN however(the red triangles), we find that they preferentially reside in star-forming galaxies in the blue cloud and largely avoid the green val-ley. The He II -only AGN fraction in the blue cloud reaches 10 − but does not reach the AGN fraction seen in the green valley us-ing other methods – this is due to the large numbers of underlyingstar-forming galaxies in the blue cloud. We verify this behaviour byplotting the u − r colour histograms of the BPT and He II AGN inFigure 4.This observation can be further explored by plotting the He II -only AGN hosts on the “main sequence” diagram of stellar massversus star formation rate (Noeske et al. 2007; Elbaz et al. 2011;Lee et al. 2015). We show the main sequence in Figure 5 with theHe II -only AGN hosts as red triangles. As these objects are classi-fied as star-forming on the standard BPT diagram, we can interprettheir H α -derived star formation rates as reliable. We note that above M ∗ (log M ∗ ∼ II -only AGNhosts are not just on the main sequence, but systematically elevatedabove it.All this points to a picture where the He II AGN selection ismore sensitive to AGN activity in highly star-forming galaxies thanthe standard BPT method, and the He II -only AGN hosts missed bythe BPT method are highly star-forming galaxies in the blue cloudand on the main sequence, with the most massive He II -only AGNhosts being significantly elevated from the main sequence. MNRAS000
With the Venn diagram we show the relationship between the BPTand He II AGN candidates selections. From our sample of 63’915 galaxiesthe BPT diagnostic identified 1’075 AGN candidates and the He II diagnos-tic, according to Shirazi & Brinchmann (2012), identified 559 AGN candi-dates. 325 AGN candidates are found by both methods. The He II methodidentifies 234 AGN candidates with are not detected by the BPT method,which represents an increase of 22% to the number of AGN candidatesidentified by the BPT diagram. forming region of the BPT diagram can be explained by strong starformation in the host galaxies overwhelming the AGN signal.We provide a complete list of the objects selected by bothmethods in Table 1. II -selected AGN In order to summarize the analysis performed in Figure 1 wepresent the results in a Venn diagram in Figure 2. We can distin-guish four different groups of AGN candidates:(i) 1,075 galaxies identified as AGN by the standard BPT diag-nostic diagram only (hereafter BPT AGN)(ii) 559 galaxies identified as AGN by the HeII diagnostic dia-gram (hereafter HeII AGN)(iii) 325 galaxies identified as AGN by both diagrams (hereafterHeII BPT AGN)(iv) 234 galaxies identified as AGN candidates by the He II di-agnostic diagram only (hereafter He II -only AGN)As we show here, there exists a group of AGN candidates inthe He II selected sample which the BPT method misses. Before wemade the detailed analysis we visually inspected the He II region of their SDSS spectra and eliminated 9 spectra where we deemed thefits to the He II line to be unreliable following Oh et al. (2015).From the total sample of 63,915 galaxies we identify with theBPT method 1,075 AGN candidates and with the He II method 559AGN candidates. Of these 559 AGN 325 overlap with BPT selectedAGN candidates; this means that they are selected by both methods.However the He II method selected 234 He II -only AGN candidatesnot selected by the the BPT method. This corresponds to an in-crease in the detection rate of 22 % compared to the 1,075 AGNcandidates identified by the BPT method (see Figure 2). To analyze the characteristics of the different groups of AGN candi-dates we plot the galaxy colour-mass diagram in Figure 3. We showboth the distribution of AGN host galaxies from the two selectionmethods and the implied AGN fraction as a function of colour andmass. We recover the well-known result that AGN host galaxiestend to lie in the optical green valley both in terms of distribution,and more strongly in terms of AGN fraction. (Nandra et al. 2007;Salim et al. 2007; Schawinski et al. 2007, 2009, 2010, 2014a; Sil-verman et al. 2008; Hickox et al. 2009).Both the standard BPT selection and the He II AGN exhibitthis behaviour. When we consider the He II -only AGN however(the red triangles), we find that they preferentially reside in star-forming galaxies in the blue cloud and largely avoid the green val-ley. The He II -only AGN fraction in the blue cloud reaches 10 − but does not reach the AGN fraction seen in the green valley us-ing other methods – this is due to the large numbers of underlyingstar-forming galaxies in the blue cloud. We verify this behaviour byplotting the u − r colour histograms of the BPT and He II AGN inFigure 4.This observation can be further explored by plotting the He II -only AGN hosts on the “main sequence” diagram of stellar massversus star formation rate (Noeske et al. 2007; Elbaz et al. 2011;Lee et al. 2015). We show the main sequence in Figure 5 with theHe II -only AGN hosts as red triangles. As these objects are classi-fied as star-forming on the standard BPT diagram, we can interprettheir H α -derived star formation rates as reliable. We note that above M ∗ (log M ∗ ∼ II -only AGNhosts are not just on the main sequence, but systematically elevatedabove it.All this points to a picture where the He II AGN selection ismore sensitive to AGN activity in highly star-forming galaxies thanthe standard BPT method, and the He II -only AGN hosts missed bythe BPT method are highly star-forming galaxies in the blue cloudand on the main sequence, with the most massive He II -only AGNhosts being significantly elevated from the main sequence. MNRAS000 , 1–10 (2016)
GN from HeII u − r c o l o u r [ A B ] BPT selected BPT BPT selected BPT u − r c o l o u r [ A B ] HeII selected HeII HeII selected HeII
Stellar Mass log(
M/M fl ) u − r c o l o u r [ A B ] HeII, not BPT selected HeII-only
Stellar Mass log(
M/M fl ) HeII, not BPT selected HeII-only -2 -1 A G N f r a c t i o n -2 -1 A G N f r a c t i o n -2 -1 A G N f r a c t i o n Figure 3.
We show galaxy colour-mass diagrams with the various AGN selections shown in the different panel. In the top panel of the left column, the BPTAGN candidates are shown as blue diamonds. They are predominantly located in the green valley. In the middle panel of the left column, the He II selectedAGN candidates are shown as red points. They are predominantly located in the green valley and scatter into the blue cloud. In the bottom panel of the leftcolumn we show those He II selected AGN candidates which are not identified by the BPT method as red triangles. They are concentrated in the blue cloud.The He II - only AGN candidates host galaxies are less massive than those selected by BPT diagram. In the right hand column we show the respective fractionsof AGN from the various selection methods. These AGN fraction colour-mass diagrams highlight the concentration of AGN hosts in the green valley, withtheHe II only AGN being the only population with an elevated fraction in the blue cloud. The fraction plots also highlight that even the prevalence of He II - only AGN in the blue cloud do not lead to an elevated overall AGN fraction in highly star-forming galaxies – there are too many underlying star-forminggalaxies. II line luminosity Berney et al. (2015) have shown that the He II line luminosity cor-relates with the AGN hard X-ray luminosity as well as other linescommonly used (Veilleux & Osterbrock 1987; Sarzi et al. 2010;Schawinski et al. 2010), such as [OIII]. We can therefore explore whether the He II -only AGN are more or less luminous than theHe II AGN which are also detected by the standard BPT method.We plot the histogram of He II line luminosities in Figure 6 for boththese populations. We find that the mean luminosity of the He II -only AGN is 40.71 erg s − and 41.10 erg s − for the AGN de-tected by both. This means that the He II -only AGN are intrinsically MNRAS , 1–10 (2016)
Rudolf E. B¨ar et al. u-r color N u m b e r BPTAll Galaxies u-r color N u m b e r HE AGNAll Galaxies
Figure 4.
We show the u − r colour histograms of the BPT selected and He II AGN host galaxies, and compare them to the total galaxy sample. The He II selected AGN candidates are bluer than the BPT selected AGN host galaxies. Stellar Mass log(
M/M fl ) l og ( S F R / M fl y r − ) HeII-only
Figure 5.
We show that the He II -only AGN which are not selected on thestandard BPT diagram reside predominantly on the main sequence. TheHe II -only AGN hosts show increased star formation rates above the mainsequence for stellar masses above . M (cid:12) . According to Rodighieroet al. (2011) star bursts in high mass galaxies are thought to be mergerdriven; they may represent a critical phase towards the quenching of starformation and morphological transformation in galaxies. They grey-shadedcontours show galaxies classified as star-forming according to the BPTemission line diagram. less luminous, even though they are hosted by more highly star-forming galaxies. If they were more luminous, they likely wouldalso be detected by the BPT method. This does however not ex-plain why the host galaxies are bluer: this could due to the BPTselection bias against low luminosity AGN in star forming galax-ies; another explanation could be that the black hole masses of bluecloud AGN hosts are generally lower. log L HEII erg sec − N u m b e r HeII-onlyHeII BPT
Figure 6.
We plot the histogram of the He II line luminosity for the He II -only AGN and for the objects which are selected by the standard BPTand the He II selection. The mean luminosity of the He II -only AGN is40.71 erg s − and 41.10 erg s − for the AGN detected by both. Tak-ing the He II line luminosity as a proxy for AGN luminosity (Berney et al.2015), we find that the He II only objects are ∼ . dex less luminous thanthe objects detected by both methods. Chandra
X-ray Observations
Optical emission line diagnostics have to be supplemented withobservations at other wavelengths to confirm AGN activity of ourHe II -only AGN candidates. In order to obtain archival X-ray obser-vations for our sources we cross-matched the sample of 234 He II -only AGN candidates with the Chandra
Source Catalog (Evanset al. 2010). We visually crossmatched the optical positions withthe X-ray images using a 2 arcsec search radius and obtained theX-ray observations for 12 He II -only AGN candidates. MNRAS000
Source Catalog (Evanset al. 2010). We visually crossmatched the optical positions withthe X-ray images using a 2 arcsec search radius and obtained theX-ray observations for 12 He II -only AGN candidates. MNRAS000 , 1–10 (2016)
GN from HeII L Halpha erg sec − L X − r a y e r g s e c − L Halpha erg sec − L X − r a y e r g s e c − Figure 7.
We compare the observed X-ray luminosity of those He II -only AGN with archival observations and compare them to the observed H α luminosityto test whether the observed X-ray emission could come from star formation alone. In the left panel, we show the 5 He II -only AGN for which we could obtainX-ray spectra. The dashes line shows the expected level of X-ray emission given the H α luminosity from star formation alone from Ranalli et al. (2003). Theplot shows that the X-ray derived luminosity for the 5 objects are above the dashed line, so star formation as a source for the X-ray emission can be ruledout. In the right panel, we show the X-ray luminosity upper limits for 7 other He II -only AGN with X-ray coverage. For each object the upper limit for the 2models with Intrinsic N H of Galactic plus zero and of Galactic plus cm − is shown. 6 objects have the upper limits in both cases above the dashed line,1 object has the upper limit for the higher N H value below the dashed line. Thus 6 out of 7 objects are consistent with their X-ray emission coming from anAGN rather than star formation. The Observation IDs are shown next to the symbols in the legends.Observation ID Exposure Counts Model red χ Flux 2–10 keV Luminosity 2–10 keV NH/100ksec erg s − cm erg s − cm − phabs*zpow . × − , × phabs*zpw . × − . × phabs*diskbb . × − . × phabs*zpow . × − . × phabs*zpow . × − . × Table 2.
In this table we provide the information for the 5 He II -only AGN for which we found archival Chandra observations and which had sufficientcounts to extract spectra. The dashed line corresponds to the expected X-ray luminosity assuming that all the H α luminosity comes from star formation.Observation ID F[2-10]keV gal+none F[2-10]keV gal + [erg/s/cm ] [erg/s/cm ]3340 . × − . × − . × − . × − . × − . × − . × − . × − . × − . × − . × − . × − . × − . × − Table 3.
For the 7 He II -only AGN for which we have no detection in archival Chandra observations, we provide the X-ray flux in the 2–10 keV range for 2alternative Intrinsic N H values, Galactic plus none and Galactic plus cm − .MNRAS , 1–10 (2016) Rudolf E. B¨ar et al.
The goal of this analysis was to determine the X-ray flux ofthe AGN candidates and, if possible, their spectral shape. First, thedata were reduced with the CIAO 4.7 software (Fruscione et al.2006, SPIE, 6270) following standard procedure. Since AGN areconsidered to be point sources in the X-ray but the
Chandra
PSFvaries with off-axis angle, we then determined the extraction aper-ture to be considered for each source according to their off-axis an-gle and the Encircled Energy Average Radius reported in the Chan-dra Proposer’s Observatory Guide. For the background estimationwe considered three source-free circular regions with a radius of 15arcsec.Among 12 sources with X-ray data available, 5 were detectedwith sufficient counts to allow us to extract a spectrum and fit it.We extracted the spectra with the CIAO tool
SPECEXTRACT andbinned them with a minimum of 3 cts/ bin. We then fitted the ob-tained spectra with the HEARSAC package
Xspec (Arnaud 1996)using Cash statistics (Gehrels 1986). All 5 spectra show a clearpower law with photon indices between 1.62 and 2.11. For eachobject we determined the intrinsic flux in the 2.0 - 10.0 keV rangeusing the zpow model of
Xspec and derived the respective X-rayluminosity. We show the results in table 2. We provide the
Chan-dra
Observation ID, the exposure time the number of counts andthe model which we used to fit the spectrum. In addition we showthe χ red and the Flux . − obtained for the respective model, theresulting luminosity and the absorbing column N H .While the power law X-ray spectra are indicative of AGNemission, the observed X-ray luminosities in the range of − erg s − are low for AGN and could potentially be due tostar formation. Following Kennicutt (1998), we therefore computedwhat the expected X-ray emission from star formation would be ifall the intrinsic X-ray and H α luminosity is due to star formation in-stead of AGN activity (Ranalli et al. 2003, Kennicutt 1998). In theleft panel of Figure 7, we plot the intrinsic 2.0 - 10.0 keV Lumi-nosity versus H α luminosity for the 5 He II -only AGN with X-rayspectra and find that the X-ray luminosity of three objects ( Chan-dra
OBS ID 11468, 5666 and 11482) exceed the expected emissionfrom star formation by more than 2 orders of magnitude; the other 2objects (
Chandra
OBS ID 2224 and 10729) are also above the line,but their emission may still be consistent with star formation. For 7further He II -only AGN for which we detected no emission we de-termined upper limits for the 2.0–10.0 keV flux. First we estimatedthe noise level (in counts per second) in the source region fromthe measured background counts and computed the 3 σ upper limitsas 3 times the noise level. We then converted the obtained countsrate to flux with the WebPIMMS simulator assuming a power lawmodel with a photon index Γ = 1.9. For this calculation we deter-mined the Galactic N H using the Colden Galactic Neutral Hydro-gen Calculator and applied 2 different values for the intrinsic N H :N H intr = 0 and N
H intr = cm − . The results for these 7 objectsare given in table 3, where we list the Chandra
Observation ID, andthe F − KeV for both alternatives given above.Similar to what we did for the sources with detected X-rayemission, we compared the observed X-ray luminosity upper limitsto the X-ray emission from star formation alone in the right panel ofFigure 7. For the model with N
H intr = cm − the upper limitsare 2 to 4 orders of magnitude above the dashed line and so thesenon-detected He II -only AGN candidates are consistent with beingAGN. https://heasarc.gsfc.nasa.gov/cgi-bin/Tools/w3pimms/w3pimms.pl In this paper, we have applied the Shirazi & Brinchmann (2012)He II diagnostic diagram to the local Universe galaxy population toperform a more complete census of black hole growth in galaxies,particularly in highly star-forming galaxies. This analysis shouldbe seen as an addition to the ongoing discussion about galaxy evo-lution scenarios, in particular to star formation quenching mecha-nisms.The analysis of He II -only AGN host galaxies compared toAGN host galaxies from the standard BPT selection (Figure 3)shows that the host galaxies of AGN missed by the BPT selec-tion and detected only by the He II method are situated primarilyin the blue cloud in the colour-mass diagram. This is equivalentto the He II -only AGN residing primarily on the main sequenceof star formation (Figure 5) The position of AGN candidates incolour-mass space is important for understanding their role in starformation quenching.If AGN reside exclusively in green valley galaxies, then itwould imply that they are not relevant for the quenching processsince their host galaxies have already experienced the quenching”event” several hundred million years in the past (Schawinski et al.2007, 2009, 2014a; Wild et al. 2007, 2010). However, if there areAGN in galaxies which are still blue and star-forming, then theyare plausible sites for AGN-driven quenching.As we have discussed above, previous results based on theBPT selection find that BPT selection is biased against AGN in blueand star forming galaxies (Trump et al. 2015; Sartori et al. 2015).Therefore finding AGN mainly in the green valley might only bethe result of detecting AGN preferentially in host galaxies in whichthe star formation has dropped enough to allow the detection ofthe AGN. The He II -selected AGN are significantly less affected bythis effect, and so the He II -only AGN reside preferentially in lowermass, blue cloud galaxies on or above the main sequence. In fact, atthe high mass end of the main sequence above M ∗ , the He II -onlyAGN are hosted by galaxies which are above the main sequence;these galaxies are prime sites to look for evidence of AGN-drivenoutflows and quenching.The He II diagnostic diagram is shown to be a useful tool forAGN selection; it highlights a population of AGN which were pre-viously difficult or impossible to identify because they are locatedin starforming galaxies. In surveys with high quality spectroscopicdata, the He II can be measured and used for AGN selection, thoughthe weakness of the He II line makes it less useful in low signal-to-noise data and at higher redshift. We used the He II diagnostic diagram developed by Shirazi &Brinchmann (2012) to search nuclear activity in the galaxy pop-ulation of the local universe. We find: • In a sample of 63,915 SDSS galaxies, we find 1,075 AGNusing the standard BPT selection whereas we find with the HeIIdiagnostic diagram 559 AGN; of these 234 AGN are only identifiedby the HeII method (HeII-only), representing an increase of 22%in the AGN population. • We investigate archival Chandra X-ray observations for 12 HeII selected AGN candidates, 5 objects (42%) are confirmed as AGNbased on their X-ray luminosity and power law nature; of the re-maining 7 objects, 6 objects have X-ray luminosity upper limits
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GN from HeII consistent with being AGN. This small sample is of course not rep-resentative of the whole HeII-only population. • The host galaxies of the He II -only AGN are systematicallybluer than those selected by the BPT method and they reside in theblue cloud rather than green valley host galaxies. • The He II -only host galaxies lie on the main sequence of starformation and scatter above it at host galaxy masses above M ∗ . • The fact that the host galaxies of the He II -only AGN arehighly star-forming makes them prime sites for searching for ev-idence of AGN quenching. • The He II AGN selection method clearly yields valuable in-sights, but can be challenging to apply to low quality spectroscopicdata due to the He II line being weak. ACKNOWLEDGEMENTS
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