Search for HI emission from superdisk candidates associated with radio galaxies

TL;DR

This study searched for neutral hydrogen in superdisk structures around radio galaxies but found no evidence, setting upper limits on hydrogen density and challenging the idea that superdisks are HI-rich.

Contribution

First observational attempt to detect neutral hydrogen in superdisks of radio galaxies, providing constraints on their composition and ruling out HI-dominated models.

Findings

No positive HI detection in four superdisk candidates.

Upper limits on neutral hydrogen density and column density established.

Results suggest superdisks are unlikely to be primarily HI-rich structures.

Abstract

Giant gaseous layers (termed "superdisk") have been hypothesized in the past to account for the strip-like radio emission gap (or straight-edged central brightness depression) observed between the twin radio lobes, in over a dozen relatively nearby powerful Fanaroff-Riley Class II radio galaxies. They could also provide a plausible alternative explanation for a range of observations. Although a number of explanations have been proposed for the origin of the superdisks, little is known about their material content. Some X-ray observations of superdisk candidates indicate the presence of hot gas, but a cool dusty medium also seems to be common in them. If they are made of entirely or partly neutral gas, it may be directly detectable and we report here a first attempt to detect/image any neutral hydrogen gas present in the superdisks that are inferred to be present in four nearby radio galaxies. We have not found a positive HI signal in any of the four sources, resulting in tight upper limits on the HI number density in the postulated superdisks, estimated directly from the central rms noise values of the final radio continuum subtracted image. The estimated ranges of the upper limit on neutral hydrogen number density and column density are $10^{-4}-10^{-3}$ atoms per cm$^3$ and $10^{19}-10^{20}$ atoms per cm$^{2}$, respectively. No positive HI signal is detected even after combining all the four available HI images (with inverse variance weighting). This clearly rules out an HI dominated superdisk as a viable model to explain these structures, however, the possibility of superdisk being made of warm/hot gas still remains open.

Tables

Table 1. Table 1 : Basic parameters of the target radio galaxies (GKW2000 and references therein): (i) Name, (ii) Coordinates, (iii) Redshift, (iv) Distance (angular dia. distance D A subscript 𝐷 𝐴 D_{A} and luminosity distance D L subscript 𝐷 𝐿 D_{L} ), (v) Inferred superdisk size (minimum dia. d 𝑑 d and width w 𝑤 w ), and (vi) Full width at half maximum (FWHM) of the synthesized beam (see, Figure 3 ). See table footnote, for the references for z 𝑧 z , D A subscript 𝐷 𝐴 D_{A} , D L subscript 𝐷 𝐿 D_{L} , d 𝑑 d , w 𝑤 w .

Source	Coordinates		Redshift	$D_A$	$D_L$	Superdisk sizeg		FWHM
	R.A. (J2000)	Dec. (J2000)	($z$)	(Mpc)	(Mpc)	$d$ (kpc)	$w$ (kpc)	of the synthesized beam
3C 98	${03}^h{58}^m{54.0}^s$	${10}^h{26}^m{03}^s$	0.0304a	126.5d	134.3d	30	15	$56.3\text{arcsec}\times 46.7\text{arcsec}$
3C 192	${08}^h{05}^m{34.9}^s$	${24}^h{09}^m{50}^s$	0.0597b	239.5d	269.0d	80	25	$76.6\text{arcsec}\times 46.5\text{arcsec}$
4C+32.25	${08}^h{31}^m{27.5}^s$	${32}^h{19}^m{26}^s$	0.0512a	207.5f	229.3f	150	52	$55.2\text{arcsec}\times 41.5\text{arcsec}$
3C 227	${09}^h{47}^m{45.0}^s$	${07}^h{25}^m{20}^s$	0.0858c,e	334.0f	393.8f	85	33	$59.5\text{arcsec}\times 50.7\text{arcsec}$

Table 2. Table 2 : The 3 σ 𝜎 \sigma upper limits to the H  i number density, column density and mass of the putative superdisks in the four observed radio galaxies, assuming a velocity width of ∼ 100 similar-to absent 100 \sim 100  km s -1 . Note that the listed RMS noise values refer to the velocity width resulting from spectral smoothing over 15 channels, in order to obtain a velocity resolution of 100  km s -1 . These are ∼ 5 − 10 similar-to absent 5 10 \sim 5-10 times above the confusion level in the D configuration for VLA L-band observations (74 μ 𝜇 \mu Jy/beam).

Source	RMS noise	n(H i)$>$	N(H i)$>$	M(H i)
	($\mu$Jy/beam)	(cm-3)	(cm-2)	(M${}_{\text{sun}}$)
3C 98	$698$	$1.6\times {10}^{-3}$	$1.5\times {10}^{20}$	$4.2\times {10}^8$
3C 192	$456$	$3.5\times {10}^{-4}$	$8.7\times {10}^{19}$	$1.1\times {10}^9$
4C+32.25	$398$	$2.4\times {10}^{-4}$	$1.1\times {10}^{20}$	$5.4\times {10}^9$
3C 227	$745$	$7.1\times {10}^{-4}$	$1.9\times {10}^{20}$	$3.3\times {10}^9$

Full text

1 Introduction

Conspicuous radio emission gaps with sharp, quasi-linear edges have been noticed in the middle of the twin radio lobes of nearly 20 powerful double radio sources. These strip-like emission gaps (or sharp-edged brightness depressions) oriented roughly orthogonally to the axis defined by the radio lobe pair, have a typical width of about 30 kpc and length of at least 50-100 kpc, and are suspected to be caused due to a giant layer of thermal gas enveloping the massive elliptical galaxy hosting the double radio source (Gopal-Krishna & Wiita 1996, 2000; Gopal-Krishna et al. 2007; Gopal-Krishna & Wiita 2009). As argued in these papers, such postulated superdisks can provide viable alternative explanations to a range of well known phenomena associated with radio galaxies and quasars, namely (i) the radio lobe depolarization asymmetry, popularly known as Laing-Garrington effect (Laing 1988; Garrington et al. 1988); (ii) correlated radio-optical asymmetry of double radio sources (McCarthy et al. 1991); (iii) the occurrence of absorption dips in the Lyman-$\alpha$ (Ly-$\alpha$) emission profiles of high-$z$ radio galaxies with a total radio extent of up to $\sim 50$ kpc (van Ojik et al. 1997; Binette et al. 2006); and (iv) the apparent asymmetry of the extended Ly-$\alpha$ emission found associated with the lobes of high-$z$ radio galaxies (Gopal-Krishna & Wiita 2000, hereafter GKW2000). Since the sharp edges of the radio brightness gaps/depressions would only be conspicuous when the axis of the double radio source is oriented near the plane of the sky and hence the twin radio lobes do not appear partially overlapping, the (inferred) superdisks probably exist in many more radio galaxies than are actually observed.

Although the postulated superdisks (or, ‘fat-pancakes’) might play a significant role in causing several well known correlations exhibited by radio galaxies, as mentioned above, progress in understanding their nature is severely hampered due to the lack of information about their content. Early hints for the presence of dusty material in the superdisks emerged from the observed asymmetry of the extended Ly-$\alpha$ emission in high-$z$ radio galaxies and from the above cited correlated radio-optical asymmetry (GKW2000 and references therein). Such a cool component (with dusty clumps) would also be consistent with the curious result (van Ojik et al. 1997; Binette et al. 2006) that absorption dips in the Ly-$\alpha$ emission profiles of radio galaxies are mostly seen when the overall radio extent of the lobes is under $\sim 50$ kpc, i.e., comparable to the typical width of the putative superdisks (Gopal-Krishna & Wiita 1996, 2000, 2009). An observation particularly relevant to the superdisk hypothesis is the detection of an enormous H i ring (of diameter $\sim 200$ kpc) surrounding an early-type galaxy NGC 612 hosting a double radio source (Emonts et al. 2008, and references therein). Although a superdisk signature is not evident in the radio images of this particular radio galaxy, this could be understood if the aforementioned requirement of the radio axis lying close to the sky plane remains unsatisfied in this case.

Motivated by the aforementioned findings and correlations, a number of physical scenarios have been proposed wherein the superdisks are envisioned to be: (a) A layer of dusty neutral and/or ionized gas blocking (at least partially) the backflow of the synchrotron plasma of the radio lobes, which is the tidally stretched remnant of a disk galaxy captured by the elliptical galaxy that hosts the double radio source (Gopal-Krishna & Nath 1997, see also, Black et al. 1992); (b) A thermal wind bubble blown by accretion disk of the active galactic nucleus (AGN), which is sandwiched and compressed by the backflowing synchrotron plasma of the twin radio lobes (Gopal-Krishna et al. 2007); (c) A segment of the gaseous filaments of the cosmic web in which the galaxy hosting the radio source happens to reside (Gopal-Krishna & Wiita 2000, 2009). In this scheme, the putative superdisks must have already existed prior to the birth of the double radio source (see also, Hardcastle et al. 2007); (d) In a radically different scenario, superdisks are carved out as two galaxy cores, each containing a supermassive black hole (SMBH), merge and the associated pair of relativistic plasma jets undergoes a rapid precession during the later stage of the merger (Gergely & Biermann 2009). Some additional mechanisms proposed for the origin of superdisks are mentioned in §4.

While the different proposed scenarios seem plausible, choice of the leading contender(s) is contingent upon a direct detection of the putative superdisks, possibly in emission. With the aim to address this critical information deficit, we have performed a sensitive search for H i emission from four nearby FR-II (Fanaroff-Riley Class II; Fanaroff & Riley 1974) radio galaxies whose high-resolution maps show that the twin radio lobes have a nearly straight edge on the side facing the host galaxy, thus marking a strip-like radio emission gap between the lobe pair (Table 1). It is worth pointing out that although the existence of ’superdisk’ is not established through direct observations, we are not aware of any satisfactory alternative explanation for the observational results/correlations mentioned in §1. These four radio galaxies were selected from the first sample of ten superdisk candidates (GKW2000), which was assembled through a visual inspection of the then available radio galaxy images. The selected 4 galaxies are among the nearest ones in the sample.

As an example, we have displayed in Fig 1 for one of them (3C 227), the grey scale image and the radio brightness profile taken along the major radio axis. The sharply bounded central brightness depression marks the location of the superdisk inferred to be associated with this source (see also Baum et al. 1988, Black et al. 1992, and “An Atlas of DRAGNs”111http://www.jb.man.ac.uk/atlas/ edited by J. P. Leahy, A. H. Bridle & R. G. Strom, for high resolution radio continuum images of these sources). To the best of our knowledge, no targeted H I observations of superdisk candidate radio galaxies had been carried out, prior to the ones reported here.

It may be noted that out of the ten radio galaxies listed in GKW2000, deemed to possess superdisks, five lie at $z<0.1$. Here we report H i search for four of these five radio galaxies, the fifth source (3C 33) could not be observed due to scheduling considerations. The observational details of this project are provided in §2. In §3, we present the results. After a brief discussion, our main conclusions are summarized in §4.

2 Observation and data analysis

The four superdisk candidates in radio galaxies ($0.03<z<0.09$) were observed during January to March, 2013, with the NSF’s Karl G. Jansky Very Large Array (VLA) in its D-configuration, yielding a baseline coverage from $0.035$ to $1.03$ km (VLA Project code 13A-194). The total observation time was 11 hours including the calibration and other overheads (total 5 hrs including 4 hrs on-source for 3C 227, and total 2 hrs including 1.5 hr on-source for each of the remaining three sources).

To achieve a reasonable sensitivity and uv-coverage, the observations were carried out with a bandwidth of 16 MHz centered at the redshifted H i frequency and using 512 spectral channels. This yielded an overall velocity width of $\sim 3500$  km s*-1at a spectral resolution of $\sim 7$  km s-1*. The relatively high spectral resolution enabled a more effective RFI flagging. We opted for a large velocity coverage considering the total lack of a priori information about the width of the putative H i emission line. The 512 channels provided enough “line-free” (and RFI-free) channels to be able to properly subtract the continuum and thus establish a flat spectral baseline. The data reduction was done in the Common Astronomy Software Applications (CASA) package version - 4.6.0 (McMullin et al. 2007) and the Astronomical Image Processing System (AIPS) version - 31DEC17 (van Moorsel et al. 1996) following standard procedures. As a consistency check, the initial flagging and calibration for one of the targets (namely, 3C 98) were carried out both manually and using the VLA Scripted Calibration Pipeline version - 1.3.8 (McMullin et al. 2007). Subsequently, for flagging and calibration of the rest of the data, we only used the pipeline. Additional data flagging, initial imaging and self-calibration of the continuum data, and the final continuum imaging were done using AIPS. The AIPS task UVSUB was then used to subtract the continuum, CVEL to accurately convert the observed frequency to Heliocentric velocity, and thereafter, IMAGR was run on the residual data to make image cubes at various spectral resolutions. Residual spectral baseline was removed by using the task IMLIN on the spectral data cube. Finally, we visually inspected the cube for any emission signatures and to extract the spectrum of the central radio emission gap. For 3C 227, the data from the two observing sessions were analyzed separately and then combined with inverse variance weighting at the final stage subsequent to the continuum subtraction.

3 Results

Despite a fairly high sensitivity, the present VLA observations have not yielded a positive signal of H i 21 cm emission from any of the four superdisk candidates. Figure 2 displays the spectra extracted at their locations, for a $\pm 300$  km s*-1velocity range, at the original spectral resolution. Even when the spectral cubes were smoothed to a resolution of $\sim 100$  km s-1*, no significant signal of H i emission was detected for any of the four targets (Figure 3). The cross in each panel shows the central position of the emission gap at which the spectrum was extracted. We note that even after a careful flagging, calibration and imaging there are some residual low-level artifacts, probably due to uv-coverage limitations or residual RFI in some of the images. These, however, do not significantly affect the central region from where the spectra have been extracted. We set the 3-sigma noise as the upper limit to H i mass under the assumptions of optically thin line emission (Draine 2011) and a line width of 100  km s*-1*. We then used the estimated size of the putative superdisk (Table 1) to compute the corresponding upper limit to H i number density and (averaged) H i column density (by a spatial averaging performed over the inferred superdisk region). These results are summarized in Table 2. It should be remembered that the size estimate for each superdisk is quite approximate, as the superdisk may extend even beyond the lateral extent of the radio emission gap. In each case, the spatial resolutions of the present observations is such that the synthesized beam size is very similar to the size of the emission gap and hence the beam should encompass much of the putative superdisk. Lastly, in a bid to further enhance the sensitivity, we have combined the final four H i images, all smoothed to a common resolution of $76.6\arcsec\times 76.6\arcsec$ (the largest among the synthesized beams for the four sources), with inverse variance weighting. However, as seen from Figure 4, even this stacking of the H i spectra of the four superdisk candidates has not yielded a significant H i signal.

4 Summary and discussion

The present VLA H i imaging observations have provided the first information on the H i content of the putative superdisks, For this, we have carried out H i imaging of a sample of four nearby radio galaxies in which a superdisk has been inferred on the basis of high-resolution radio continuum maps (Table 1; §1). Although the derived upper limits to H i mass ($10^{8}-10^{9}$ $M_{\sun}$) for these superdisks are similar to the H i content of the Milky Way, the corresponding average H i number densities have upper limits of only $10^{-4}-10^{-3}$ atoms per cm3, thanks to the immense volumes of the inferred superdisks. We reiterate that the superdisk dimensions adopted here are approximate as they refer only to the region of the strip-like radio emission gap (or brightness depression) seen between the radio lobe pair; in reality the superdisk may extend well beyond the lobe width, resulting in even tighter upper limits on H i number densities. Considering this, upper limits on the average H i column density might be more meaningful (Table 2). Thus, while extension of H i searches to more superdisk candidates and to deeper levels would be very desirable, the present study does point to a scarcity of H i in superdisks, the highly suggestive evidence for dust in superdisks notwithstanding (§1). Here it may be mentioned that the presence of dust or dusty globules/filaments, per se, does not rule out a superdisk medium dominated by a warm/hot gas. For instance, dust emission has been convincingly detected in the central parts of the X-ray emitting clusters of galaxies (e.g., Yamada & Kitayama 2005; Montier & Giard 2005; Kitayama et al. 2009). It is also important to clarify that our observations do not exclude the existence of a small, kpc-scale H i disk in these galaxies. For comparison, the typical H i column density of such disks can be of the order of $0.6-3.1\times 10^{19}$ atoms per cm2 (Serra et al. 2012), which would remain undetected with the surface-brightness sensitivity of the present wide-beam images targeted at much larger H i structures (i.e., superdisks) that are postulated to explain the observed huge radio emission gaps. The present observations only imply that the volume density of H i in the putative superdisks must be at least an order of magnitude lower than that estimated for the kpc-scale H i disks seen in some early-type galaxies (e.g., Serra et al. 2012).

One class of gaseous structures that might have some bearing on the superdisk scenario is the so called “gas belts” (Mannering et al. 2013, and references therein). These X-ray detected structures have been identified in a few radio galaxies located in group/cluster-like environments, namely 3C 35 (Mannering et al. 2013), 3C 285 (Hardcastle et al. 2007), 3C 386 (Duffy et al. 2016), and 3C 442A (Hardcastle et al. 2007; Worrall et al. 2007). Broadly, the belt-like structures of hot gas associated with these radio galaxies are seen to overlap with the radio gap/depression between their twin radio lobes and are oriented roughly orthogonally to the axis defined by the radio lobe pair. No consensus has yet emerged about their formation mechanism. Hardcastle et al. (2007) have argued that the central X-ray ridge, i.e., the gas belt, in 3C 285 is a ridge of thermal plasma that pre-existed the commencement of the radio activity, an inference also reached in Gopal-Krishna & Wiita (2009), from the observed greater lobe-length symmetry when measured relative to the mid-plane of the superdisk, rather than when referenced to the host galaxy. On the other hand, for the case of 3C 442A, Worrall et al. (2007) have argued that the gas belt is the hot gas being stripped from the merging galaxies and is getting aligned into a belt shape as it pushes the twin radio lobes straddling it (see also, Gopal-Krishna et al. 2007, for a scenario involving a dynamical interaction of the hot thermal gaseous halo with the radio lobe pair). A yet another explanation, advanced by Mannering et al. (2013) for the case of 3C 35, posits that its gas belt has formed out of the gas belonging to the fossil galaxy group but driven outward due to the expansion of the twin radio lobes. More recently, a temperature gradient has been detected along the projected 53 kpc length of the gas belt observed in 3C 386, with a cooler gas core ($T\sim 0.73\pm 0.09$ keV) found closer to the host elliptical and the hotter ($T\sim 1.72\pm 0.46$ keV) outer portion of the belt interpreted as the group’s atmosphere which is undergoing an inward flow triggered by the buoyant rise of the twin radio lobes through the circumgalactic medium (Duffy et al. 2016). While all these physical processes may be operating to varying degrees, the striking straightness of the lobes’ brightness contours marking the edges of the central radio emission gap/depression which we identify as superdisk, continues to pose a challenge and its origin remains to be properly understood. Perhaps even more enigmatic are the several radio galaxies, highlighted in Gopal-Krishna & Wiita (2009), where the host elliptical galaxy is located nearly at one edge of the strip-like central radio depression (i.e., superdisk), or even outside it, i.e., within a radio lobe (this puzzling extreme situation is witnessed in the radio galaxy 3C 16, e.g., Harvanek & Hardcastle 1998). Since superdisks are likely to contain both ionized and dusty cool gas, a multi-pronged observational follow-up will be important, including the possibility of H i search through sensitive absorption measurements against any available background radio source.

Acknowledgements.

We are grateful to Eric Greisen, Sanjay Bhatnagar, Arnab Rai Choudhuri and Prateek Sharma for useful discussions. This research has made use of NASA’s Astrophysics Data System. The results reported here are based on observations made with the Karl G. Jansky Very Large Array of the National Radio Astronomy Observatory. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. Most of the work was done when AA was at Indian Institute of Science as a undergraduate student. NR acknowledges support from the Infosys Foundation through the Young Investigator grant. G-K thanks the Alexander von-Humboldt Foundation for financial support and Prof. A. Zensus, Director, Max-Planck-Institut für Radioastronomie, Bonn for the local hospitality.