N. Kanekar 1 - J. N. Chengalur 2
1 - Kapteyn Astronomical Institute, University of Groningen,
Post Bag 800, 9700 AV Groningen, The Netherlands
2 -
National Centre for Radio Astrophysics, Post Bag 3,
Ganeshkhind Pune - 411 007, India
Received 5 November 2004 / Accepted 26 November 2004
Abstract
We report a deep search for HI 21cm emission from
the
z= 0.00632 sub-DLA toward PG1216+069 with the Giant
Metrewave Radio Telescope. No emission was detected and
our
upper limit on the mass of any associated galaxy
is
,
nearly 3
orders of magnitude less than
.
The
absorber is thus the most extreme known
deviation from the standard paradigm in which high column
density quasar absorption lines arise in the disks of
gas-rich galaxies.
Key words: galaxies: evolution - galaxies: formation - galaxies: ISM - cosmology: observations - radio lines: galaxies
Neutral atomic gas clouds at high redshift can currently be detected
only via the absorption lines they produce if they happen to lie
along the line of sight to an even more distant quasar. Although
present technology allows one to detect Lyman-absorption lines from
clouds with as low an HI column density as
cm-2,
the bulk of the neutral gas at
turns out to be in extremely
rare systems, with column densities higher by several orders of magnitude,
viz.
cm-2. At such large
,
the optical depth is substantial even in the Lorentzian wings of the
Lyman-
profile; these systems are called damped
Lyman-
absorbers or DLAs.
In the local universe, the disks of spiral galaxies
have characteristic HI column densities 1020 cm-2 over a
large spatial scale. High z DLAs are thus natural candidates
for the precursors of today's large galaxies (e.g. Wolfe 1988).
The nature of high redshift DLAs and their evolution with redshift
have hence been of much interest to astronomers studying
galaxy formation and evolution. Unfortunately, cosmological dimming
makes it impossible to directly image high redshift DLAs; the information
obtainable is hence limited to the gas that lies along the narrow
pencil beam that is illuminated by the background quasar. It is
primarily for this reason that, despite decades of study, the nature
of high redshift damped absorbers remains an unsettled issue, with
models ranging from large, rotating disks (e.g. Prochaska & Wolfe 1997)
to small, merging proto-galaxies (e.g. Haehnelt et al. 1998). Detailed
absorption studies with 10-m class optical telescopes have
established that current samples of high redshift DLAs typically
have low metal abundances (
0.1 solar; Pettini et al. 1997)
and that these abundances evolve slowly, if at all, with redshift
(e.g. Prochaska et al. 2003; Kulkarni et al. 2004). This is somewhat surprising
if the absorbers are indeed typically the precursors of large
galaxies like the Milky Way.
Low redshift (
)
DLAs offer the possibility of
direct identification of the absorbing galaxy by optical imaging,
(e.g. le Brun et al. 1997; Rao et al. 2003; Chen & Lanzetta 2003). Since the HI mass function is
dominated by bright galaxies (e.g. Zwaan et al. 1997), one might
a priori expect that low z DLAs should primarily be associated
with large spirals. Interestingly enough, however, it has been found
that a wide variety of galaxy types are responsible for damped
absorption at low redshifts (e.g. Rao et al. 2003; Bowen et al. 2001), with no
particular type dominating the current (admittedly rather small)
low z sample. One possible explanation is that, although the
total HI mass is dominated by large spiral galaxies, the HI cross-section at the
1020 cm-2 threshold varies across
galaxy type in such a way as to yield an even distribution of
absorbers across a range of galaxy luminosities (Zwaan et al. 2002).
On the other hand, it is also possible that, although the optical
counterparts to these low redshift DLAs are faint, they have
unusually large HI envelopes and thus, a large total mass.
Such a preferential selection might well occur since absorption
surveys are biased toward objects with a large HI cross-section.
Unfortunately, it is very difficult to observationally test the
latter scenario as this requires detection of the HI 21-cm line
and current radio instrumentation limits searches for 21-cm
emission from galaxy-sized objects to the very nearby
universe,
.
Almost no DLAs are known at these
low redshifts and hence, searches for 21-cm emission have
been carried out in only three absorbers; in
all cases, the HI mass has been found to be lower than
,
the typical mass of local bright spirals
(Lane 2000; Kanekar et al. 2001; Bowen et al. 2001; Chengalur & Kanekar 2002). Identifying such
DLAs is of much importance as it is only
these systems that can be followed up in detail in all wavebands,
to estimate the total mass (as opposed to the luminous mass). We
note, in passing, that such DLAs cannot be identified (except
by accident) using the MgII selection criterion of Rao &
Turnshek (2000), as the MgII lines are redshifted into optical
wavebands only for redshifts z > 0.1.
Recently, Tripp et al. (2004) reported the discovery
of an extremely low redshift (
z = 0.00632) absorption system toward
the quasar PG 1216+069, with a high HI column density,
cm-2. While the
absorption profile shows clear damping wings, its column density is
somewhat smaller than the cut-off of
cm-2 that
has traditionally been used to define DLAs (e.g. Wolfe et al. 1986).
This system is hence classified as a sub-DLA. However, it should
be emphasized that the column density threshold used to define DLAs
is entirely arbitrary and that the absorber properties do not show any
sharp qualitative differences here.
Interestingly enough, although the new absorber is at a very low
redshift, it has an extremely low metallicity, 1/40 Solar
(Tripp et al. 2004), one of the lowest measured in the gas phase in
the nearby universe and, in fact, similar to that of high z
DLAs (e.g. Prochaska et al. 2003). No optical counterpart can be
seen in the Hubble Space Telescope (HST) image of the field
(Tripp et al. 2004); the nearest L* galaxy has a projected
separation of
kpc
. We report
here the results of a search for HI 21-cm emission from the
sub-DLA, using the Giant Metrewave Radio
Telescope (GMRT; Swarup et al. 1991).
The GMRT observations were conducted on the 28th and 30th
of August 2004. The total on-source time was 8 h, with
29 antennas. The observing bandwidth of 2 MHz, centred at 1411.5 MHz,
was divided into 128 spectral channels, yielding a spectral
resolution of
15.6 kHz. The total velocity coverage was thus
425 km s-1, with a resolution of
3.3 km s-1. Flux and
bandpass calibration were done using short scans on 3C 147 and 3C 286 at
the start and end of each
observing run, while the compact source 1150-003 was used
for phase calibration. The flux density of 1150-003 was
measured to be
Jy.
Data reduction was carried out using standard tasks in "classic''
AIPS. Visibilities from the different days were calibrated separately
and then combined together using the AIPS task DBCON; the combined
data were used to make the final images, with the task IMAGR.
We note that the GMRT has a hybrid configuration, with fourteen antennas
in a central array (the "central square'') and the remaining sixteen
distributed in the three arms of a "Y'' (Swarup et al. 1991). The central
square antennas yield baselines of 1 km (i.e. U-V coverage out
to
at 1411.5 MHz), while longer baselines (up to a
maximum length of
25 km, or a U-V coverage to
at 1411.5 MHz) are obtained with the arm antennas. Continuum images and
spectral cubes were hence made with a variety of U-V ranges and tapers,
allowing a search for HI and continuum emission at various spatial
resolutions ranging from
3'' to
40''.
The continuum emission was subtracted using two independent approaches: (1)
in the U-V plane, using the task UVSUB and (2) in the image plane,
using the task IMLIN. The two strategies gave very similar results; the
numbers quoted below are for the U-V plane approach.
Finally, the GMRT does not do on-line Doppler tracking, implying that any required Doppler shifts must be applied to the data off-line. In the present case, however, it was not necessary to apply a differential Doppler shift to the data of the two observing runs since this shift was found to be small, relative to the channel separation. No off-line Doppler corrections were hence applied while making the image cubes.
Table 1:
rms noise levels and
HI mass limits.
The central ninety channels of the band were averaged together to
map the continuum, at various spatial resolutions. The low resolution
(
)
image shows good agreement with the NRAO
VLA Sky Survey map of the same region (Condon et al. 1998). Our somewhat
higher sensitivity allows us to detect faint emission from the quasar
itself; we measure a flux density of
mJy. The
peak flux density in the image is
mJy.
The data cubes were examined for line emission at a variety of
spectral resolutions; in all cases, no significant line emission was
found in the vicinity of the quasar line-of-sight. Besides a visual
inspection, the AIPS task SERCH was used to search for line emission
in the different cubes. No statistically significant emission features
were detected in the cube, except from the galaxy VCC 297; this system
is discussed later. The highest resolution (
)
cube did show a weak (
)
emission feature at the quasar
location (albeit
70 km s-1 away from the optical redshift); if
real, it corresponds to an HI mass of
.
However,
this is not very believable as the feature is spatially unresolved
and seen only in a single velocity channel, after smoothing the data
to a resolution of 13 km s-1. Further, if real, this concentration of gas
would imply an HI column density
1021 cm-2, (i.e.
considerably larger than the column density of the sub-DLA) and
it would be surprising that no associated metal lines were found
at this velocity.
Table 1 summarizes our results, for a representative
selection of spatial resolutions; the data have been smoothed
to a velocity resolution of 20 km s-1 in all cases, a fairly typical
velocity width for a small galaxy. The different
columns in the table are: (1) the half-power beam width (HPBW) of the
synthesized beam (i.e. the spatial resolution, in arcsec), (2) the
rms noise (in mJy/Bm) at this spatial resolution, (3) the physical
distance at
z = 0.00632 (in kpc), corresponding to the synthesized
HPBW, and (4) the
upper limit on the HI mass of the
sub-DLA at this spatial resolution (in units of
), assuming that
the HI profile has a top-hat shape, with a velocity width of 20 km s-1.
Note that lower spatial resolutions are obtained by weighting down
data from the more distant antennas; this implies that the mass limit
improves at higher spatial resolution.
The
upper limits on the HI mass of the galaxy listed in
the last column of Table 1 are extremely small, nearly
3 orders of magnitude lower than
.
This is
quite remarkable, given the high HI column density estimated from
the Lyman-
line. Of course, it should be pointed out that
the mass limits quoted in the table are based on the assumption
that the absorber is entirely contained within the synthesized beam.
It is, in principle, possible that the HI emission is actually
spread over a larger angular scale and is not detected in our
interferometric observations either because the flux per synthesized
beam falls below our detection threshold or because the flux is resolved
out or due to a combination of both effects. We initially consider this
possibility, before discussing the implications of our results
for the nature of the
z = 0.00632 sub-DLA.
If the absorbing galaxy were far larger than the GMRT synthesized beam,
the mass limits of Table 1 would refer only
to the HI mass within the beam and not to that of the entire galaxy.
For example, if the galaxy were a face-on disk of diameter 10 kpc,
twice our coarsest resolution, the upper limit to its total
HI mass would be a factor of 4 larger than that listed in
the first line of Table 1, i.e.
.
However, dwarf irregular galaxies with
low HI masses (
)
have fairly
small HI diameters (e.g. Stil & Israel 2002a,b; Begum et al. 2003). For example,
all ten dwarf irregulars with
in the sample of Stil & Israel (2002a,b) have size
5 kpc,
our coarsest resolution. In fact, of the 27 dwarfs in
the Stil & Israel sample with estimates of HI extent, the only
galaxies that have physical sizes larger than 11 kpc are the five
systems with
(i.e.
considerably larger than the upper limit in Table 1).
Further, if we assume that
cm-2
throughout the absorber, the total HI mass in an area 5 kpc in
diameter is
,
comparable to our detection
threshold. The latter is a severe under-estimate of the true HI
mass, if the absorption arises in a normal galaxy, since large
fractions of even dwarf galaxy disks have
1020 cm-2. It is
thus highly unlikely that the absorption arises in a dwarf galaxy
whose HI disk is so extended that the flux per synthesized beam
falls below our detection threshold. Similarly, the second scenario,
that the flux is resolved out because it arises from a highly
uniform HI distribution, is also rather improbable as this would
require a very small velocity gradient over the entire spatial
extent of the gas, for emission to not be detected even at the
highest velocity resolutions.
The nearest known HI-rich galaxy to the sub-DLA is VCC 297,
which has
(Impey et al. 1999),
and a velocity width W50 of 145 km s-1 (Giovanelli et al. 1997);
this is
kpc away from the QSO sightline. VCC 297 lies
at the edge of our field of view, where imaging is particularly difficult
because of the asymmetry of the edges of the GMRT primary beam. The
emission velocity of VCC 297 also lies at the edge of the observing
band where the sensitivity is lower; further, there appears to be some
low level interference at these edge channels (note that none of
these problems are present at the centre of our field of
view and observing band, where the emission from the sub-DLA
would arise). Next, the optical extent of VCC 297 is a
factor of
3 larger than our synthesized beam; its HI extent
is likely to be at least this large, reducing the flux per synthesized
beam. Despite these problems, HI emission from
VCC 297 is reliably detected in the spectral cubes
(see Fig. 1). The emission was readily
apparent to the eye and was also detected by the automatic
search algorithm SERCH, at a signal-to-noise ratio of
12.
Finally, as a concrete counter-example, a dwarf galaxy at
z=0.0051 with
was
serendipitously discovered in a different GMRT observation,
with essentially identical observational setup and rms noise
(Chengalur et al. 2004). From all of the above, we conclude that
it is highly unlikely that the sub-DLA arises in a
galaxy with HI mass substantially larger than the limits
quoted in Table 1. Further, if one assumes a uniform
HI column density of
cm-2 throughout the
absorber (and a disk geometry), our HI mass limit of
implies an upper limit of
9 kpc to the size of any associated
galaxy. It is still not impossible, of course, that the sub-DLA
arises in a more massive, highly extended, smooth HI cloud, but this
would require it to be completely unlike any known low redshift galaxy.
![]() |
Figure 1:
Integrated HI 21-cm emission profile toward VCC 297,
made from the
![]() |
Open with DEXTER |
As discussed in detail by Tripp et al. (2004), the
sub-DLA is highly unusual in having a very low metallicity
([O/H] =
-1.6+0.09-0.11) despite being at low redshift and
lying in a region of high local galaxy density (the outskirts of the
Virgo cluster). The sub-DLA also shows an under-abundance of nitrogen
and an over-abundance of iron (implying a lack of dust) which also argues
in favour of a system that is at an early stage of chemical evolution
(Tripp et al. 2004). These abundance patterns are very different from
those characteristic of local
spirals, and are consistent
with the absence of a luminous optical counterpart to the sub-DLA
in the HST image. Of course, the lack of a detected optical counterpart
does not entirely exclude a scenario in which the sub-DLA arises
in a massive galaxy, since such
a system might well be hidden beneath the point spread function of
either the QSO itself or a nearby foreground star located near the quasar
(Tripp et al. 2004); alternatively, as discussed in the introduction,
the absorber might be anomalously gas-rich for its luminosity. However,
the stringent upper limit on the HI mass of the absorber provided by
our observations indeed rules out this possibility.
The abundance patterns and low metallicity of the
absorber are consistent with an origin in (i) a low metallicity,
gas-rich, blue compact dwarf (BCD) like I Zw 18 or
SBS0335-052; (ii) a high velocity cloud (HVC),
(iii) a dwarf spheroidal galaxy; or (iv) a dark mini-halo left over from the
epoch of reionization (see Tripp et al. 2004 and references therein).
However, the mass limits in
Table 1 are significantly lower than HI masses typical
of low metallicity BCDs (
;
van Zee et al. 1998,Pustilnik et al. 2001). It
is also unlikely that the sub-DLA is associated with an HVC,
given that the nearest
galaxy (NGC 4260)
is
kpc away (in projection) and HVCs
have not been detected at such large distances from their
parent galaxies, despite deep searches (e.g. Pisano et al. 2004).
In fact, for M 31 (the only galaxy that has been sensitively searched for
large-scale diffuse HI emission), the typical HI column density
falls to values smaller than 1015 cm-2
by the time one reaches galacto-centric distances of 200 kpc
(Braun & Thilker 2004). Similarly, Miller & Bregman (2004) place a
limit of
25 kpc on the distance of HVCs from their
parent galaxy; this implies that the absorption is also
unlikely to arise from an HVC associated with the fainter
galaxy VCC 297, which is
kpc away.
The most likely counterpart is hence an extreme dwarf galaxy
(or a dark mini-halo), which is surprising, given the relatively
small size of these systems. How likely is it that such a system would
show up in an unbiased search for absorption?
While there are no existing measurements of the typical size of
faint dwarf galaxies at HI column densities of
cm-2,
a linear extrapolation of the results of Zwaan et al. (2002),
for systems with higher mass and column density, (and further
assuming the faint end slope of the HI mass function to be -1.2)
indicates that the HI cross-section offered by
107
galaxies is significantly more than an order of magnitude smaller than that
of
galaxies. The implied low probability of
a sub-DLA arising in the former class of systems may suggest that
our understanding of the HI mass function is incomplete at the low mass
end.
In summary, contrary to the a priori expectation that
systems found via absorption line searches should be biasedtoward
galaxies with large HI mass, our upper limit to the total HI content
of the
sub-DLA is
three orders of magnitude
lower than
.
This is by far the smallest HI
mass limit placed on gas associated with DLAs or sub-DLAs.
The
z= 0.00632 sub-DLA toward PG1216+069 thus appears to be
the most extreme known deviation from the standard paradigm for
high column density absorbers.
Acknowledgements
The data presented in this paper were obtained using the GMRT, which is operated by the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research. We thank the referee, Martin Zwaan, for useful comments on the manuscript.