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Accretion Properties of High- and Low-Excitation Young Radio Galaxies

Young radio galaxies (YRGs) provide an ideal laboratory to explore the connection between accretion disk and radio jet thanks to their recent jet formation. We investigate the relationship between the emission-line properties, the black hole accretion rate, and the radio properties using a sample of 34 low-redshift (z<0.4) YRGs. We classify YRGs as high-excitation galaxies (HEGs) and low-excitation galaxies (LEGs) based on the flux ratio of high-ionization to low-ionization emission lines. Using the H{\alpha} luminosities as a proxy of accretion rate, we find that HEGs in YRGs have \sim1 dex higher Eddington ratios than LEGs in YRGs, suggesting that HEGs have higher mass accretion rate or higher radiative efficiency than LEGs. In agreement with previous studies, we find that the luminosities of emission lines, in particular H{\alpha}, are correlated with radio core luminosity, suggesting that accretion and young radio activities are fundamentally connected.


INTRODUCTION
Active galactic nuclei (AGN) play an important role in galaxy evolution by feeding their host galaxies with radiative and/or kinetic energy, leading to the observed scaling relations between black hole (BH) mass and galaxy properties (e.g., Ferrarese & Merritt 2000;Gültekin et al. 2009;Woo et al. 2010). Thus, investigating the radiative and kinetic processes of AGN is of fundamental importance for understanding AGN physics as well as feedback mechanisms.
The formation of relativistic jets and its connection to accretion disk remain as an open issue in AGN physics (e.g., Rees 1984;Meier 2003;McKinney 2006;Komissarov et al. 2007;McKinney et al. 2012). Thanks to their short dynamical timescale, X-ray binaries in various states have been observed in detail, revealing that individual sources occupy a particular accretion state with various X-ray and radio luminosities (e.g., Fender et al. 2004;. By analogy to X-ray binaries, the power of AGN jets may also depend on the physical states of accretion disk. However, the disk-jet connection is more complicated in AGNs. It has been known that the radio power is correlated with narrow emission line luminosities, i.e., [O iii], which is a proxy for the accretion power (e.g., Baum & Heckman 1989;Rawlings & Saunders 1991), indicating that the jet launching mechanism is connected with accretion disk. However, the correlation shows a large scatter (e.g., Morganti et al. 1992;Labiano 2008), implying that the nature of the disk-jet connection is complicated and that other physical parameters are necessary to constrain.
The properties of the accretion disk seem to play an 8 corresponding author; woo@astro.snu.ac.kr additional role in the disk-jet connection. At a given radio luminosity, high-excitation galaxies (HEG) classified with high [O iii]/Hα ratio (e.g., Laing et al. 1994) have an order of magnitude higher [O iii] luminosity than low-excitation galaxies (LEG). Using a sample of 3C radio galaxies, Buttiglione et al. (2010) showed that there are two sequences in the radio-emission line luminosity plane, suggesting that systematically different accretion rates or accretion modes are responsible for the separation between HEGs and LEGs. One of the limiting factors in interpreting the disk-jet connection in powerful radio galaxies is that the lifetime of large radio sources is much longer than the transition timescale of the physical states in the accretion disk (O'Dea et al. 2009). Thus, comparing radio and accretion powers in radio galaxies with large-scale jets may suffer systematic uncertainties, leading to large scatters in the correlation between optical and radio properties (see Punsly & Zhang 2011).
In contrast, compact radio galaxies with small jets triggered by recent activity are very useful to investigate the disk-jet connection, since radio and disk activities are contemporaneous. The ages of compact radio galaxies are typically estimated as t age = 10 2 − 10 3 years (e.g., Orienti et al. 2007;Fanti 2009;Giroletti & Polatidis 2009), while the lifetime of extended (up to a few hundred kpc) radio sources are about 10 6 − 10 7 years (e.g., Alexander & Leahy 1987;Carilli et al. 1991;Fanti et al. 1995;O'Dea et al. 2009).
Recently, a substantial amount of such compact radio galaxies have been detected and classified with various characteristics: compact symmetric objects (CSO) with a linear scale 1 kpc, gigahertz-peaked spectrum (GPS; 1 kpc) sources, medium-size symmetric objects (1 − 10 kpc), and compact steep-spectrum (CSS; 20 kpc) sources (See O'Dea et al. 2009). These young radio galaxies (YRGs) are a good laboratory to investigate the physical link between AGN jets and accretion disks. YRGs show a close connection between emission-line gas and radio properties. For example, it has been reported that emission-line gas (e.g., [O ii], [O iii]) is well aligned with the radio jet (e.g., de Vries et al. 1999;Axon et al. 2000) and that emission-line luminosities (e.g., [O iii]) are correlated with the radio power (e.g., Labiano 2008;Buttiglione et al. 2010;. The optical emission-line diagnostics of the narrow-line region (NLR) can constrain accretion properties, since the NLR is photoionized by the nuclear continuum radiation (e.g., Kawakatu et al. 2009).
In this work, using a sample of YRGs covering a large range of luminosities, we investigate the properties of NLRs and accretion by directly measuring narrow emission-line luminosities from optical spectra, and compare them with radio properties for constraining the diskjet connection. In Section 2, we describe the sample selection, spectroscopic observations, data reduction, and radio data collection. The measurements of emission-line fluxes and the stellar velocity dispersions are described in Section 3. Main results are presented in Section 4 and Section 5 contains discussions and summary. Throughout the paper, we used cosmological parameters, H 0 = 70 km s −1 Mpc −1 , Ω m = 0.3, and Ω Λ = 0.7.

OBSERVATIONS AND DATA
2.1. Sample Selection To investigate the properties of optical emission lines and their connection to radio activities, we selected a sample of 34 YRGs from the literature. Initially, we selected 19 known YRGs listed by O'Dea (1998) and Kawakatu et al. (2008), by limiting a redshift range z < 0.4. This choice was made in order to include narrow emission lines from [O II] to [Ar III] (3727 − 7136Å in the rest-frame) in the observed wavelength range for comparing emission-line flux ratios with the photoionization models.
During our observing campaign, new YRGs with relatively low luminosity have been reported by  based on the unresolved and isolated morphology in Faint Images of the Radio Sky at Twenty-centimeters (FIRST). To enlarge the sample size and the luminosity range, we selected additional 15 YRGs at z < 0.3 from their list, for which optical spectra with the same rest-frame wavelength range (3727 − 7136 A) were available in the archive of the Sloan Digital Sky Survey (SDSS) Data Release 7 (DR7; Abazajian et al. 2009). Thus, combining new data with SDSS archival data, we compiled a sample of 34 low-redshift YRGs for investigating optical and radio properties. Table 1 lists the properties of individual YRGs, including optical and radio AGN classifications.

New Optical Data
Among 34 objects in the sample, SDSS spectra are available for 15 objects, while for the remaining 19 objects, new spectroscopic data have been obtained from the Lick and Palomar telescopes. In this section, we describe new observations and data reduction.

Observations
We observed 19 YRGs to obtain high-quality optical spectra. Ten objects were observed with the Kast double spectrograph at the Lick 3 m telescope (Miller & Stone 1993). We used the D55 dichroic beam splitter to pass the light into blue and red side detecters. Depending on the redshift of the target, different gratings were used in the red spectrograph, covering the wavelength range of 5500 − 10000Å with a wavelength scale of 1.7 − 4.6 A pixel −1 and spatial scale of 0.78 arcsec pixel −1 . For the blue spectrograph, the 600/4310 grism was used, covering the wavelength range 3600 − 5500Å with a wavelength scale of 1.0Å pixel −1 and spatial scale of 0.43 arcsec pixel −1 .
The other nine galaxies were observed withthe Double Spectrograph for the Palomar 200-inch Telescope (DBSP; Oke & Gunn 1982) using the D55 or D68 dichroic mirrors depending on the redshift of the targets. The 158/7500 and 316/7500 gratings were used on the red spectrograph, covering 5200−10000Å with a wavelength scale of 4.8 or 2.4Å pixel −1 and spatial scale of 0.62 arcsec pixel −1 , while the 600/4000 grating was used on the blue spectrograph, covering the wavelength range of 3200−5500Å or 4000−6800Å with a wavelength scale of ∼ 1.1Å pixel −1 and spatial scale of 0.39 arcsec pixel −1 .
For all observations, a 2 ′′ -wide slit was centered on the nucleus of the targets after aligned with a parallactic angle. Several flux photometric standard stars, namely, BD+262606, BD+284211, Feige 34, G138-31, G191B2B, HD192281, or Wolf1346 were observed during each night. A0V stars with similar airmass and hour angle to each target were observed for telluric correction. The properties of each YRG along with exposure time and S/N are presented in Table 1, while the details of instrumental setup, observing date, and sky conditions are listed in Table 2 .

Data Reduction
Standard spectroscopic data reduction procedures, including bias subtraction, flat-fielding, spectral extraction, wavelength calibration, and flux calibration, were carried out for each data set using a series of IRAF scripts developed for long-slit data (e.g., Woo et al. 2005Woo et al. , 2006. Telluric A and B bands were corrected using an A0V star spectrum obtained for each target as similarly performed in the previous studies (e.g., Woo et al. 2006). After combining multiple exposures in each blue and red sides, the blue-and red-side spectra were merged into one final spectrum for each target. Galactic extinction corrections were applied using the extinction law by Cardelli et al. (1989) with the E(B −V ) values listed in Table 1. We measured the spectral resolution by averaging the width of sky or arc lines for each instrumental setup (see Table 2). In the case of the SDSS archival data, the spectra were obtained with a 3 ′′ diameter fiber and a spectral resolution of λ/∆λ ≈ 1800.

Radio Data
To compare with optical spectroscopic properties, we collected the radio luminosity and the jet size of the sample from the literature, after correcting for the adopted cosmological parameters. The 1.4 GHz integrated flux Note. -Columns: (1) Target name; (2) R.A.; (3) Decl..; (4) Redshift; (5) Galactic extinction; (6) Spectroscopic AGN type -1: Type 1 AGN with broad emission lines; 2: Type 2 AGN without broad emission line; (7) YRG type; (8) Classification by excitation -HEG: high excitation galaxy, LEG: low excitation galaxy; (9) 1.4 GHz integrated flux density of radio source; (10) Reference for the flux density -F: from the Faint Images of the Radio Sky at Twenty-centimeters (FIRST) catalog, N: from the NRAO/VLA Sky Survey (NVSS) catalog; (11) Jet size; (12) Table 2; (14) Exposure time; (15) Signal-to-noise ratios near 5100Å in the rest frames. densities of radio core were extracted from the FIRST catalog, while the NRAO/VLA Sky Survey (NVSS) catalog was used if FIRST data were not available. We confirmed that these integrated flux densities were measured from radio core by checking the presence of unresolved radio cores in the FIRST and NVSS images. The integrated flux densities and their references are listed in Table 1.
The jet size is generally defined by a half of the linear scale or the hot spot distance from the core. We collected the linear jet sizes from the literature after correcting for the adopted cosmological parameters. The radio luminosity and the jet size of each object are listed in Table 1. 3. ANALYSIS In this section, we present AGN emission and stellar absorption line measurements based on the optical spectra. First, we describe AGN classification based on the presence of the broad Hα line in Section 3.1. Second, we present the emission-line fitting procedure and mea-surements in Section 3.2. Then, we describe the stellar velocity dispersion measurements in Section 3.3.

The Spectra
In Figure 1, we present the spectra of 19 YRGs obtained from the Lick and Palomar telescopes. For completeness, we also present SDSS spectra of 15 additional YRGs in Figure 2. We classified YRG as Type 1 and Type 2 based on the presence of the broad Hα line. The sample is composed of 10 Type 1 (i.e., broad-line) and 24 Type 2 (i.e., narrow-line) AGNs as listed in Table 1. For Type 1 objects, the broad Hα component was clearly present although the broad Hβ were not detected for three objects, presumably due to the weak flux level of the broad Hβ component (for details, see Figure 3). In Type 2 objects, stellar absorption lines were dominant as expected, confirming that subtracting stellar absorption lines is important to precisely measure AGN emission line fluxes. While one object 0810+077 was previously classified as Type 1 AGN, the broad Hα was not detected in our spectra, hence we classified it as a Type 2 AGN. The optical spectra of 0428+205, 1203+645, 1225+442, 1233+418, and 1323+321 are newly presented in this paper. The spectra of other objects have higher spectral resolution and/or better S/N than spectra presented by previous studies (e.g., Gelderman & Whittle 1994: 0134+329, 0221+276, 0345+337, and 1250+568; Labiano et al. 2005and O'Dea et al. 2002Buttiglione et al. 2009: 0345+337, 0605+480, and 1807+698).

Emission Lines
We measured the flux of all emission lines in the rest- , in order to constrain physical properties of NLR. We fitted each emission line with Gaussian profiles using the IDL routine mpfit (Markwardt 2008). The fitting code determined the best χ 2 fit, and measures the peak intensity, central wavelength, and line dispersion (σ). After subtracting a linear continuum, emission lines were modeled with single-or multi-Gaussian com-ponents. For the Hβ+[O iii] and [N ii]+Hα+[S ii] regions, we simultaneously fitted individual lines accounting for line blending. The flux and uncertainty were evaluated by integrating the fitted Gaussian function and the propagation of errors, respectively.
In the case of Type 1 AGNs, it was necessary to simultaneously fit broad and narrow components of the Hα and Hβ emission lines, in order to properly measure the narrow line fluxes. One Type 1 object, 0134+329 showed strong Fe II emission in the Hβ+[O iii] region, thus, we subtracted the Fe II emission features, by fitting them with a series of Fe II templates convolved with various Gaussian velocities as performed in our previous studies (Woo et al. 2006;McGill et al. 2008; see also, Boroson & Green 1992). For Type 2 AGNs, stellar absorption lines were subtracted before emission line fitting (see their Section 3.3). In Figure 4 we present emission-line model fits around the Hβ and Hα regions. The measured emissionline fluxes are listed in Table 3.

Stellar Velocity Dispersions
We were able to measure the stellar velocity dispersions (σ * ) of 24 YRGs including two Type 1 objects, by comparing strong stellar lines, e.g., the G band (4300Å), Mg b triplet (5172Å), and Ca II H & K lines, with stellar tem- plates (e.g., Woo et al. 2004Woo et al. , 2005Bennert et al. 2011). For most of Type 1 objects, stellar lines are relatively weak or not clearly detected due to the higher AGN continuum luminosity than stellar luminosity. To measure stellar velocity dispersion, we utilized a Python-based code vdfit, which employs Bayesian statistical estimation using Markov Chain Monte Carlo sample with a set of stellar templates from the INDO-US library comprised of G and K giant stars (see Suyu et al. 2010). AGN emission lines, i.e., Hγ+[O iii]λ4363 and [N i]λ5200 were masked out before the fitting procedure. The continuum was fitted with a low-order polynomial function, and then the width of absorption lines in the normalized spectra was compared to a series of stellar templates convolved with various Gaussian velocities. The measured stellar velocity dispersions were corrected for the instrumental resolution by subtracting the instrumental resolution from the measured velocity dispersion in quadrature (e.g., Barth et al. 2002;Woo et al. 2004). . The observed spectra (black lines) were compared with the best stellar template models (red lines). When the quality of the fit is poor (thin dashed lines), the measurements are discarded. Note that non-stellar lines, e.g., Hγ, [O iii]λ4363, and [N i]λ5200 lines were masked out before the fitting procedure. (A color version of this figure is available in the online journal.) Figure 5 presents the best-fit models and the observed spectra. In most cases, the stellar velocity dispersions measured from the G band and Mg b triplet regions were consistent within the measurement errors (see Figure 5). Thus, we adopted the average of the two measurements as a final value. For several objects, e.g., 0019+000, 1233+418, 1323+321, 1558+536, 1807+698 and 1943+546, the Mg b triplet region is not acceptable for measuring stellar velocity dispersion due to the contamination of AGN emission lines and/or low S/N ratios. Thus, we adopted velocity dispersion measured from the G-band region. In the case of 1225+442, both G band and Mg b regions have low S/N, thus we measured σ * from the Ca II H & K region. The measured stellar velocity dispersions will be used to estimate BH masses for each galaxy in next section.

RESULTS
In this section, we investigate the physical properties of YRGs by estimating BH masses in Section 4.1, explore the properties of the NLR and the accretion rate in Section 4.2, and compare radio and emission-line properties in Section 4.3.

Black Hole Mass
We determine BH masses (M BH ) using two different methods for Type 1 and Type 2 AGNs, respectively. For 10 Type 1 AGNs with detected broad emission lines, we estimate M BH based on the virial assumption of the broad-line region (e.g., Woo & Urry 2002;McGill et al. 2008;Shen et al. 2008). In practice, we used the single-epoch mass where σ Hα is the line dispersion of broad Hα, and L Hα is the broad Hα luminosity. Table 4 where σ Hβ is the line dispersion of broad Hβ, and L 5100 is AGN continuum luminosity at 5100Å. Since broad Hβ is relatively weak and detected only for 7 objects, we inferred σ Hβ from σ Hα , using the correlation between Hα and Hβ line widths (Equation (3) in Greene & Ho 2005). In the case of L 5100 , we used the correlation between L Hα and L 5100 given by Greene & Ho (2005). M BH estimated in this method is systematically larger than that determined with Equation (1) by ∼ 26% due to the different calibrations between luminosities and/or line widths adopted in the mass estimators. However, the systematic difference is relatively small compared to the large range of mass and luminosity of the sample, and does not significantly affect the main results.

Excitation and Accretion
We classify YRGs into two groups: HEGs and LEGs based on the flux ratios between high-and low-excitation lines using the excitation index (EI) suggested by Buttiglione et al. (2010)  were measured, thus, we used only one or two available lines for classification as suggested by Buttiglione et al. (2010).
1943+546 is excluded since none of the three lowionization lines were detected. We note that 0345+337 is classified as LEG, while it was previously classified as HEG by Buttiglione et al. (2010), presumably owing to the lower quality of their spectra.
In summary, the YRG sample consists of 16 HEGs, 17 LEGs, and 1 unclassified object. All Type 1 AGNs with broad Hα belong to the HEG class except for 1601+528, while Type 2 AGNs belong to both HEG and LEG classes. These trends are similar to those found in FR I and FR II galaxies. For example, Buttiglione et al. (2010) showed that all Type 1 objects in their 3CR sample were HEGs, while Type 2 objects were composed of both HEGs and LEGs.

Ratio
We investigate whether HEGs and LEGs have systematically different accretion activity by comparing their Eddington ratios. Since bolometric luminosity cannot be directly measured for Type 2 AGNs, first, we use the [O iii] line luminosity as a proxy for bolometric luminosity. In Figure 7  is used as a proxy for AGN bolometric luminosity, the difference of bolometric luminosity between HEGs and LEGs would be overestimated.
In Figure 9 we compare M BH with the Hα luminosity. The average difference between HEGs and LEGs decreases compared to Figure 7. However, we find that the average Eddington ratio of HEGs is still larger than that of LEGs by ∼1.0 dex (a factor of 9.0). The difference of the Eddington ratio suggests that HEGs may have higher mass accretion rate at fixed M BH than LEGs or that radiative efficiency is systematically different if the mass accretion rate normalized by M BH is similar.

Comparison with Photoionization Models
We investigate whether HEGs and LEGs have different types of photoionizing continua by comparing observed emission-line flux ratios with photoionization models. Using Cloudy version 08.00 (Ferland et al. 1998), we calculate the ratios of various emission lines arising in the NLR gas clouds. We assume the NLR metallicity Z NLR = 2Z ⊙ since such a super-solar metallicity has been generally reported for the NLRs (e.g., Nagao et al. 2006), and investigate line flux ratios with a density range of n H = 10 2.0 − 10 5.0 and an ionization parameter range of log U = 10 −4.0 − 10 −2.5 that are typical for NLRs (e.g., Nagao et al. 2001). In the case of photoionizing sources, we used two types of SEDs: SED with big blue bump (BBB) and SED without BBB (see Nagao et al. 2002;Kawakatu et al. 2009). Note that the SED without BBB can be expressed with a single powerlaw continuum (harder spectrum) in the range of ∼10 12 to ∼10 20 Hz. Such a harder spectrum of the optically thin disk (radiatively inefficient accretion flow) generates different line ratio compared to the SED with BBB, as presented in Figure 10. At fixed U and n H , the photoionization models using SED with BBB (blue lines) predict lower [O i]λ6300/[O iii]λ5007 than the models using SED without BBB (red lines), as also summarized by Kawakatu et al. (2009;see Section 4.1 Kawakatu et al. 2009). Thus, high-excitation YRGs and Seyfert 2 galaxies may have similar accretion properties. Kawakatu et al. (2009) suggested that YRGs have radiatively inefficient accretion disk based on the small sample of YRGs, of which the oxygen line flux ratios were consistent with photoionization model without BBB. However, note that their sample was mostly composed of LEGs. In contrast, in this work, we include many Type 1 AGNs with relatively higher Eddington ratios, which are mostly HEGs, leading to a more general view on the accretion properties of YRGs.
The different location on the oxygen line ratio diagram can also be interpreted as that LEGs have relatively lower ionization parameter than HEGs, while both classes have similar ionizing SEDs. We cannot rule out this possibility with the current data. To break the degeneracy between the SED shape and the ionization parameter, we attempted to measure [Ar iii]/[O iii] ratio, which is a robust indicator of the ionization parameter, independent of SED shapes (Nagao et al. 2002). Unfortunately, [Ar iii] is generally very weak and we are not able to detect [Ar iii] in most of LEGs, leading to inconclusive results (see Table 3).

Gas Properties
We estimated the electron temperature (T e ) and density (n e ) of the NLR using emission-line ratios. The [O iii] flux ratio, I(4363)/I(4959+5007) is generally used as an indicator of T e , while the flux ratio of [S ii], I(6716)/I(6731) is used for estimating n e (Osterbrock & Ferland 2006). We used the temden task in the IRAF nebular package to calculate T e and n e . For six objects we were able to estimate T e in the ranges of 4.0 ≤ log T e ≤ 4.6, assuming n e = 10 3 cm −3 .
For measuring electron density, [S ii] ratios were determined for 20 objects, ranging from 0.7 to 1.4, which corresponded to log n e ≈ 1.6 − 3.4 cm −3 , assuming n e = 10 4 K. For the remaining galaxies, the electron density could not be calculated due to the failure of deblending of [S ii]λ6717 & [S ii]λ6731 lines, We find that electron temperature and density of the NLRs in YRGs are similar to typical RQ AGNs. The estimated electron temperature and density of NLR are listed in Table 4 and 5.

Radio jet properties
We investigate the connection between radio and accretion activities by comparing radio luminosity and jet size with [O iii] and Hα luminosities. Figure 11 compares the radio core luminosity with the line luminosities of [O iii] and Hα. In general there is a broad correlation between radio and narrow emission-line luminosities. If we consider narrow-line luminosity as a proxy for AGN bolometric luminosity as discussed in Section 4.2, the broad correlation indicates that radio and accretion activities are fundamentally connected in YRGs. A similar trend has been reported in large-scale radio galaxies (i.e., FR Is and FR IIs) by a number of previous studies (e.g. Baum & Heckman 1989;Rawlings & Saunders 1991;Buttiglione et al. 2010).
Comparing HEGs and LEGs at fixed radio luminosity, we find HEGs have higher [O iii] luminosity than LEGs, suggesting that accretion properties are different between HEGs and LEGs although radio activities are similar. It is possible that there is a second parameter to change the accretion power at given radio power. These trends were also reported for large-scale radio galaxies by Buttiglione et al. (2010), who investigated the radio and emissionline luminosities of a sample of low-z 3C radio galaxies (see also . They argued that the separation between HEGs and LEGs is due to the different temperature of the accreting gas. While HEGs have inflow of cold gas, LEGs accrete hot gas, resulting in a harder photoionizing spectrum and stronger low-excitation lines than those of HEGs. If the [O iii] line flux is systematically higher than the Hα line flux in HEGs as discussed in Section 4.2 (see Figure 8), then the difference between HEGs and LEGs may be overestimated. To overcome this systematic uncertainty, we also use the Hα luminosity as a proxy for bolometric luminosity for investigating whether HEGs and LEGs have different accretion properties at fixed radio luminosity. When [O iii] is replaced by Hα (bottom panel of Figure 11), the separation between HEGs and LEGs is less clear although on average HEGs have higher Hα luminosity than LEGs. Since the division between HEGs and LEGs is not distinct, we derive the correlation for the combined sample of HEGs and LEGs using a least square fitting method: log L Hα = (21.9 ± 3.0) + (0.5 ± 0.1) log νL 1.4 . (5) The correlation has 0.4 dex scatter, indicating that accretion luminosity can vary by more than a factor of two at given radio luminosity. The correlation between narrowline and radio luminosities found in YRGs seems similar to that of large-scale radio galaxies. However, the slope of the correlation in large-scale radio galaxies is close to 1, which somewhat steeper than that of YRGs (Buttiglione et al. 2010).
By comparing the linear jet size with the [O iii] and Hα luminosities in Figure 12, we find a weak correlation between emission-line luminosity and the projected jet size, as similarly reported by Labiano (2008) for GPS and CSS sources. However, the correlation is not very tight with considerably large scatter in the case of [O iii]. When the Hα luminosity is compared with the jet size, the relation becomes slightly tighter with 0.5 dex scatter, probably due to the systematic difference of [O iii]/Hα ratios between HEGs and LEGs. We derive the correlation between Hα luminosity and jet size as: log L Hα = (41.6 ± 0.1) + (0.2 ± 0.1) log(R jet /kpc), (6) where R jet is the size of jet in kpc. Note that since the jet size is measured as a projected size, the true jet size may be larger, implying that the correlation is even weaker. The shallow slope of the relation indicates that YRGs with similar emission line luminosities and accretion rates can have dramatically different jet sizes, implying that the jet size may be determined by other mechanisms, e.g., the properties of interstellar medium in the host galaxies than accretion properties. We further investigate whether the HEG/LEG ratio changes as a function of the jet size using our YRG sample and 3CR radio galaxies from Buttiglione et al. (2010). No significant change of the ratio has been detected over the large range of the jet size (∼10 pc to ∼1 Mpc), suggesting that the jet size is not directly connected to the properties of the accretion flows. These findings are consistent with a scenario that accretion properties can change over the lifetime of radio jets.

DISCUSSIONS AND SUMMARY
To investigate spectral properties of YRGs and compare them with radio properties, we construct a sample of 34 YRGs at relatively low redshift (z < 0.4) for measuring narrow emission-line properties, M BH , and Eddington ratio. We determined M BH from the width and luminosity of the broad Hα line using single-epoch mass estimators for Type 1 (broad-line) AGNs, or from the measured stellar velocity dispersion using the M BH − σ * relation for Type 2 (narrow-line) AGNs. The estimated M BH ranges from 10 7.0 to 10 9.2 M ⊙ , indicating YRGs have relatively massive BHs, similar to the large-scale radio galaxies.
Based on the narrow emission-line flux ratios (e.g. [O iii]/Hβ, [N ii]/Hα, [S ii]/Hα, and [O i]/Hα), we classified YRGs as HEG and LEG. Most of Type 1 AGNs belong to HEGs, while Type 2 AGNs are composed of HEGs and LEGs. We find that the Eddington ratio of HEGs is higher by ∼1.0 dex than that of LEGs, using the Hα line luminosity as a proxy for AGN bolometric luminosity. The difference in Eddington ratios and comparison with photoionization models suggest that HEGs are high Eddington ratio AGNs with an optically thick accretion disk, which are similar to QSOs or Seyfert 1 galaxies, while LEGs have lower Eddington ratios with radiatively inefficient accretion flow. This interpretation is similar to the division between Seyfert galaxies and LINERs in RQ AGNs (Kewley et al. 2006;Ho 2008), suggesting that YRGs have a various range of accretion activities over 2-3 orders of magnitude in the Eddington ratio. Kawakatu et al. (2009) investigated whether the optical narrow emission-line ratios of YRGs are systematically different from those of RQ Seyfert 2 galaxies by comparing the observed line ratios (e.g., [O i] ) with photoionization models. Using a limited sample of YRGs, they concluded that YRGs favor SED without a strong BBB, i.e., optically thin advectiondominated accretion flow, while RQ AGNs are consistent with the models adopting SED with a strong BBB, i.e., a geometrically thin, optically thick disk. In this study with an enlarged sample including Type 1 AGNs with higher Eddington ratios, we find that there are various levels of accretion activity in YRGs and that both SEDs with/without BBB are required to reproduce the observed line flux ratios of YRGs.
Low luminosity AGNs, i.e., LINERs, generally tend to be radio-loud (Ho 1999, Terashima & Wilson 2003, implying that radio activity may be related with radiatively inefficient accretion flow, similar to the low-state in Xray binaries ). In the case of YRGs, we find a large range of Eddington ratios, including HEGs with high accretion power and Seyfert-like emission-line flux ratios. Thus, the connection between radio jet and radiatively inefficient accretion flow is not strong in YRGs. Instead, YRGs are probably composed of heterogeneous objects representing various accretion states.
By comparing narrow emission-line properties with radio luminosity and jet size, we investigated the disk-jet connection in YRGs. The [O iii] and Hα line luminosities show broad correlations with the radio core luminosity, indicating that accretion and radio activities in YRGs are fundamentally linked. However, at fixed radio luminosity, HEGs have higher line luminosities (particularly for [O iii]) than LEGs, indicating that HEGs have higher accretion activity than LEGs for a given radio activity. These results may suggest that at a given radio activity there is a continuous distribution of accretion powers due to various mass accretion rate.