Outflowing material in the compact steep-spectrum source quasar 3C 48: evidence of jet-cloud interaction?

Neeraj Gupta 1 D. J. Saikia 1 0 Inter-University Centre for Astronomy and Astrophysics , Pune 411 007 , India 1 National Centre for Radio Astrophysics, TIFR , Pune 411 007 , India We report on the detection of a z abs = 0.3654 associated absorption-line system in the ultraviolet spectrum of the compact steep-spectrum source (CSS) quasar 3C 48. The absorbing material is blueshifted with respect to the quasar emission-line redshift, z em = 0.3700, suggesting an outflow velocity of 1000 km s1. We detect absorption lines over a range of ionization states from Ly, Ly , C IV, N IV, S VI to O VI and possibly O IV and Ne VIII. The kinematical properties of the absorption-line system are similar to the blueshifted emission-line gas seen in [O III] 5007, which is believed to have interacted with the radio jet. We study the properties of the absorbing material using CLOUDY and find that photoionization models with solar abundance ratios (with overall metallicity in the range 0.1 Z /Z 1.3) are enough to explain the observed column densities of all the species except Ne VIII, detection of which requires confirmation. Because the cooling and recombination time for the gas is 105 yr, the consistency with the photoionization models suggests that any possible interaction of absorbing material with the jet must have taken place before 105 yr. The abundance ratio of nitrogen to carbon is close to solar values, unlike in the case of most quasars, especially at high redshifts, which have supersolar values. We observed 3C 48 with the Giant Metrewave Radio Telescope (GMRT) to search for redshifted 21-cm H I absorption. We did not detect any significant features in our spectra and estimate the 3 upper limit to the optical depth to be in the range 0.001-0.003. However, due to the diffuse nature of the radio source, optical depths as high as 0.1 towards individual knots or compact components cannot be ruled out. 1 I N T R O D U C T I O N Compact steep-spectrum sources (CSSs) are defined to be radio sources with projected linear size 20 kpc ( H 0 = 71 km s1 Mpc1, m = 0.27, = 0.73; Spergel et al. 2003) and having a steep high-frequency radio spectrum [ 0.5, where F () ]. Highresolution radio images of CSSs show that there is a wide variety of structures, ranging from double-lobed and triple sources to those which are very complex and distorted (e.g. Phillips & Mutel 1982; Spencer et al. 1989; Fanti et al. 1990; Conway et al. 1994; Wilkinson et al. 1994; Dallacasa et al. 1995; Sanghera et al. 1995; Readhead et al. 1996a; Taylor, Readhead & Pearson 1996; Snellen, Schilizzi & van Langevelde 2000a; Stanghellini et al. 2001; Orienti et al. 2004). Although there is a consensus of opinion that most CSSs are young radio sources (Fanti et al. 1995; Readhead et al. 1996b; ODea 1998 E-mail: (NG); (RS); (DJS) for a review; Snellen et al. 2000b), a small fraction could be confined to small dimensions as a result of either a dense medium or interactions of the jet with dense clouds. Radio studies have shown that CSSs tend to be more asymmetric than the larger sources, possibly due to interaction with an asymmetric distribution of gas in the central regions (Saikia et al. 1995, 2001; Saikia & Gupta 2003). These authors speculated that this might be due to collisions with clouds of gas, some of which fuel the nuclear activity. Arshakian & Longair (2000) also concluded that intrinsic/environmental asymmetries are more important for sources of small physical sizes. Radio polarization studies often indicate large Faraday depths (e.g. Mantovani et al. 1994; Fanti et al. 2004), and also sometimes show evidence of asymmetrically located clouds as a huge differential rotation measure between the oppositely directed lobes as in the CSS quasar 3C 147 (Junor et al. 1999), or high rotation measure where the jet bends sharply suggesting collision of the jet with a dense cloud as in 3C 43 (Cotton et al. 2003). Optical studies of CSSs have also often shown evidence of interaction of the radio jets with the external medium (e.g. Gelderman & Whittle 1994; de Vries et al. 1999; Axon et al. 2000; ODea et al. 2002). The physical conditions under which a jet may be bent or distorted by such clouds, as well as the effects of an asymmetric gas distribution on opposite sides of the nucleus, have been explored using analytical calculations as well as numerical simulations (Carvalho 1998; Higgins, OBrien & Dunlop 1999; Wang, Wiita & Hooda 2000; Jeyakumar et al. 2005). An interesting way of investigating the interaction of the jet with the external clouds/medium could be via the absorption lines arising from the clouds accelerated by the jet. Baker et al. (2002) have studied the absorption spectra in a sample of quasars selected from the Molonglo Reference Catalogue, and find a slight excess of blueshifted C IV absorption lines in CSSs compared with the larger objects. In a sample of high-redshift radio galaxies, van Ojik et al. (1997) reported a high incidence of H I absorption (nine out of 10) in the smaller objects (<50 kpc) compared with the larger ones (two out of eight). Their study also showed an excess of blueshifted Ly absorption lines, and showed strong evidence of interaction of the radio source with the external environment. Although blueshifted absorption lines could arise due to either halo gas or circumnuclear gas kinematically affected by nuclear winds and/or radiation pressure, such studies suggest that jetcloud interaction may also play a significant role in these objects. To explore this theme, we have studied the well-known CSS quasar 3C 48, which has a highly complex and distorted radio structure, speculated to be due to disruption of the jet by interaction with a cloud (e.g. Wilkinson et al. 1991; Worrall et al. 2004; Feng et al. 2005). We explore evidence of blueshifted absorption line gas at radio and ultraviolet (UV) wavelengths using the Giant Metrewave Radio Telescope (GMRT) and archival, but previously unpublished, data from the Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) and the Goddard High Resolution Spectrograph (GHRS), the Far Ultraviolet Spectroscopic Explorer (FUSE) and the International Ultraviolet Explorer (IUE). We report on the detection of blueshifted absorption line gas, examine the physical properties of the absorber and discuss the possibility that this might be accelerated by interaction with the radio jet. 3C 48, the second quasar to be identified (Mathews & Sandage 1963), is an enigmatic CSS source. It has unusually strong infrared (IR) emission (Neugebauer, Soifer & Miley 1985), comparable with most luminous IR galaxies. The detection of CO emission (Scoville et al. 1993; Wink, Guilloteau & Wilson 1997) at z = 0.3695 implies the presence of about twice as much molecular gas as in ultra-luminous infrared galaxies (ULIRGs). At radio wavelengths, the very long baseline interferometry (VLBI) images resolve this highly compact radio source into a one-sided c (...truncated)