PG 1543+489

PG 1543+489, also known as QSO B1544+4855 and PGC 2325245, is a quasar located in the constellation of Boötes. At the redshift of 0.399, the object is located 4.5 billion light-years away from Earth. It was first discovered in 1983, by researchers who presented 114 objects in the Palomar-Green bright quasar survey, as one of the best studied samples of active galactic nuclei (AGN).

Characteristics
PG 1543+489 is classified as a radio-quiet quasar (RQQ) with weak 1.3 mm emission and soft X-ray spectra ( g = 2.58 +/- 0.05 for z < 0.5). It contains a high X-ray luminosity which it was studied by X-ray satellites like ROSAT and XMM-Newton, who observed its continuum emission is modelled by a power-law component <Γ2-12 keV > = 1.89±0.1 featuring a strong, broad excess that is below 2 keV.

The quasar is also classified as a narrow-line Seyfert 1 galaxy, a type of AGN that shows all properties of normal Type 1 Seyfert galaxies but has peculiar characteristics such as narrowest Balmer lines with a full width at half-maximum (FWHM) of 1630 km s−1, strong Fe II emission, and extreme properties presented in the X-rays. The measured hard power-law continua in PG 1543+489 is said to have photon indices spanning the range 1.6-2.5 with a mean of 2.1, found only slightly steeper than the norm for 'broad-line' Seyfert 1s. Furthermore, it shows a soft excess typically modelled as blackbody emission (T_bb~100-300eV) that is superposed on the underlying power law. According to data collected from ROSAT, the photon index of the power law is between 2 and 3 for PG 1543+489, with an absorption of cold interstellar gas.

Apart from its Seyfert properties, PG 1543+489 is also classified as an luminous infrared galaxy with its luminosity going up as high as LIR > 1011 L_sun, as observed in the 2-10 keV energy band through new and archival data. According to observations by BeppoSAX, the source in PG 1543+489 is completely Compton thick (N_H > 1025 cm−2).

Observations
Researchers reanalyzed mid-infrared (5-40 μm) Spitzer spectra of 86 low-redshift (z < 0.5) Palomar-Green quasars for nature of polycyclic aromatic hydrocarbon (PAH) emission and its utility as a star formation rate (SFR) indicator. Upon decomposing the spectra with their recently developed template-fitting technique to measure PAH fluxes and its upper limits, by interpreting a mock spectra simulating the effects of AGN dilution, they found the PAHs weak in some sources considered as gas-rich forming stars. This shows PG 1543+489 is a powerful quasar destroying the PAH molecules. Moreover, the bolometric luminosity of PG 1543+489 is ≲1046 erg s−1 suggesting PAHs can also trace star formation activities but increasingly underestimates the SFR, typically by ~0.5 dex. In comparison to low-luminosity AGNs and starburst galaxies, PG 1543+489 has a PAH 11.3 μm/7.7 μm ratio but with a characteristically lower ratios of 6.2 μm/7.7 μm, 8.6 μm/7.7 μm, and 11.3 μm/17.0 μm, indicating the quasar as a powerful AGN does descry small grains but enhances the PAH ionization fraction.

Researchers also found a peculiar feature in PG 1543+489. The quasar shows a blueshift of the [O III] 5007 Å line that is 1150 km s−1 with respect to the systemic velocity of the galaxy as well as the blue asymmetry of its profile. The large [O III] blueshift or so-called 'blue outliers' by researchers, is found theoretically interpreted by the end result of intense outflows whose receding parts are obscured by an optically thick accretion disc or possibly a scenario which the narrow-line region clouds are entrained by decelerating winds, potentially associated with the high Eddington ratio typical of the 'blue outliers'.

Absorption system
Through observations from Hubble Space Telescope, researchers were able to find an absorption-line system at z = 0.07489. Looking at it, they found the sightline passes within ρ = 66 kpc of an edge-on 2{L}* disk galaxy at a similar redshift, belonging to four other galaxies in the group within ρ = 160 kpc. From the absorption-line system, they detected H I [log N(H I/cm-2) = 19.12 ± 0.04] as well as N I, Mg II, Si II, and Si III, from which we measure a gas-phase abundance of [N/H] = -1.0 ± 0.1. The photoionization models indicate that the nitrogen-to-silicon relative abundance is solar, yet magnesium is found underabundant by a factor of ≈2. By extracting out its rotational curve and reporting emission-line spectroscopy of the nearby galaxy, researchers suggests the metallicity is ≈8× higher compared to [N/H] in the absorber. Interestingly, the absorber velocities in the galaxy suggests gas at ρ = 66 kpc is corotating with the galaxy's stellar disk, possibly with an inflow component. Although indicating the sub-damped Lyα absorber system is responsible in causing cold accretion flow, the absorber abundance patterns are quite peculiar. Researchers hypothesized gas was probably ejected from its home galaxy or result of tidal debris from interactions between the group galaxies, with solar nitrogen abundance, but mixed with the gas in the circumgalactic medium or group. If the gas is bound to the nearby galaxy, this system may become an example of the gas "recycling" as predicted by theoretical galaxy simulations.

Dampen Lyα absorber
The Lyα absorber in PG 1543+489 has H I column density of NH I ≥slant 1020.3 cm−2, implying there is an incidence per absorption length, X = 0.017-0.008+0.014 at median survey path redshift of z = 0.623. According to researchers, the estimated H I mass density in PG 1543+489 at z~ 0.6, is of ρH I DLA = 0.25-0.12+0.20× 108 M⊙ Mpc−3. This is found significantly lower compared to previous estimates from targeted DLA surveys with Hubble Space Telescope, but consistent with results from low-z H I 21 cm observations, suggesting that the neutral gas density of the universe has been decreasing over the past 10 billion years.

Host galaxy
The host galaxy of PG 1543+489 is a spiral galaxy  with a gas fraction similar to massive early-type galaxies and an estimated ongoing star formation rate of ~1-250 M⊙ yr−1 according to researchers analyzing 1-500 μm spectral energy distributions whom they found the total infrared emission yields 8-1000 μm. This makes PG 1543+489 a starburst galaxy given the star population is an average age of ~200 Myr and such stars showing stellar winds can affect the circumgalactic medium out to a large radius as 200 kpc.

In additional, the host galaxy has a dusty torus surrounding its AGN whom it contributes significant fraction (~70%) of total infrared (1-1000 μm) luminosity. Apart from that, the torus also contains tidally disrupted clumps that represents the source of the broad-line region gas. In the case of NGC 1068, another Seyfert galaxy, PG 1543+489 is suggested a small amount of dust might exist inside its inner edges of the torus and usually dominated by its far-infrared luminosity. If this theory is correct, then it might be considered a "type 2" obscured quasar rather than a type 1 quasar.

According to researchers, the host galaxy is in the midst of a transitional stage between an ultraluminous infrared galaxy and quasar, which the stage normally lasts <~300 Myr. The host galaxy is also found near a companion galaxy with a separation of p' ≃ 2-113 h-1 kpc. With bridge of gas connecting both objects and post-starburst populations, all of this properties suggested a strong interaction between PG 1543+489 and its companion galaxy. In a short period of time, these galaxies will eventually merge.

Black hole
The supermassive black hole in PG 1543+489 has an estimated solar mass of 1–2.4 × 108M⊙ through optical and ultraviolet mass scaling by researchers, with a larger black hole mass expected by using a spectral energy distribution (SED) fitting approach. The black hole is found to grow at a rapid rate with a high Eddington ratio [defined as Lbol/LEdd, where Lbol is the bolometric luminosity and LEdd= (1.3–3.1) × 1046 erg s−1 is the Eddington luminosity] of ≈1.3–3.7 vs ≈2.3, according to researchers who adopted the bolometric correction for quasars. This values are higher as expected compared to previous estimations from the observed 2–10 keV luminosity ≈1.1 × 1044 erg s−1 using the average spectral energy distribution of broad-line quasars which is ≈0.1–0.3.