EDP Sciences
Free Access
Issue
A&A
Volume 507, Number 2, November IV 2009
Page(s) 757 - 768
Section Extragalactic astronomy
DOI https://doi.org/10.1051/0004-6361/200912546
Published online 15 September 2009

A&A 507, 757-768 (2009)

H I in nearby low-luminosity QSO host galaxies

S. König1 - A. Eckart1,2 - M. García-Marín1 - W. K. Huchtmeier2

1 - I. Physikalisches Institut, Universität zu Köln, Zülpicher Strasse 77, 50937 Köln, Germany
2 - Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received 20 May 2009 / Accepted 2 September 2009

Abstract
We searched for 21 cm H I emission in a sample of 27 previously CO detected nearby galaxies hosting low-luminosity quasi - stellar objects (QSOs). In this paper we investigate the relationship between the H I and infrared properties of these host galaxies, compare the atomic and molecular gas content and look for connections to the optical and FIR properties. The single dish observations have been made with the Effelsberg 100-m telescope with a beam size of 9.5$^\prime $. The sample objects have been drawn from a wide-angle survey for optically bright QSOs (HES), which have declinations $\delta >-30\hbox{$^\circ$ }$ and redshifts up to z = 0.06. 12 host galaxies from the sample have been detected in the H I 21 cm emission line. Eight of them have a spiral geometry, whereas the other four are bulge dominated and probably of elliptical type (E/S0). Three of the objects seem to be in a phase of merging/interaction. The neutral atomic gas masses range from $1.1 \times 10^{\rm 9}~M_{\hbox{$\odot$ }}$ up to $3.8 \times 10^{\rm 10}~M_{\hbox{$\odot$ }}$. The median H I gas mass in the whole sample is of the order of $11.4 \times 10^{\rm 9}~M_{\hbox{$\odot$ }}$, which is a factor of two higher than the H I content of our galaxy. We find no strong correlation between H I mass and IR luminosity. The objects agree well within the expectations from the Tully-Fisher relation. In the color-color diagram we find all sources in the estimated locations. With the non-detected sources we clearly sample an upper envelope of this mass distribution.

Key words: galaxies: active - radio lines: galaxies - galaxies: nuclei - galaxies: Seyfert

1 Introduction

Molecular gas is the fuel for any activity process, whether it is in the form of vigorous bursts of star formation or an intensively accreting super massive central black hole (active galactic nuclei, AGN). The study of its morphology and kinematics is of key importance to understand the fueling mechanisms that keep the activity alive over cosmologically significant time scales. As important as studying the ongoing activity processes is studying the trigger that leads to it in the first place. It is assumed that galaxy interaction thereby plays a dominant role, initializing the transport of gas from the outer parts of the involved galaxies into their centers and so igniting the starburst and/or AGN activity. Sanders et al. (1988) already suggested early on that there might be even an evolutionary sequence between the onset of starburst and AGN activity with starbursts as precursors for AGN. This hypothesis is mainly based on the remarkable resemblance between luminous starburst galaxies (so called ultra-luminous-infrared-galaxies, or ULIRGs) and galaxies with a pronounced AGN signature (so called QSO host galaxies), such as their enormous amounts of molecular gas. However, while ULIRGs often exhibit significant signs of galaxy interaction, this is not necessarily true for QSO host galaxies. In fact, optical studies on QSO host galaxies (e.g., Boyce et al. 1998; Bahcall et al. 1997) find the majority to be rather morphologically normal without any clear signs of a past or ongoing interaction, though $\sim$50% of their sample live with (projected) nearby companions. Much effort has been invested in studying the molecular gas emission in QSO host galaxies (e.g., Krips et al. 2006b; Bertram et al. 2007; Scoville et al. 2003; Krips et al. 2006a). Atomic hydrogen, on the other hand, tracing the cold atomic molecular gas, may be a much better indicator for galaxy interaction. Only little is known so far on the atomic gas content in QSO host galaxies. Lim & Ho (1999), Lim et al. (2001) and Ho et al. (2008a,b) present one of the first studies of H I emission in nearby QSO host galaxies. They find that while most of their observed sources show signs of a past or ongoing tidal interaction, they do not yet physically merge with another galaxy. This is in contrast to the proposed sequence by Sanders et al. (1988) in which QSOs are thought be the merging or merged descendants of ULIRGs. In this paper we present the study of neutral atomic hydrogen in 27 nearby low-luminosity quasi-stellar objects. The observations are part of an ongoing project aimed at enhancing the statistics on nearby QSOs.

In Sect. 2 we describe the sample selection, Sect. 3 reports on the observations and data reduction. The results of the observations and the conclusions/discussion follow in Sects. 4 and 5.

Unless otherwise stated, $H_{\rm0} = 75$ km s$^{\rm -1}$ Mpc$^{\rm -1}$ and $q_{\rm0}=0.5$ are assumed throughout the paper.

Table 1:   List of sources observed at 21 cm.

2 The sample

We have selected a volume limited sample of sources. As the sole selection criterion an upper redshift limit of z < 0.06 was applied. This value has been set to ensure the observability of the important diagnostic CO (2-0) rotation vibrational band head absorption line in the K-band. The sources have been selected from the Hamburg / ESO survey of optically bright QSOs (HES; Wisotzki et al. 2000). These 27 sources, with a declination $\delta > - 30$$^\circ$(by means of the observability from the Effelsberg 100-m telescope), have been searched for CO emission with the IRAM 30 m telescope on Pico Veleta (Spain) and SEST (Swedish ESO-Submillimeter Telescope) in La Silla (Chile). 27 objects (Table 1) were detected in 12CO(1 - 0) and 12CO(2 - 1) (see Bertram et al. 2007). They feature recession velocities in the range 2500 km s $^{-1}\!\!<\!\!v\!\!< 18~000$ km s-1. Bertram et al. (2007) confirmed that the majority of galaxies hosting low-luminosity QSOs are rich in molecular gas. In 2007 and 2008 we searched this subsample for 21 cm H I emission. This sample complements the study of Ho et al. (2008a). But contrary to their sample our sources have positions further to the south.

A more detailed description of the ``nearby QSO sample'' can be found in Bertram et al. (2007).

\begin{figure}
\par\includegraphics[width=15.5cm,clip]{12546f1a.ps}
\end{figure} Figure 1:

H I spectra of detected host galaxies from the ``nearby QSO sample'' observed with the Effelsberg 100-m telescope and optical DSS images. The images in the middle extend over 2$^\prime $  and the ones to the right contain 9.5$^\prime $, which is roughly the size of the beam at 21 cm. North is up and east to the left. Each source is identified by its HES name ( top left corner of the spectrum) and the redshift ( top right corner). The spectral resolution of the spectra in this figure range from 0.3 km s$^{\rm -1}$ up to 32 km s$^{\rm -1}$. If regions in the spectrum are affected by RFI (Radio Frequency Interference) and are not used for baseline-fitting and noise estimates, they are set to zero intensity.

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\begin{figure}
\par\includegraphics[scale=0.92,clip]{12546f1b.ps}
\vspace*{4mm}
\end{figure} Figure 1:

continued.

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\begin{figure}\par\includegraphics[scale=0.92,clip]{12546f1c.ps}
\vspace*{5mm}
\end{figure} Figure 1:

continued.

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3 Observations and data reduction

The observations of the H I 21 cm emission line were carried out with the Effelsberg 100-m telescope in 2007 and 2008. The 18-21 cm (1.29-1.72 GHz) two - channel L-band HEMT facility receiver was used as the frontend, in conjunction with the 8192-channel-autocorrelator (AK 90) and the Fast Fourier Transform Spectrometer (FFTS) as backends. With bandwidths of 10 MHz (AK 90) and 20 MHz (AK 90 + FFTS), centered on the redshifted rest frame frequency for each galaxy, we were able to obtain data with velocity ranges $\Delta$v from $\sim$4200 to $\sim$21 000 km s$^{\rm -1}$ and a velocity resolution between 0.2577 and 2.061 km s$^{\rm -1}$, depending on the backend and settings. The measurements were done in position - switch mode with 5 mn on and 5 mn off the source positions, with an off - position 45$^\prime $  away. This provides the advantage of better baselines and the reduction of atmospherical influences. The beam efficiency was $\sim$1.15 for a beam size of 9.5$^\prime $  at 21 cm. The on-source integration times range between 30 and 90 minutes. Daily pointing checks were performed using sources from the Effelsberg Catalog of pointing and flux density calibration (Ott et al. 1994). As flux calibrators we used Holmberg I, NGC 4303, Sextans B and UGC 2345 (fluxes taken from Tifft & Huchtmeier 1990 and from Springob et al. 2005).

The Effelsberg 100-m telescope site is known for exhibiting low level radio frequency interference (RFI) in the 21 cm band. However as the RFI signals are situated well outside the range of the expected central velocity or can be filtered out, these disturbances have no influence on the observational results.

Table 2:   Summary of the H I properties.

\begin{figure}
\par\includegraphics[origin=rb,angle=-90,width=12cm,clip]{12546fg2.ps}
\end{figure} Figure 2:

H I mass as a function of infrared luminosity for the nearby QSOs. Stars represent the sample from this paper. Squares and triangles represent sources taken in order to enhance the statistics from Ho et al. (2008a): squares represent the sources from the Ho sample itself and triangles represent a literature sample assembled by Ho et al. (2008a). Filled squares and triangles mark available values for $L_{\rm IR}$, whereas open ones with arrows represent upper limits of $L_{\rm IR}$ or $M_{\rm H {\sc i}}$. Whereas filled stars for our sample represent H I and CO detected sources and unfilled ones denote upper mass limits for the CO but not H I detected sources. Arrows show upper limits for mass and infrared luminosity.

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The data were reduced and analyzed with the CLASS package of IRAM's GILDAS[*] software. All spectra have been averaged, when applicable. To obtain a better signal to - noise ratio, most of the spectra were Hanning smoothed. For data taken on different dates the subscans of each observation were calibrated with respect to a suitable flux calibrator (see Fig. 1 and Table 1). Baselines were fitted and subtracted from each subscan individually before resampling and averaging of the spectra. Another baseline fit was performed after the final resampling and averaging of all available data for the source. All polynomial fits to the baselines have been of the order of one. The errors $\Delta I$ for the given intensities I were determined following the procedure from Bertram et al. (2007). The geometric average of the line error $\Delta I_{\rm l}$ and the baseline error $\Delta I_{\rm b}$ were taken into account.

4 Analysis

The galaxies observed during this survey do not form a complete sample, but the results can still be used to improve our knowledge of the population of nearby AGNs. In order to derive the atomic neutral hydrogen gas mass $M_{\rm H {\sc i}}$, the following formula (e.g., Shostak 1978) was applied:

\begin{displaymath}M_{\rm H {\sc i}} = 2.36 \times 10^{\rm 5} D^{\rm 2}_{\rm L}~\int~S_{\nu}~{\rm d}v~M_{\hbox{$\odot$ }}.
\end{displaymath} (1)

$D_{\rm L}$ denotes the luminosity distance in Mpc and $\int~S_{\nu}~{\rm d}v$ the flux integral in Jy km s$^{\rm -1}$. For a comparison we measured the dynamical mass via

\begin{displaymath}M_{\rm dyn} =\frac{v_{H {\sc i}}^2~r}{G}~{M_{\hbox{$\odot$ }}}
\end{displaymath} (2)

( $v_{H {\sc i}}$ is the velocity width from which 90% of the H I line emission arises and r is the estimated H I radius) under the assumption of a disk geometry in virial equilibrium, which is appropriate for gas rich objects. Since the size of the H I extension is not known we estimated the area of the sources from the DSS images (Fig. 1) by circular apertures. These apertures were centered on the nucleus of each galaxy. The optical radius was taken as the value where the counts of the galaxy emission in the DSS image were 3$\sigma$ larger than the background. This radius was multiplied by the factor for the median $r_{H {\sc i}}$/ $r_{\rm opt}$ of 1.50 from Haan et al. (2008), and assuming a circular area we then derived the area by $A = \pi ~r^{\rm 2}$ which in turn was used for the determination of the dynamical mass. As expected the values for $M_{\rm dyn}$ are up to two orders of magnitude larger than the H I masses determined from the integrated intensity (Table 2). The infrared luminosity was determined from the IRAS fluxes (12, 25, 60 and 100 $\mu $m fluxes were taken from the IRAS Faint Source Catalog, Moshir et al. 1990) and with the following formulae (Sanders & Mirabel 1996):

\begin{displaymath}F_{\rm IR} \!=\! 1.8 \times 10^{\rm -14}~\left(13.48~f_{{\rm ...
...{\rm 60}}\!+\!f_{{\rm 100}}\right)~~\frac{{\rm W}}{{\rm m}^2}
\end{displaymath} (3)

\begin{displaymath}L_{\rm IR} = 4 \pi~D_{\rm L}^2~F_{{\rm IR}}~~~ {L_{\hbox{$\odot$ }}}.
\end{displaymath} (4)

These results are summarized in Table 2. Given are the integrated line intensity $I_{H {\sc i}}$ (Col. 3), the line width at half maximum intensity $W_{\rm 50}$ (Col. 4), an estimate on the line shape (Col. 5), the atomic neutral hydrogen mass $M_{H {\sc i}}$ (Col. 6), the dynamical mass deduced from the H I data (Col. 7), the total hydrogen gas mass of the host galaxy (Col. 8), as well as the molecular to atomic gas mass fraction (Col. 9) and the infrared luminosity $L_{\rm IR}$ (Col. 10).

5 Results and discussion

In Fig. 1 we show the obtained H I spectra together with the associated DSS images of the region with a diameter of 2$^\prime $  and 9.5$^\prime $  around the source position. The images should serve as a means to estimate the source confusion. The spectrum of HE 1248-1356 is remarkable in the sense that the velocity range 4200-4350 km s-1 shows strong CO emission (see Bertram et al. 2007) but no H I emission. As this source has a close companion (see the DSS images), it is possible that the H I gas was partially more disturbed/stripped/disrupted, by the companion. This could cause the ``one-sidedness'' of the spectrum indicating that the part of the host galaxy moving towards us has less neutral atomic hydrogen than the one receding. Why wasn't the CO affected? It could just be that the disturbance was only affecting the more outwards lying H I gas and not yet the molecular CO in former times. Since then H I was converted into H$_{\rm 2}$ (CO) and the H I line in its present shape could be a remnant signature. The coordinate labels of RA and Dec (J2000), as well as the flux in Jy and the velocity in km s$^{\rm -1}$ are indicating the scales. Each source name is denoted above the spectrum. The H I properties and results are summarized in Table 2: Integrated line intensities cover a range from 0.5 up to 7.6 Jy km s$^{\rm -1}$. The intensity upper limits and errors for non-detections represent 3$\sigma$ values. The line widths ( $W_{\rm 50}$) are of the order of hundreds of km s$^{\rm -1}$. The sources HE 0232-0900 ( $W_{\rm 50} = 559$ km s$^{\rm -1}$) and HE 2302-0857 ( $W_{\rm 50} = 338$ km s$^{\rm -1}$) are two special cases: They both have extraordinary large line widths both in CO (FWZI: 597 km s$^{\rm -1}$ and 653 km s$^{\rm -1}$, from Bertram et al. 2007) and H I emission. The H I line emission detected from HE 2302-0857 is even more extraordinary if we look at the full width at zero intensity (FWZI) of the H I line, with a width of 616 km s$^{\rm -1}$, which is comparable to the line width (FWZI) in CO ( $\Delta v_{\rm CO} = 653$ km s$^{\rm -1}$), although the galaxy is not seen edge-on. For HE 0232-0900 however the H I line is narrower than the CO line. This indicates that the outer regions of the galactic disk were stripped from H I, or are simply depleted from the atomic gas component. As the DSS images (Fig. 1) show a peculiar morphology of this object it is also possible that the current geometry is due to a previous interaction/merger with another galaxy. The H I masses range between 1.1 and $37.5 \times 10^{\rm 9}~M_{\hbox{$\odot$ }}$, whereas the upper limits for the dynamical masses have values from $0.9 \times 10^{\rm 10}$ up to $1.3 \times 10^{\rm 12}~M_{\hbox{$\odot$ }}$.

Figure 2 shows a plot of the atomic hydrogen mass versus infrared luminosity containing sources from this work and the work of Ho et al. (2008a). In comparison to Ho et al. (2008b) we concentrate more on the radio - IR properties, rather than the optical H I relations. The position of the objects in this graph are in good agreement with the work of Young et al. (1989) about infrared bright galaxies: In general galaxies with high IR luminosities have high neutral atomic hydrogen masses. Although their work places an emphasis on even more nearby sources ( $D_{\rm L} =36.3$ Mpc), the mean luminosity distance for the enlarged sample (this work + Ho et al. 2008a) plotted here was 219.5 Mpc. The comparison of these results shows no evolutionary effect in redshift due to $M_{\rm H {\sc i}}$ or $L_{\rm IR}$ (see also Table 3 and Haan et al. 2008). The comparison between the molecular gas content (taken from Bertram et al. 2007) and the atomic one results in a median fraction of $L^{\prime}_{\rm CO}$/ $M_{\rm H {\sc i}} = 0.06$ for our sample (including the upper limits for the non-detections). Maiolino et al. (1997) measured a value of 1.79 for their $M_{\rm H_2}$/ $M_{\rm H {\sc i}}$ (for galaxy types up to Sy 1.5). Since we are not sure if Bertram et al. (2007) and Maiolino et al. (1997) used the same CO - to - H$_{\rm 2}$ conversion factor we simply compare the $L^{\prime}$$_{\rm CO}$ - to -  $M_{\rm H {\sc i}}$ ratios. The median $L^{\prime}$$_{\rm CO}$/ $M_{\rm H {\sc i}}$ value given by the data of Maiolino et al. (1997) results in 0.14, which is about a factor of two higher than the value obtained for our sample. Since the median CO luminosity $L^{\prime}$$_{\rm CO}$ is about the same in both samples, $5.55 \times 10^{\rm 8}$  $M_{\hbox{$\odot$ }}$ for Maiolino et al. (1997) and $5.40 \times 10^{\rm 8}$  $M_{\hbox{$\odot$ }}$ (CO data taken from Bertram et al. 2007), the galaxies in their sample are less rich in atomic hydrogen than the ones in this study. In a more recent publication Haan et al. (2008) compiled a study of H I properties in AGN host galaxies taken from the NUGA sample. We compared these sources, which lie a step lower in redshift range ( $D_{\rm L} = 6.6$ ... 53.9 Mpc, see Table 3), with our sample. They find more ring structures and dynamically disturbed H I disks in LINERs than in Seyfert host galaxies. Since our sample consists only of Seyfert galaxies we just account for part of the Haan sample.

Table 3:   Sources from Haan et al. (2008).

The distribution of data points in Fig. 2 shows that we are sampling an upper envelope of the $M_{\rm H {\sc i}}$/ $L_{\rm IR}$ distribution. The sources that we detected in H I emission are those that have the largest amount of atomic hydrogen, compared to the molecular gas mass, in their hosts. This could be an explanation for the lack of more H I detections in this work. Upper limits of the non-detected sources show, that with deeper and longer integrations we would be able to correct for this deficiency in our observations.

\begin{figure}
\par\includegraphics[origin=rb,angle=-90,width=12cm,clip]{12546fg3.ps}
\end{figure} Figure 3:

H I flux vs. CO intensity (taken from Bertram et al. 2007). Crosses mark the upper limits of H I non-detected sources. Filled circles represent the intensities for detected sources. Each object is identified by a number, which is related to the source name given in the figure legend.

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In Fig. 3 we compare the H I and the CO intensities of the detected sources in our sample. Similar to Fig. 2 it is obvious from Fig. 3 that we have sampled an upper envelope of the H I mass distribution. If we consider only the certain H I detected sources from this work then a high H I mass is present almost independently of the molecular mass content. Remarkable about this plot is, that the two sources with the largest CO intensity values (HE 0433-1028 and HE 1108-2813) are not detected in atomic hydrogen. This could mean that most of the available neutral atomic gas was already converted to molecular gas, i.e. H$_{\rm 2}$ (CO). The number of upper limits in the H I line strength is larger for galaxies with low molecular gas content. This implies that a correlation may be present, such that low molecular gas mass also implies a low atomic gas mass. On a statistical basis the H I detection fraction above and below the median value of the molecular gas masses (taken from Bertram et al. 2007) could give a hint for the confidence limit of this trend in our sample. The median value results in $M_{\rm H_2} = 2.10 \times 10^{\rm 9}$  $M_{\hbox{$\odot$ }}$. Then we studied the distribution of the H I masses of the sources above the median of the molecular gas mass and below it. The median H I mass above the median molecular gas mass is $M_{\rm H {\sc i}} = \big(9.24 \pm 4.84\big) \times 10^{\rm 9}$  $M_{\rm\hbox{$\odot$ }}$ and the median H I mass below the median molecular gas mass is $M_{\rm H {\sc i}} = \big(2.22 \pm 0.56\big) \times 10 ^{\rm 9}$  $M_{\rm\hbox{$\odot$ }}$. These calculations do not take the non-detections into account. We defined estimated values for error bars by determining the median deviation from the medians of the H I masses. Taking these error bars into account, we find that the sources above the median molecular mass also have the larger H I masses, the sources below the median molecular gas mass have lower H I masses. Within the error bars the two populations of H I detected sources do not overlap. Nonetheless our sample is very small and hence not suitable for making statistically significant statements.

5.1 Source confusion

We can not exclude the possibility of source confusion (see Fig. 1). The half power beam width (HPBW) at 21 cm is 9.5$^\prime $. Since the whole beam lobe is (reduced) sensitive for radio signals out to twice the primary beam diameter, an H I rich galaxy at, the appropriate red shift, can cause disturbances out to angular distances of up to 18$^\prime $. Asymmetries in the line profiles could possibly indicate the presence of companions or phases of merging that influence the host galaxies. One help to shed light on this matter may also be provided by the DSS images (see Fig. 1). Here we can look for indications for interactions or companions within a field of view representing the 9.5$^\prime $  Effelsberg beam size. An outstanding example for this circumstance is the source HE 0150-0344. Its H I spectrum shows two emission lines close in velocity. In the DSS image the source shows an elongated structure with two peaks, indicating two very close by sources. With the large Effelsberg beam we are not able to distinguish between the objects. Due to the close proximity of the sources interferometrical data would be needed to achieve the differentiation. Source confusion has a strong influence on the line shape, as well as on the line width, as mentioned earlier. Two objects, which exhibit emission at the observed frequency, close in redshift, hence also in recession velocity would cause a broadening of the studied emission line (always under the assumption that the two objects are galaxies). Therefore the target source would feature a much broader line width than it shows in reality (see Fig. 1, HE 0150-0344). This would then cause wrong numbers for the observed intensity and the resulting gas mass. Cross-checking with other observational tools, for e.g., optical images, is therefore essential to get the right result.

5.2 Morphology

A comparison of the shape of the spectra shows good agreement between line shape and associated morphology in e.g., the optical (Fig. 1, Tables 1, 2): for 11 of the 12 detected sources (the exception is HE 1248-1356) we find that the morphology from the optical DSS images and the line shape determined from the H I spectra are the same. But one has to remark that the asymmetry of the line shape in some of the spectra makes it difficult to achieve an accurate differentiation. From the optical DSS images (Fig. 1) we find spiral morphology in 15 (55%) host galaxies, elliptical morphology in 11 (41%) host galaxies and a ring morphology in 1 (4%) host galaxy. In 33% of the sources the H I spectra show triangular shaped line profiles, that is expected for turbulent line-of-sight dispersion (as in elliptical galaxies), and 67% have boxy or double-horn shaped line profiles, which is an indicator for emission from an inclined, rotating disk (as in spirals). In comparison Haan et al. (2008) find that 44% of the sources in their AGN sample are of spiral morphology, 28% are centrally peaked, 22% ringed and 5% of their host galaxies show a warped geometry. This hints at a preference for spirals in terms of morphological properties of QSO host galaxies. In their Seyfert subsample a slight excess of galaxies with a spiral geometry is evident.

\begin{figure}
\par\includegraphics[origin=rb,angle=-90,width=12cm,clip]{12546fg4.ps}
\end{figure} Figure 4:

Infrared luminosity $L_{\rm IR}$ vs. compactness f. Filled circles represent the H I detected sources from this work, filled stars however reperesent detected objects form the sample of Haan et al. (2008). Unfilled symbols with arrows mark sources with upper limits for their infrared luminosities. Each detected object from this work is identified by a number, which is related to the source name given in the figure legend.

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In 1980 Dressler discovered a morphology-density ($T-\Sigma$) relation for galaxies: the fraction of elliptical like (E + S0) galaxies increases strongly with increasing local galaxy density, whereas the numbers of spiral galaxies decrease (see e.g., Table 1 in Pimbblet 2003). In good agreement with further studies of this relationship he also derives values for different environments: from galaxies in the field over poor and rich groups to clusters. Furthermore Dressler et al. (1997) report a redshift evolution in the $T\!\!-\!\!\Sigma$ relation: an additional increase of the fraction of S0 galaxies with redshift whilst the number of elliptical galaxies stays nearly constant. The distribution of spiral, elliptical (E) and lenticular (S0) galaxies in our H I data is comparable to the numbers for field galaxies or poor groups. Dressler et al. (1985) expand the statistical studies of clusters by also looking at emission line galaxies. They find a much higher emission line frequency in field galaxies than in cluster galaxies. AGN occur at a rate of $\sim$5% in field galaxies whereas in clusters only $\sim$1% of the galaxies harbor an AGN. Martini et al. (2006) searched for low-luminosity AGN in low redshift clusters. They determined an AGN fraction of $\sim$5%, in clusters arguing that through X-ray emission more AGN towards lower luminosities can be identified. They concluded that if the AGN fraction in the field is indeed 5 times the AGN cluster fraction they would identify $\sim$25% of the field galaxy population to show characteristics of AGN. On the other hand, Popesso & Biviano (2006) find an average AGN fraction of 18% in clusters. They, too, argue that their estimate includes low-luminosity AGN, probably missed in previous studies. Scaling the $T\!\!-\!\!\Sigma$ relation from Dressler (1980) to AGN this would, by a strong simplification, mean that most AGN in our sample reside in host galaxies of spiral morphological type, whereas in clusters the fraction of AGN in elliptical and lenticular hosts would increase significantly.

For analytical purposes we introduce an estimate for the compactness of gas confined to the host galaxies within our sample by the derivation of the fraction of the CO line width to the H I line width as follows:

\begin{displaymath}{\rm compactness} f = \frac{v_{\rm 90}\left({\rm CO} \right)}{v_{\rm 90}\left(H {\sc i}\right)} \cdot
\end{displaymath} (5)

$v_{\rm 90}$ is the velocity width from which 90% of the emission arises. We show the values for the compactness in Table 2. We want to study the distribution of CO and H I in the host galaxies with this quantity f. We assume that H I is found all over the place. If CO is mostly limited to the central inner region, filling the flat part of the rotation curve, f will be smaller than 1. For f = 1 the line widths in both emission lines should be equally large. Galaxies with compactness factors f > 1 have broader line widths in CO than in H I. As we see a large fraction of objects with companions or signs of interactions f may be used as an additional indicator for dynamical peculiarities (due to merging or disturbance). But as the beam size for the 21 cm observations was very large, other galaxies within the beam size contribute to the detected emission. Here the exceptional host galaxies HE 0232-0900 and HE 2302-0857, with their very broad emission lines at 21 cm have to be mentioned again. No clear trend could be seen in Fig. 4. There we plotted the infrared luminosity against the determined compactness of each object. All sources from Haan et al. (2008) have compactness factors close to one, except for NGC 5953 and NGC 4321, which show factors f < 1 (Table 3). Usually a compactness factor close to one is expected as the CO distribution normally reaches the maximal rotation velocity. In this sense our sample is exceptional. For example the sources HE 0045-2145, HE 0224-2834, HE 0949-0122, HE 1248-1356 and HE 2302-0857 exhibit compactness factors strongly differing from a value of f = 1. But as the fraction of objects with companions and/or indications for mergers or interactions is extraordinary high in this source sample a compactness of f < 1, together with the line shape, may also provide early hints for distortions in the outer regions of the host galaxies. The best example for this issue is HE 0224-2834. The small compactness of 0.3 means that the atomic gas is much more extended than the molecular gas. The asymmetry in the shape of the H I line tells us that there are distortions in the outer H I disk. The comparison to the optical DSS images shows that we are right, HE 0224-2834 has a close companion and it shows morphological signs of inter action. The same holds true for HE 0045-2145: Its line shape is asymmetrical, the compactness factor is only 0.6 and in the DSS images there is another galaxy close by. Hence the H I distribution can serve as an early indicator for interactions. The compactness for objects with spiral features seems to be lower (the CO emitting regions have a smaller extension than H I emitting ones) as they have a larger outspread than the ones showing an elliptical geometry.

As another analytical tool we use the surface mass density:

\begin{displaymath}\Sigma = \frac{M_{\rm H {\sc i}}}{\rm area} \left[{M_{\hbox{$\odot$ }}~{\rm pc}^{-2}}\right].
\end{displaymath} (6)

The area of the sources again was estimated from the DSS images (Fig. 1, see Sect. 4 again for the description of the area extraction process). The sample sources show areas between 55 and 76 000 $^{\prime\prime}$2 resulting in $\Sigma$ values of 1.0 up to 434  $M_{\hbox{$\odot$ }}$ pc$^{\rm -2}$. The median surface mass density, not taking the non-detections into account, is of the order of 20.1  $M_{\hbox{$\odot$ }}$ pc$^{\rm -2}$, which is a factor of two lower compared to the value estimated for the Milky Way (50-75  $M_{\hbox{$\odot$ }}$ pc$^{\rm -2}$, Romano et al. 2000). The data from Haan et al. (2008) however result in an even lower average surface mass density of 1.77  $M_{\hbox{$\odot$ }}$ pc$^{\rm -2}$. Here the assumption of circular morphology leads to an overestimation of the area: We use the maximum H I radius and assume a circular morphology of the host galaxy. Since the galaxies usually are not circular but more elliptical or exhibit other structures we make an imprecise assumption of the morphology. This leads to significantly smaller surface mass densities for the sources than expected.

\begin{figure}
\par\includegraphics[origin=rb,angle=-90,width=12cm,clip]{12546fg5.ps}
\vspace*{5mm}
\end{figure} Figure 5:

Blue magnitude $M_{\rm B}$ vs. rotation velocity $v_{\rm rot}$. The median value is plotted as an empty star. The dashed lines show functions for the Tully-Fisher relation from the literature (Schöniger & Sofue 1997; Ziegler et al. 2002; Sakai et al. 2000). The H I detected sources from this work are coded with grey-scale. Each object is identified with its name given in the figure legend.

Open with DEXTER

\begin{figure}
\par\includegraphics[origin=rb,angle=-90,width=12cm,clip]{12546fg6.ps}
\vspace*{5mm}
\end{figure} Figure 6:

IRAS color-color diagram according to Helou (1986). The underlying model was taken from Desert (1986). Filled circles show H I detected sources from this work, unfilled ones mark H I non-detected sources with available IRAS fluxes. Arrows are indicators for upper limits. Each object is identified by a number, which is related to the source name given in the figure legend. The location of our sample sources in the plot shows that almost all of them are starburst dominated hosts that have high dust temperatures (sources to the upper left). The galaxies to the lower right of the plot have colors that are often found for elliptical galaxies. The two sources HE 0150-0344 and HE 0212-0059 are such objects. These objects are dominated by the 100 $\mu $m emission implying lower dust temperatures.

Open with DEXTER
In Fig. 5 we show blue magnitude $M_{\rm b}$ vs. rotation velocity $v_{\rm rot}$. This plot of the Tully-Fisher relation allows us to test the quality of the FWHM values ( $W_{\rm 50}$) for the emission lines. No severe outliers are seen that could not be explained as a consequence of the inclination. As the inclinations for the sources in this sample are not defined, we can only assume a range of rotation velocities. By inserting graphs for the Tully-Fisher relation from literature sources (see e.g., Ziegler et al. 2002; Sakai et al. 2000) we can estimate where our host galaxies should be found in this graph. Since the majority of the 45$^\circ$ points as well as the value for the median are located to the left of the literature Tully-Fisher functions our objects should feature lower inclination angles of about 30$^\circ$, which is in good agreement with our assumption of a nearly face on inclination for low luminosity AGN.

5.3 IRAS color-color diagram

Figure 6 represents an IRAS color-color diagram of the sources from this sample after Helou (1986), overlaid with the computations of Desert (1986). Our sample sources fit well into the model by Desert (1986). The location of our sample sources in the IRAS color-color plot shows that almost all of them exhibit signs which are typical for starburst dominated hosts that have elevated dust temperatures i.e. show bright 60 $\mu $m emission compared to their 100 $\mu $m emission, and show faint 12 $\mu $m emission compared to their 25 $\mu $m emission. The far-IR color is particularly sensitive to foreground Galactic ``cirrus'' emission, which can cause artificially small values of $f_{\rm\nu}$(60)/ $f_{\rm\nu}$(100). Only 2 objects (HE 0150-0344, HE 0212-0059) are dominated by the 100 $\mu $m emission implying lower dust temperatures. They have colors that are often found for elliptical galaxies. A visual inspection of their images shows that an identification with an elliptical host is plausible. Objects to the upper left of this plot are thought to be starburst galaxies.

6 Conclusions

We present results on the neutral atomic gas content of 27 nearby low-luminosity QSO host galaxies. 12 of the target sources were detected in H I emission. The atomic gas masses cover a range from $1.1 \times 10^{\rm 9}$  $M_{\hbox{$\odot$ }}$ up to $3.8 \times 10^{\rm 10}~M_{\hbox{$\odot$ }}$ with a median mass of the order of $6.2 \times 10^{\rm 9}~M_{\hbox{$\odot$ }}$. Considering the upper limits for non-detections this value changes to $11.4 \times 10^{\rm 9}~M_{\hbox{$\odot$ }}$. Taking this median H I mass for the whole sample into account, the atomic hydrogen mass is a factor of two higher for our sample than the H I content of the Milky Way (Hartmann & Burton 1997, $M_{\rm H {\sc i}} = 5.5 \times 10^{\rm 9}~M_{\hbox{$\odot$ }}$). Though it is not possible to discuss the nature of the objects without high resolution imaging in more detail, we were able to draw some interesting conclusions with the help of the profiles of the H I spectra and optical DSS images. For several galaxies we found possible companions in the vicinity, as well as indications for merger / interactions. We find no distinct correlation between the IR luminosity of a galaxy and its H I gas content. In general galaxies with high IR luminosities have high neutral atomic hydrogen masses. The compactness for objects with spiral features seems to be lower (the CO emitting regions have a smaller extension than H I emitting ones) as they have a larger outspread than the ones showing an elliptical geometry. Only for our sample there are CO and H I measurements done for the sample together. For other samples, like e.g., for the Haan et al. sample, there exist CO measurements with different telescopes/techniques in the literature by separate authors. To our knowledge an estimated number of about 50 sources have H I and CO data taken. Therefore our sample contributes to the, at the moment, rather small contingency of hosts with atomic and molecular masses. The observed median surface mass density is about a factor two lower than the values for our Milky Way (50- $75~M_{\hbox{$\odot$ }}$ pc$^{\rm -2}$). From the Tully-Fisher relation we deduced a median inclination of $\sim$30$^\circ$, in good agreement with the assumption of nearly face-on morphology. Interferometrical data through follow-up observations for two out of the twelve H I detected sources have been obtained. For the other ten sources these follow-up observations are in planning.

Acknowledgements
M.G.-M. is supported by the German federal department for education and research (BMBF) under the project numbers: 50OS0502 & 50OS0801. Based on observations with the 100-m telescope of the MPIfR (Max-Planck-Institut für Radioastronomie) at Effelsberg. This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.

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Footnotes

... GILDAS[*]
http://www.iram.fr/IRAMFR/GILDAS

All Tables

Table 1:   List of sources observed at 21 cm.

Table 2:   Summary of the H I properties.

Table 3:   Sources from Haan et al. (2008).

All Figures

  \begin{figure}
\par\includegraphics[width=15.5cm,clip]{12546f1a.ps}
\end{figure} Figure 1:

H I spectra of detected host galaxies from the ``nearby QSO sample'' observed with the Effelsberg 100-m telescope and optical DSS images. The images in the middle extend over 2$^\prime $  and the ones to the right contain 9.5$^\prime $, which is roughly the size of the beam at 21 cm. North is up and east to the left. Each source is identified by its HES name ( top left corner of the spectrum) and the redshift ( top right corner). The spectral resolution of the spectra in this figure range from 0.3 km s$^{\rm -1}$ up to 32 km s$^{\rm -1}$. If regions in the spectrum are affected by RFI (Radio Frequency Interference) and are not used for baseline-fitting and noise estimates, they are set to zero intensity.

Open with DEXTER
In the text

  \begin{figure}
\par\includegraphics[scale=0.92,clip]{12546f1b.ps}
\vspace*{4mm}
\end{figure} Figure 1:

continued.

Open with DEXTER
In the text

  \begin{figure}\par\includegraphics[scale=0.92,clip]{12546f1c.ps}
\vspace*{5mm}
\end{figure} Figure 1:

continued.

Open with DEXTER
In the text

  \begin{figure}
\par\includegraphics[origin=rb,angle=-90,width=12cm,clip]{12546fg2.ps}
\end{figure} Figure 2:

H I mass as a function of infrared luminosity for the nearby QSOs. Stars represent the sample from this paper. Squares and triangles represent sources taken in order to enhance the statistics from Ho et al. (2008a): squares represent the sources from the Ho sample itself and triangles represent a literature sample assembled by Ho et al. (2008a). Filled squares and triangles mark available values for $L_{\rm IR}$, whereas open ones with arrows represent upper limits of $L_{\rm IR}$ or $M_{\rm H {\sc i}}$. Whereas filled stars for our sample represent H I and CO detected sources and unfilled ones denote upper mass limits for the CO but not H I detected sources. Arrows show upper limits for mass and infrared luminosity.

Open with DEXTER
In the text

  \begin{figure}
\par\includegraphics[origin=rb,angle=-90,width=12cm,clip]{12546fg3.ps}
\end{figure} Figure 3:

H I flux vs. CO intensity (taken from Bertram et al. 2007). Crosses mark the upper limits of H I non-detected sources. Filled circles represent the intensities for detected sources. Each object is identified by a number, which is related to the source name given in the figure legend.

Open with DEXTER
In the text

  \begin{figure}
\par\includegraphics[origin=rb,angle=-90,width=12cm,clip]{12546fg4.ps}
\end{figure} Figure 4:

Infrared luminosity $L_{\rm IR}$ vs. compactness f. Filled circles represent the H I detected sources from this work, filled stars however reperesent detected objects form the sample of Haan et al. (2008). Unfilled symbols with arrows mark sources with upper limits for their infrared luminosities. Each detected object from this work is identified by a number, which is related to the source name given in the figure legend.

Open with DEXTER
In the text

  \begin{figure}
\par\includegraphics[origin=rb,angle=-90,width=12cm,clip]{12546fg5.ps}
\vspace*{5mm}
\end{figure} Figure 5:

Blue magnitude $M_{\rm B}$ vs. rotation velocity $v_{\rm rot}$. The median value is plotted as an empty star. The dashed lines show functions for the Tully-Fisher relation from the literature (Schöniger & Sofue 1997; Ziegler et al. 2002; Sakai et al. 2000). The H I detected sources from this work are coded with grey-scale. Each object is identified with its name given in the figure legend.

Open with DEXTER
In the text

  \begin{figure}
\par\includegraphics[origin=rb,angle=-90,width=12cm,clip]{12546fg6.ps}
\vspace*{5mm}
\end{figure} Figure 6:

IRAS color-color diagram according to Helou (1986). The underlying model was taken from Desert (1986). Filled circles show H I detected sources from this work, unfilled ones mark H I non-detected sources with available IRAS fluxes. Arrows are indicators for upper limits. Each object is identified by a number, which is related to the source name given in the figure legend. The location of our sample sources in the plot shows that almost all of them are starburst dominated hosts that have high dust temperatures (sources to the upper left). The galaxies to the lower right of the plot have colors that are often found for elliptical galaxies. The two sources HE 0150-0344 and HE 0212-0059 are such objects. These objects are dominated by the 100 $\mu $m emission implying lower dust temperatures.

Open with DEXTER
In the text


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