We report the discovery of two small intergalactic HII regions in the loose
group of galaxies around the field elliptical NGC 1490. The HII regions are
located at least 100 kpc from any optical galaxy but are associated with a
number of large HI clouds that are lying along an arc 500 kpc in length and
that have no optical counterpart on the Digital Sky Survey. The sum of the HI
masses of the clouds is almost 10^10 M_sun and the largest HI cloud is about
100 kpc in size. Deep optical imaging reveals a very low surface brightness
counterpart to this largest HI cloud, making this one of the HI richest optical
galaxies known (M_HI/L_V~200). Spectroscopy of the HII regions indicates that
the abundance in these HII regions is only slightly sub-solar, excluding a
primordial origin of the HI clouds. The HI clouds are perhaps remnants
resulting from the tidal disruption of a reasonably sized galaxy, probably
quite some time ago, by the loose group to which NGC1490 belongs.
Alternatively, they are remnants of the merger that created the field
elliptical NGC1490. The isolated HII regions show that star formation on a very
small scale can occur in intergalactic space in gas drawn from galaxies by
tidal interactions. Many such intergalactic small star formation regions may
exist near tidally interacting galaxies.Comment: To appear in the proceedings of the IAU Symposium #217, Recycling
  Intergalactic and Interstellar Matter, eds. P.-A. Duc, J. Braine, and E.
  Brinks, 6 pages with low resolution figures. The full paper with high
  resolution images can be downloaded from
  http://www.astron.nl/~morganti/Papers/cloud.ps.g

Ferguson, A.

Jerjen, H.

Morganti, R.

Oosterloo, T.

Sadler, E. M.

van der Hulst, J. M.

English

arXiv

Hulst, J. M.

University of Groningen

   University of GroningenTidal Remnants and Intergalactic HII RegionsOosterloo, T.; Morganti, R.; Sadler, E. M.; Ferguson, A.; Hulst, J. M.; Jerjen, H.Published in:Default journalIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2003Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Oosterloo, T., Morganti, R., Sadler, E. M., Ferguson, A., Hulst, J. M., & Jerjen, H. (2003). Tidal Remnantsand Intergalactic HII Regions. Default journal.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 12-11-2019Recycling intergalactic and interstellar matterIAU Symposium Series, Vol. 217, 2004Pierre-Alain Duc, Jonathan Braine and Elias Brinks, eds.Tidal Remnants and Intergalactic H II RegionsTom Oosterloo, Raaella MorgantiNetherlands Foundation for Research in Astronomy, Dwingeloo, NLElaine M. SadlerSchool of Physics, University of Sydney, NSW 2006, AustraliaAnnette FergusonMax-Planck Institute fu¨r Astrophysik, Karl-Schwarzschild-Str. 1, 85748Garching, GermanyThijs van der HulstKapteyn Astronomical Institute, RuG, Landleven 12, 9747 AD,Groningen, NLHelmut JerjenResearch School of Astronomy & Astrophysics, Australian NationalUniversity, ACT 2611, Canberra, AustraliaAbstract.We report the discovery of two small intergalactic H II regions in theloose group of galaxies around the eld elliptical NGC 1490. The H IIregions are located at least 100 kpc from any optical galaxy but are asso-ciated with a number of large H I clouds that are lying along an arc 500kpc in length and that have no optical counterpart on the Digital SkySurvey. The sum of the H I masses of the clouds is almost 1010 M andthe largest H I cloud is about 100 kpc in size. Deep optical imaging revealsa very low surface brightness counterpart to this largest H I cloud, mak-ing this one of the H I richest optical galaxies known (MHI/LV  200).Spectroscopy of the H II regions indicates that the abundance in theseH II regions is only slightly sub-solar, excluding a primordial origin of theH I clouds. The H I clouds are perhaps remnants resulting from the tidaldisruption of a reasonably sized galaxy, probably quite some time ago,by the loose group to which NGC 1490 belongs. Alternatively, they areremnants of the merger that created the eld elliptical NGC 1490. Theisolated H II regions show that star formation on a very small scale canoccur in intergalactic space in gas drawn from galaxies by tidal interac-tions. Many such intergalactic small star formation regions may existnear tidally interacting galaxies.12 Oosterloo et al.1. IntroductionHierarchical accretion and merging of small clumps appears to be a good de-scription of the formation of early-type galaxies. In many early-type galaxiessignatures of recent hierarchical assembly can be observed, particularly in eldgalaxies. Optical ne structure -shells, dust lanes, \X" structures- revealedthrough un-sharp masking of optical images suggests a recent accretion event.These ne structures are correlated with the presence of an intermediate age(1-2 Gyr) population of stars (e.g. Schweizer et al. 1990). Moreover, compar-ing detailed stellar population models with high signal-to-noise optical spectraindicates the presence of an intermediate age population in several early-typegalaxies (e.g. Trager et al. 2000). A small but signicant fraction of early-typegalaxies contain neutral hydrogen. The origin of this neutral hydrogen is gener-ally thought to be external and due to recent accretion of companions. Hence thepresence of H I in early-type galaxies is generally thought to t into the pictureof continued accretion of companions and the associated intermittent episodesof star formation in some early-type galaxies.We are undertaking a systematic H I survey, using the Australia TelescopeCompact Array (ATCA), of all early-type galaxies south of δ < −25 withV <6000 km s−1 (based on HIPASS, the Parkes All Sky H I Survey, see Barnes etal. 2001 and elsewhere in this volume) to obtain an unbiased sample of H I-richearly-type galaxies. Such a complete sample will allow us to study the eectsof H I on the evolution of early-type galaxies in dierent environments, and therelation between H I and other indicators of recent accretion and star formation(see also Sadler et al. 2002). We can also study whether H I always has anexternal origin in early-type galaxies or whether in some early-type galaxies theH I may be related to an earlier phase in the evolution of the galaxies.During this HIPASS follow-up program we discovered two intergalactic H IIregions near the eld elliptical NGC 1490 that are probably related to merg-ing/accretion within the loose group of galaxies around NGC 1490, or to themerger that created the eld elliptical NGC 1490. Although unusual, these re-gions seem to t into the generally accepted picture regarding H I in early-typegalaxies. We discuss these H II regions in section 2. However, we nd someevidence that some early-type galaxies do not t into the scheme of continuedaccretion. This we briefly discuss in section 3.2. Intergalactic H II regions near NGC 1490The HIPASS spectrum of NGC 1490 shows a strong detection with the presenceof almost 1010 M of neutral hydrogen. This detection was followed up with deepH I observations performed with the ATCA to image the H I in the region aroundNGC 1490. Five objects were detected with a total H I mass of 8.9  109M(Figure 1). Only one H I cloud has an obvious counterpart (cloud 4) on theDigital Sky Survey in the form of a small galaxy. The other H I clouds seem tolie along an arc of more than 500 kpc long that bends around NGC 1490. Abouthalf of the total amount of H I is found in one cloud of about 100 kpc in size(cloud 1). Fig. 2 shows a position-velocity plot of two H I clouds, as marked inFig. 1. The kinematics of the H I clouds indicates that they are separate entities.Intergalactic HII regions around NGC 1490 3Figure 1. View of the entire region around NGC 1490, displaying the5 objects detected in H I. Contour levels: 2, 4, 6, ...  1019 cm−2. Thelines indicate the orientation along which the position-velocity plots ofFig. 2 are taken.E.g. the velocity gradient in cloud 3 is perpendicular to the line connecting theneighbouring clouds. The \U-shaped" pattern observed in the largest H I cloudindicates that this cloud is not in rotational equilibrium.Deep optical imaging was performed in a number of standard lters of theregion of this largest H I cloud, using FORS1 on the VLT. This imaging revealsa very low surface brightness optical counterpart (the \OC"). The peak surfacebrightness in the V band is 24.5 V mag arcsec−2. The low surface brightnessnature of the OC is illustrated in Fig. 3. The total magnitude is about 20.5in V. This implies an extremely high gas-to-light ratio of MHI/LV  200. Forthe colour indices we nd UB = 0.5, BV = 0.1 VR = 0.4 and RI = 0.6. Thesecolours indicate the presence of a young stellar population.We further investigated the nature of the main H I cloud and its OC bysearching for the presence of Hα emission using narrow-band Hα imaging withthe NTT. These images reveal the presence of Hα emission in the OC at thelocation of the peak of the H I, indicating that some star formation is occurringin the OC. Most interestingly, two small regions of Hα emission were found atthe edge of the main H I cloud, at a distance of about 100 kpc from the OC(indicated by 1 & 2 in Fig. 3). The broad-band images indicate that there is noextended optical continuum light near these two Hα regions.The Hα luminosities of the H II complexes are 3−51038 erg s−1 (i.e. 30-50times Orion). The rate of star formation estimated from these luminosities isvery modest: around 0.005 Myr-1 per region. All the stars corresponding to the4 Oosterloo et al.Figure 2. Position-velocity plots of clouds 1 and 3, taken along theline indicated in Figure 1. Contour levels are {1, 1, 2, 3, ... mJybeam−1diuse optical counterpart can be formed over a Hubble Time with such a starformation rate.Optical spectroscopy of the three Hα regions was performed with FORS1on the VLT. Preliminary analysis of the data suggests that the abundances inthe Hα regions are only slightly below solar (in the range 0.5-1.0 Z). Thisrelatively high abundance excludes a primordial nature of the H I clouds.2.1. The nature of the clouds near NGC 1490The observations point to a tidal- or merger-related origin for the system ofH I clouds near NGC 1490. The alternative hypothesis, i.e. the H I clouds areprimordial, can be excluded because of the near solar abundance measured inthe H II complexes. The arc-like arrangement of the H I clouds is suggestive fora tidal origin. It bears some resemblance to other systems. There is probably aparallel between the objects discussed here and systems like e.g. the Leo Ring(Schneider et al. 1989). Tidal interactions between galaxies and merging is a verydiverse process that can produce very spectacular objects with bright tidal tails.But there are also cases where the evidence of an interaction is only detectablein neutral hydrogen. The extreme version is that of a galaxy being completelydestroyed by a larger galaxy without any merging taking place. The material ofthe original galaxy is spread out over such a large volume that, in the optical,the surface brightness is well below the detection limit and only the H I remainsobservable (e.g. Ryan-Weber et al. 2003, see also this proceedings). Perhaps theH I clouds near NGC 1490 have such an origin.An alternative hypothesis is that the H I clouds are remnants of the mergerbetween two or more gas-rich galaxies that also created the eld elliptical NGC 1490.The environment of NGC 1490 can be characterised as a loose group. The pres-ence of an elliptical galaxy in such a group often indicates that merging hasoccurred in the group.The interesting aspects of the case presented here are:1) the very large amount of H I involved, distributed in fairly large clouds forwhich only deep optical imaging reveals an optical counterpart. As the velocitystructure indicates, these clouds are separate entities, suggesting they are TidalIntergalactic HII regions around NGC 1490 5Figure 3. Left H I contours on top of an r-band image obtained withthe NTT. Contour levels are as in Fig. 1. The location of the threeH II complex discovered using the narrow-band imaging are indicated.Right Continuum optical image (top) centred on the regions 1 and 2and the continuum subtracted line image, centred on the redshiftedHα, revealing the two H II complexes (bottom)Dwarfs even though they have no bright optical counterpart. The clouds are not,or hardly, visible in the optical, but, given the large reservoir of H I, perhaps atsome point in time they will start to make stars and become brighter opticalobjects.2) the existence of an extremely gas-rich object with MHI/LV  200. This isperhaps an example of a "Tidal Gas Cloud" that started making stars recently.However, it is perhaps more likely that the optical material originates in a galaxythat was destroyed in an interaction.3) the presence of small H II complexes that are located far away from any opti-cally detectable galaxy, in our case more than 100 kpc. Only very few examplesof such H II regions are known. The observations presented here suggest that inmore systems intergalactic H II complexes may exist.3. Non-accretion origin of H I in some early-type galaxies?As a nal remark, we note that, although the results discussed above t very wellinto the picture continued accretion by early-type galaxies, and the associatedexternal origin of H I in early-type galaxies, we have found some indications thatsome early-type galaxies do not t into this scheme. Our survey has revealed asurprisingly large number of early-type galaxies that have very large (up to 200kpc in size), regular disks of low column density H I. In Fig. 4 we present threeexamples of such large regularly rotating H I disks. The largest of these diskshave H I masses up to 1010 M. Given their size and regular appearance, these6 Oosterloo et al.Figure 4. Examples of very large regular disks of H I distributionaround \normal" early-type galaxies obtained from the ATCA followup of HIPASS detections (see text for details). The disks shown herehave sizes between 150 and 200 kpc. Contour levels: 2, 4, 8, 16  1019cm−2.disks must be quite old, in many cases well over 5  109 yr. Hence, they arenot related to recent accretions and may be related to a much earlier formationphase of these galaxies. Moreover, no large accretion can have occurred bythese galaxies, as these would likely have destroyed these large H I disks, or havetriggered signicant star formation in the H I disk. The H I column density inthese disks peaks at only 1020 atoms cm−2, so, despite the large H I reservoir,no signicant star formation is occurring. These H I disks will evolve only veryslowly and will remain as purely gaseous disks for very long periods of time,provided the environment is not too hostile for these disks.ReferencesBarnes et al. 2001, MNRAS 322, 486Ryan-Weber, Webster & Bekki, 2003, ASSL Conf. Proc.281, p223Sadler E.M., Oosterloo T., Morganti R., 2002 in The Dynamics, Structure &History of Galaxies, Eds. G.S. Da Costa and Helmut Jerjen, ASP Con-ference Proceedings, Vol. 273, p.215Schneider et al. 1989, AJ, 97, 666Schweizer F., Seitzer P., Faber S., Burnstein D., Dalle Ore C.M., Gonzales J.J.1990, ApJL 364, L33Trager S.C., Faber S., Worthey G., Gonzales J.J. 2000, AJ 120, 165

Tidal Remnants and Intergalactic HII Regions

University of Groningen Research Database

  
 University of Groningen
Tidal Remnants and Intergalactic HII Regions
Oosterloo, Thomas; Morganti, Raffaella; Sadler, E. M.; Ferguson, A.; van der Hulst, Jan;
Jerjen, H.
Published in:
Default journal
IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from
it. Please check the document version below.
Document Version
Publisher's PDF, also known as Version of record
Publication date:
2003
Link to publication in University of Groningen/UMCG research database
Citation for published version (APA):
Oosterloo, T., Morganti, R., Sadler, E. M., Ferguson, A., Hulst, J. M., & Jerjen, H. (2003). Tidal Remnants
and Intergalactic HII Regions. Default journal.
Copyright
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Download date: 11-02-2018
Recycling intergalactic and interstellar matter
IAU Symposium Series, Vol. 217, 2004
Pierre-Alain Duc, Jonathan Braine and Elias Brinks, eds.
Tidal Remnants and Intergalactic H II Regions
Tom Oosterloo, Raaella Morganti
Netherlands Foundation for Research in Astronomy, Dwingeloo, NL
Elaine M. Sadler
School of Physics, University of Sydney, NSW 2006, Australia
Annette Ferguson
Max-Planck Institute fu¨r Astrophysik, Karl-Schwarzschild-Str. 1, 85748
Garching, Germany
Thijs van der Hulst
Kapteyn Astronomical Institute, RuG, Landleven 12, 9747 AD,
Groningen, NL
Helmut Jerjen
Research School of Astronomy & Astrophysics, Australian National
University, ACT 2611, Canberra, Australia
Abstract.
We report the discovery of two small intergalactic H II regions in the
loose group of galaxies around the eld elliptical NGC 1490. The H II
regions are located at least 100 kpc from any optical galaxy but are asso-
ciated with a number of large H I clouds that are lying along an arc 500
kpc in length and that have no optical counterpart on the Digital Sky
Survey. The sum of the H I masses of the clouds is almost 1010 M and
the largest H I cloud is about 100 kpc in size. Deep optical imaging reveals
a very low surface brightness counterpart to this largest H I cloud, mak-
ing this one of the H I richest optical galaxies known (MHI/LV  200).
Spectroscopy of the H II regions indicates that the abundance in these
H II regions is only slightly sub-solar, excluding a primordial origin of the
H I clouds. The H I clouds are perhaps remnants resulting from the tidal
disruption of a reasonably sized galaxy, probably quite some time ago,
by the loose group to which NGC 1490 belongs. Alternatively, they are
remnants of the merger that created the eld elliptical NGC 1490. The
isolated H II regions show that star formation on a very small scale can
occur in intergalactic space in gas drawn from galaxies by tidal interac-
tions. Many such intergalactic small star formation regions may exist
near tidally interacting galaxies.
1
2 Oosterloo et al.
1. Introduction
Hierarchical accretion and merging of small clumps appears to be a good de-
scription of the formation of early-type galaxies. In many early-type galaxies
signatures of recent hierarchical assembly can be observed, particularly in eld
galaxies. Optical ne structure -shells, dust lanes, \X" structures- revealed
through un-sharp masking of optical images suggests a recent accretion event.
These ne structures are correlated with the presence of an intermediate age
(1-2 Gyr) population of stars (e.g. Schweizer et al. 1990). Moreover, compar-
ing detailed stellar population models with high signal-to-noise optical spectra
indicates the presence of an intermediate age population in several early-type
galaxies (e.g. Trager et al. 2000). A small but signicant fraction of early-type
galaxies contain neutral hydrogen. The origin of this neutral hydrogen is gener-
ally thought to be external and due to recent accretion of companions. Hence the
presence of H I in early-type galaxies is generally thought to t into the picture
of continued accretion of companions and the associated intermittent episodes
of star formation in some early-type galaxies.
We are undertaking a systematic H I survey, using the Australia Telescope
Compact Array (ATCA), of all early-type galaxies south of δ < −25 withV <
6000 km s−1 (based on HIPASS, the Parkes All Sky H I Survey, see Barnes et
al. 2001 and elsewhere in this volume) to obtain an unbiased sample of H I-rich
early-type galaxies. Such a complete sample will allow us to study the eects
of H I on the evolution of early-type galaxies in dierent environments, and the
relation between H I and other indicators of recent accretion and star formation
(see also Sadler et al. 2002). We can also study whether H I always has an
external origin in early-type galaxies or whether in some early-type galaxies the
H I may be related to an earlier phase in the evolution of the galaxies.
During this HIPASS follow-up program we discovered two intergalactic H II
regions near the eld elliptical NGC 1490 that are probably related to merg-
ing/accretion within the loose group of galaxies around NGC 1490, or to the
merger that created the eld elliptical NGC 1490. Although unusual, these re-
gions seem to t into the generally accepted picture regarding H I in early-type
galaxies. We discuss these H II regions in section 2. However, we nd some
evidence that some early-type galaxies do not t into the scheme of continued
accretion. This we briefly discuss in section 3.
2. Intergalactic H II regions near NGC 1490
The HIPASS spectrum of NGC 1490 shows a strong detection with the presence
of almost 1010 M of neutral hydrogen. This detection was followed up with deep
H I observations performed with the ATCA to image the H I in the region around
NGC 1490. Five objects were detected with a total H I mass of 8.9  109M
(Figure 1). Only one H I cloud has an obvious counterpart (cloud 4) on the
Digital Sky Survey in the form of a small galaxy. The other H I clouds seem to
lie along an arc of more than 500 kpc long that bends around NGC 1490. About
half of the total amount of H I is found in one cloud of about 100 kpc in size
(cloud 1). Fig. 2 shows a position-velocity plot of two H I clouds, as marked in
Fig. 1. The kinematics of the H I clouds indicates that they are separate entities.
Intergalactic HII regions around NGC 1490 3
Figure 1. View of the entire region around NGC 1490, displaying the
5 objects detected in H I. Contour levels: 2, 4, 6, ...  1019 cm−2. The
lines indicate the orientation along which the position-velocity plots of
Fig. 2 are taken.
E.g. the velocity gradient in cloud 3 is perpendicular to the line connecting the
neighbouring clouds. The \U-shaped" pattern observed in the largest H I cloud
indicates that this cloud is not in rotational equilibrium.
Deep optical imaging was performed in a number of standard lters of the
region of this largest H I cloud, using FORS1 on the VLT. This imaging reveals
a very low surface brightness optical counterpart (the \OC"). The peak surface
brightness in the V band is 24.5 V mag arcsec−2. The low surface brightness
nature of the OC is illustrated in Fig. 3. The total magnitude is about 20.5
in V. This implies an extremely high gas-to-light ratio of MHI/LV  200. For
the colour indices we nd UB = 0.5, BV = 0.1 VR = 0.4 and RI = 0.6. These
colours indicate the presence of a young stellar population.
We further investigated the nature of the main H I cloud and its OC by
searching for the presence of Hα emission using narrow-band Hα imaging with
the NTT. These images reveal the presence of Hα emission in the OC at the
location of the peak of the H I, indicating that some star formation is occurring
in the OC. Most interestingly, two small regions of Hα emission were found at
the edge of the main H I cloud, at a distance of about 100 kpc from the OC
(indicated by 1 & 2 in Fig. 3). The broad-band images indicate that there is no
extended optical continuum light near these two Hα regions.
The Hα luminosities of the H II complexes are 3−51038 erg s−1 (i.e. 30-50
times Orion). The rate of star formation estimated from these luminosities is
very modest: around 0.005 Myr-1 per region. All the stars corresponding to the
4 Oosterloo et al.
Figure 2. Position-velocity plots of clouds 1 and 3, taken along the
line indicated in Figure 1. Contour levels are {1, 1, 2, 3, ... mJy
beam−1
diuse optical counterpart can be formed over a Hubble Time with such a star
formation rate.
Optical spectroscopy of the three Hα regions was performed with FORS1
on the VLT. Preliminary analysis of the data suggests that the abundances in
the Hα regions are only slightly below solar (in the range 0.5-1.0 Z). This
relatively high abundance excludes a primordial nature of the H I clouds.
2.1. The nature of the clouds near NGC 1490
The observations point to a tidal- or merger-related origin for the system of
H I clouds near NGC 1490. The alternative hypothesis, i.e. the H I clouds are
primordial, can be excluded because of the near solar abundance measured in
the H II complexes. The arc-like arrangement of the H I clouds is suggestive for
a tidal origin. It bears some resemblance to other systems. There is probably a
parallel between the objects discussed here and systems like e.g. the Leo Ring
(Schneider et al. 1989). Tidal interactions between galaxies and merging is a very
diverse process that can produce very spectacular objects with bright tidal tails.
But there are also cases where the evidence of an interaction is only detectable
in neutral hydrogen. The extreme version is that of a galaxy being completely
destroyed by a larger galaxy without any merging taking place. The material of
the original galaxy is spread out over such a large volume that, in the optical,
the surface brightness is well below the detection limit and only the H I remains
observable (e.g. Ryan-Weber et al. 2003, see also this proceedings). Perhaps the
H I clouds near NGC 1490 have such an origin.
An alternative hypothesis is that the H I clouds are remnants of the merger
between two or more gas-rich galaxies that also created the eld elliptical NGC 1490.
The environment of NGC 1490 can be characterised as a loose group. The pres-
ence of an elliptical galaxy in such a group often indicates that merging has
occurred in the group.
The interesting aspects of the case presented here are:
1) the very large amount of H I involved, distributed in fairly large clouds for
which only deep optical imaging reveals an optical counterpart. As the velocity
structure indicates, these clouds are separate entities, suggesting they are Tidal
Intergalactic HII regions around NGC 1490 5
Figure 3. Left H I contours on top of an r-band image obtained with
the NTT. Contour levels are as in Fig. 1. The location of the three
H II complex discovered using the narrow-band imaging are indicated.
Right Continuum optical image (top) centred on the regions 1 and 2
and the continuum subtracted line image, centred on the redshifted
Hα, revealing the two H II complexes (bottom)
Dwarfs even though they have no bright optical counterpart. The clouds are not,
or hardly, visible in the optical, but, given the large reservoir of H I, perhaps at
some point in time they will start to make stars and become brighter optical
objects.
2) the existence of an extremely gas-rich object with MHI/LV  200. This is
perhaps an example of a "Tidal Gas Cloud" that started making stars recently.
However, it is perhaps more likely that the optical material originates in a galaxy
that was destroyed in an interaction.
3) the presence of small H II complexes that are located far away from any opti-
cally detectable galaxy, in our case more than 100 kpc. Only very few examples
of such H II regions are known. The observations presented here suggest that in
more systems intergalactic H II complexes may exist.
3. Non-accretion origin of H I in some early-type galaxies?
As a nal remark, we note that, although the results discussed above t very well
into the picture continued accretion by early-type galaxies, and the associated
external origin of H I in early-type galaxies, we have found some indications that
some early-type galaxies do not t into this scheme. Our survey has revealed a
surprisingly large number of early-type galaxies that have very large (up to 200
kpc in size), regular disks of low column density H I. In Fig. 4 we present three
examples of such large regularly rotating H I disks. The largest of these disks
have H I masses up to 1010 M. Given their size and regular appearance, these
6 Oosterloo et al.
Figure 4. Examples of very large regular disks of H I distribution
around \normal" early-type galaxies obtained from the ATCA follow
up of HIPASS detections (see text for details). The disks shown here
have sizes between 150 and 200 kpc. Contour levels: 2, 4, 8, 16  1019
cm−2.
disks must be quite old, in many cases well over 5  109 yr. Hence, they are
not related to recent accretions and may be related to a much earlier formation
phase of these galaxies. Moreover, no large accretion can have occurred by
these galaxies, as these would likely have destroyed these large H I disks, or have
triggered signicant star formation in the H I disk. The H I column density in
these disks peaks at only 1020 atoms cm−2, so, despite the large H I reservoir,
no signicant star formation is occurring. These H I disks will evolve only very
slowly and will remain as purely gaseous disks for very long periods of time,
provided the environment is not too hostile for these disks.
References
Barnes et al. 2001, MNRAS 322, 486
Ryan-Weber, Webster & Bekki, 2003, ASSL Conf. Proc.281, p223
Sadler E.M., Oosterloo T., Morganti R., 2002 in The Dynamics, Structure &
History of Galaxies, Eds. G.S. Da Costa and Helmut Jerjen, ASP Con-
ference Proceedings, Vol. 273, p.215
Schneider et al. 1989, AJ, 97, 666
Schweizer F., Seitzer P., Faber S., Burnstein D., Dalle Ore C.M., Gonzales J.J.
1990, ApJL 364, L33
Trager S.C., Faber S., Worthey G., Gonzales J.J. 2000, AJ 120, 165





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