Extended H I Disks in Dust Lane Elliptical Galaxies
ABSTRACT We present the results of H I observations of five dust lane elliptical galaxies with the Australia Telescope Compact Array. Two galaxies (NGC 3108 and NGC 1947) are detected, and sensitive upper limits are obtained for the other three. In the two detected galaxies, the H I is distributed in a regular, extended, and warped disklike structure of low surface brightness. Adding data from the literature, we find that several more dust lane elliptical galaxies have regular H I structures. This H I is likely to be a remnant of accretions and/or mergers that took place a considerable time ago and in which a significant fraction of the gas survived to form a disk. The presence of regular H I structures suggests that some mergers lead to galaxies with extended low surface brightness density gas disks. These gas disks will evolve very slowly, and these elliptical galaxies will remain gas-rich for a long period of time. One of the galaxies we observed (NGC 3108) has a very large amount of neutral hydrogen (MHI=4.5×109 Msolar MHI/LB~0.09), which is very regularly distributed in an annulus extending to a radius of ~6 Reff. The kinematics of the H I distribution suggest that the rotation curve of NGC 3108 is flat out to at least the last observed point. We estimate a mass-to-light ratio of M/LB~18 Msolar/LB,solar at a radius of ~6Reff from the center. Several of the galaxies we observed have an unusually low gas-to-dust ratio MHI/Mdust, suggesting that their cold interstellar medium, if present as expected from the presence of dust, may be mainly in molecular rather than atomic form. Based on observations with the Australia Telescope Compact Array (ATCA), which is funded by the Commonwealth of Australia for operation as a National Facility managed by CSIRO.
- VizieR Online Data Catalog. 05/1992; 7137.
Article: Active galactic nuclei[show abstract] [hide abstract]
ABSTRACT: IUE observations of the UV continua of active galactic nuclei are discussed. In extremely bright active galactic nuclei such as Fairall 9, IUE found that the C IV and Ly-alpha profiles were similar and symmetric. While IUE has limited spatial resolution, it has studied and resolved extended structures in active galaxies as well. Detailed variability studies of NGC 4151 and other AGNs were also carried out.01/1987; 129:655-669.
- Workshop on Data Analysis in Astronomy, L. Scarsi & V. di Ges` u, eds, London: Plenum. 271-45..
arXiv:astro-ph/0111141v1 7 Nov 2001
Extended H I disks in dust–lane elliptical galaxies1
Thomas A. Oosterloo
ASTRON, PO Box 2, 7990 AA, Dwingeloo, The Netherlands
ASTRON, PO Box 2, 7990 AA, Dwingeloo, The Netherlands
Elaine M. Sadler
School of Physics, University of Sydney, NSW2006, Australia
Dipartimento di Astronomia, Universit` a di Bologna, via Zamboni 33, I-40126 Bologna, Italy
Smithsonian Institution, Steward Observatory, 949 N Cherry Avenue, Tucson Arizona 85719,
We present the results of H I observations of five dust-lane ellipticals with the Aus-
tralia Telescope Compact Array. Two galaxies (NGC 3108 and NGC 1947) are detected,
and sensitive upper limits are obtained for the other three. In the two detected galaxies,
the H I is distributed in a regular, extended and warped disk-like structure of low surface
brightness. Adding data from the literature, we find that several more dust-lane ellipti-
cals have regular H I structures. This H I is likely to be a remnant of accretions/mergers
which took place a considerable time ago, and in which a significant fraction of the gas
survived to form a disk. The presence of regular H I structures suggests that some
mergers lead to galaxies with extended low surface brightness density gas disks. These
gas disk will evolve very slowly and these elliptical galaxies will remain gas rich for a
long period of time.
One of the galaxies we observed (NGC 3108) has a very large amount of neutral
hydrogen (MHI= 4.5 × 109M⊙; MHI/LB∼ 0.09), which is very regularly distributed
– 2 –
in an annulus extending to a radius of ∼6 Reff. The kinematics of the H I distribution
suggest that the rotation curve of NGC 3108 is flat out to at least the last observed
point. We estimate a mass-to-light ration of M/LB∼ 18 M⊙/LB,⊙at a radius of ∼6
Refffrom the centre.
Several of the galaxies we observed have an unusually low gas–to–dust ratio
MHI/Mdust, suggesting that their cold ISM, if present as expected from the presence of
dust, may be mainly in molecular rather than atomic form.
Theoretical models of galaxy formation and evolution have pointed out that differences between
gas content and gas supply are a key factor in explaining differences between different galaxies (e.g.
Kauffman 1996). At the same time, H I, FIR and CO observations of elliptical galaxies have
demonstrated that many of these objects have an active and interesting cold interstellar medium,
often qualitatively similar to that observed in spirals, although usually (but not always) in much
smaller quantity (see Knapp 1998 for a review). The limits in sensitivity of the available instruments
is still the main limitation for studying the cold ISM in elliptical galaxies in a detailed an unbiased
It is widely believed that when neutral hydrogen is observed in an elliptical galaxy it is the
result of a recent accretion of a gas-rich companion galaxy. Indeed, gas-rich elliptical galaxies are
often classified as ‘peculiar’. This implies that by studying such galaxies one is considering only
a subset of the whole population of early-type galaxies, namely those for which it is likely that
some interaction/accretion has occurred. But when investigating the evolution of the whole group
of early-type galaxies, it could be important not to restrict samples to ‘pure’ ellipticals with no
optical peculiarities, since gas-rich systems may represent an important phase in the evolution of
many early-type galaxies and give us important information on the formation and evolution of these
systems. At high redshift, similar processes will have been more common.
The correlation between optical ‘peculiarities’ and increased probability of finding H I can be
interpreted (van Gorkom & Schiminovich 1997) either as an effect of the evolutionary stage of these
galaxies (accretion of H I is a normal phase in the evolution of these galaxies) or as an effect of
environment (the fraction of peculiar ellipticals in the Revised Shapley-Ames Catalogue goes from
5% in clusters to almost 50% in the field) or (perhaps more likely) as a combination of the two.
If the accretion of gas is a normal event in the evolution of many or most early-type galaxies, this
raises questions like how much of the observed ISM is due to small accretion events and to what
extent parallels with spiral galaxies exist, whether in major mergers a significant fraction of the gas
1Based on observations with the Australia Telescope Compact Array (ATCA) which is funded by the Common-
wealth of Australia for operation as a National Facility managed by CSIRO
– 3 –
remains and, if so, how long does the ISM survive and will it settle in a disk-like structure.
Recently, there have been several studies of major mergers (i.e. mergers of two disk galaxies)
and the fate of gas in these systems (Hibbard & van Gorkom 1996; Hibbard & Mihos 1995). These
studies have shown the importance of the late infall of gas, possibly leading to extended, long–lived
gas–rich structures in elliptical galaxies. Indeed, an increasing number of early-type galaxies are
known that have a surprisingly large amount of H I distributed in quite regular structures (i.e. disks
or rings, e.g., in IC 2006, NGC 4278, NGC 2974, NGC 5266, IC 5063). The regular structure of
these disks and rings means that they are relatively old, indicating that some early–type galaxies
have a long-lived ISM. In such galaxies, the cold ISM is not a ‘peculiarity’, but a fundamental
Apart from the origin and evolution of the neutral hydrogen in early-type systems, studying H I
in these objects and in particular in dust–lane ellipticals, may give accurate estimates of the mass
distribution at large radii in these galaxies. Bertola et al. (1993) combined M/L values derived
from H I data with estimates for the central M/L (based on optical measurements from ionized
gas disks) to show evidence for an increasing M/L with radius, supporting the idea of a dark
halo around ellipticals. However estimates of M/L from H I data are only available for a handful
of well–studied ellipticals (e.g. IC 2006, Franx et al. 1994; NGC 1052, van Gorkom et al. 1986;
NGC 4278, Raimond et al. 1981 and Lees 1994; NGC 5266, Morganti et al. 1997). It is important
to increase the number of galaxies for which a similar analysis using H I data can be done. This
requires not only finding ellipticals with H I, but finding those in which the H I has settled into
a regular disk. For dust-lane ellipticals, regular large-scale dust could indicate the presence of a
regular disk and this is particularly valuable if we want to determine the intrinsic shape of the
galaxy and the mass–to–light ratio (M/L).
In this paper we present new observations of neutral hydrogen in dust–lane elliptical galaxies.
The presence of dust lanes is often considered a ‘peculiarity’ in an elliptical galaxy. Thus, for
the reasons mentioned above, dust–lane ellipticals are good targets in which to search for neutral
hydrogen. Systematic optical searches for dust in elliptical galaxies (Goudfrooij & de Jong 1995;
van Dokkum & Franx 1995) have found dust lanes and patches in the inner regions of about 50%
of the observed galaxies. Recent HST studies have shown that a large fraction (40dust of some sort
visible in optical images (Rest et al. 2001); either in filamentary form, nuclear disks or more large
scale disks. In this paper we study five galaxies with large-scale dust lanes (Centaurus A-like),
mainly taken from the compilation of Sadler & Gerhard (1985): NGC 1947, NGC 3108, ESO 263–
G48, NGC 7049, NGC 7070A. Table 1 summarizes the main properties of these five galaxies. H I
observations of these galaxies were made with the Australia Telescope Compact Array (ATCA) as
part of a long term project to increase the number of early–type galaxies with detailed H I images
(Morganti et al. 1997, 1998, Oosterloo et al. 1999a,b, 2001; Sadler et al. 2000). We present the
results from these new observations in §3, and in §4 we discuss them in the context of existing H I
data on dust-lane ellipticals in the literature.
– 4 –
In this paper we use H◦= 50 km s−1Mpc−1.
2. Observations and Data Reduction
All five galaxies were first observed with the most compact configuration available with the
ATCA (375m array). The two detected galaxies (NGC 1947 and NGC 3108, see below) were then
observed with other ATCA configurations to image the emission in more detail. Table 2 summarizes
the H I observations. We used a 16 MHz band with 512 velocity channels centred on the optical
velocity of each galaxy. The final velocity resolution (after Hanning smoothing the data) is ∼12
We observed each galaxy for about 12 hours in each run (with the exception of NGC 7070A),
with calibrators observed every hour to monitor gain and phase changes. The flux density scale
was set by observations of PKS 1934–638, for which we adopted a flux density of 14.9 Jy at 1400
MHz. This source was also used as bandpass calibrator.
The spectral data were calibrated with the MIRIAD package (Sault et al. 1995), which has
several features particularly suited for ATCA data. The continuum subtraction was also done in
MIRIAD, by using a linear fit through the line–free channels of each visibility record and subtracting
this fit from all the frequency channels (i.e. UVLIN). An interference spike generated by the ATCA
data acquisition system is present at 1408 MHz, but this does not compromise the interpretation
of the data.
For the objects which were observed with more than one ATCA configuration, we combined
the data from different runs after calibration and continuum subtraction. The final cubes were
made with natural, uniform or robust–Briggs’ weighting. Table 3 lists the corresponding rms noise
and size of the restoring beams.
The moment analysis of the data cube was done using GIPSY (Allen, Ekers & Terlouw 1985).
The total intensity of the H I emission were derived from a data cube produced by smoothing
spatially the original cube to a resolution about twice lower than the original. This smoothed cube
was used as mask for the original cube: pixels with signal below 3σ in the smoothed cube were set
to zero in the original cube (van Gorkom & Ekers 1989).
H I was not detected in three of the target galaxies, and for these galaxies the upper limits
on H I emission were calculated as three times the statistical error on having no signal over 50
independent velocity elements (corresponding to 600 km/s) and over four synthesised beams.
The continuum data were taken from the emission-free channels in the line data. They were
also reduced using MIRIAD, and Table 4 summarizes the results. The final continuum images were
made with uniform weighting.
– 5 –
The results of our H I and radio continuum observations are summarized in Table 5. In H I,
we detected two of the five galaxies observed, and three of the five were detected as continuum
sources. We now give some details of the individual galaxies.
3.1. NGC 1947
NGC 1947 is an E1 galaxy with a system of at least three concentric minor–axis dust lanes
(Bertola et al. 1992). The galaxy also has a ring of CO emission (Sage & Galletta 1993) and ionized
gas (M¨ ollenhoff 1982) with the same position angle as the dust lane. The stars in NGC1947 rotate
around the galaxy’s minor axis, perpendicular to the ionized gas rotation axis (Bertola et al. 1992).
The optical spectrum has central emission lines with intensity ratios typical of LINERs (Low-
Ionization Nuclear Emission-line Regions; Heckman 1980), and an Hα+[NII] image (Goudfrooij
et al. 1994) shows that the ionized gas has a regular, disk–like distribution centred on the galaxy
We detect 3.4 · 108M⊙of H I in NGC 1947, very similar to the derived H2mass. The H I
emission spans a wide velocity range (from ∼900 kms−1to ∼1450 kms−1) with a width (at 20%-
level) of ∼550 kms−1. The corresponding value for MHI/LB is 0.019 M⊙/LB,⊙. The velocity
centroid of the H I is at ∼1180 kms−1, close to the optically–measured value in Table 1. The
neutral hydrogen emission is extended and elongated along P.A.∼127◦, i.e. the same position angle
as the dust lane, as can be seen from Fig. 1, where contours in total H I are superimposed on a deep
optical image kindly provided by D. Malin. Although the H I signal is quite weak (the peak in the
cube is only ∼5 mJy beam−1), it is clear that the distribution of H I is quite regular. The H I is
not only aligned with the dust lane but rotates in the same sense and with the same amplitude as
the CO. The total extent of the H I is ∼2′(∼13 kpc) in radius, i.e. ∼6.2 times the optical half-light
Position–velocity plots along the major axis of the H I distribution (Fig. 2a), and along PA =
90◦(Fig. 2b) confirm that the H I is in a warped rotating disk–like structure associated with the dust
lane. The velocity increases rapidly in the inner part (< 30 arcsec) with a ‘discontinuity’ observed
in the centre, but the projected velocity appears to decrease in the outer parts. The observed
morphology of the dust lanes suggests that the H I might also lie in three or more concentric rings.
This may explain the ‘discontinuity’ seen near the systemic velocity. The combination of a number
of rings, together with the fact that the spatial resolution of the H I data is relatively low, can
reproduce the distribution of the H I in the position-velocity plot. The morphology of the dust
lanes suggests that the whole structure is probably warped, and the decrease in projected velocity
at large radii is most likely due to this warp.
The average H I surface density is 0.3 M⊙pc−2. The distribution peaks in the centre and it is