TY - JOUR
T1 - The ATLAS3D project - IX. The merger origin of a fast- and a slow-rotating early-type galaxy revealed with deep optical imaging
T2 - first results
AU - Duc, Pierre-Alain
AU - Cuillandre, Jean-Charles
AU - Serra, Paolo
AU - Michel-Dansac, Leo
AU - Ferriere, Etienne
AU - Alatalo, Katherine
AU - Blitz, Leo
AU - Bois, Maxime
AU - Bournaud, Frédéric
AU - Bureau, Martin
AU - Cappellari, Michele
AU - Davies, Roger L.
AU - Davis, Timothy A.
AU - de Zeeuw, P. T.
AU - Emsellem, Eric
AU - Khochfar, Sadegh
AU - Krajnović, Davor
AU - Kuntschner, Harald
AU - Lablanche, Pierre-Yves
AU - McDermid, Richard M.
AU - Morganti, Raffaella
AU - Naab, Thorsten
AU - Oosterloo, Tom
AU - Sarzi, Marc
AU - Scott, Nicholas
AU - Weijmans, Anne-Marie
AU - Young, Lisa M.
PY - 2011/10/1
Y1 - 2011/10/1
N2 - The mass assembly of galaxies leaves imprints in their outskirts, such
as shells and tidal tails. The frequency and properties of such fine
structures depend on the main acting mechanisms - secular evolution,
minor or major mergers - and on the age of the last substantial
accretion event. We use this to constrain the mass assembly history of
two apparently relaxed nearby early-type galaxies (ETGs) selected from
the ATLAS3D sample, NGC 680 and 5557. Our ultra-deep optical
images obtained with MegaCam on the Canada-France-Hawaii Telescope reach
29 mag arcsec-2 in the g band. They reveal very low surface
brightness (LSB) filamentary structures around these ellipticals. Among
them, a gigantic 160 kpc long, narrow, tail east of NGC 5557 hosts three
gas-rich star-forming objects, previously detected in H I with the
Westerbork Synthesis Radio Telescope and in UV with GALEX. NGC 680
exhibits two major diffuse plumes apparently connected to extended H I
tails, as well as a series of arcs and shells. Comparing the outer
stellar and gaseous morphology of the two ellipticals with that
predicted from models of colliding galaxies, we argue that the LSB
features are tidal debris and that each of these two ETGs was assembled
during a relatively recent, major wet merger, which most likely occurred
after the redshift z ≃ 0.5 epoch. Had these mergers been older, the
tidal features should have already fallen back or be destroyed by more
recent accretion events. However, the absence of molecular gas and of a
prominent young stellar population in the core region of the galaxies
indicates that the merger is at least 1-2 Gyr old: the memory of any
merger-triggered nuclear starburst has indeed been lost. The
star-forming objects found towards the collisional debris of NGC 5557
are then likely tidal dwarf galaxies. Such recycled galaxies here appear
to be long-lived and continue to form stars while any star formation
activity has stopped in their parent galaxy. The inner kinematics of NGC
680 is typical for fast rotators which make the bulk of nearby ETGs in
the ATLAS3D sample. On the other hand, NGC 5557 belongs to
the poorly populated class of massive, round, slow rotators that are
predicted by semi-analytic models and cosmological simulations to be the
end-product of a complex mass accretion history, involving ancient major
mergers and more recent minor mergers. Our observations suggest that
under specific circumstances a single binary merger may dominate the
formation history of such objects and thus that at least some massive
ETGs may form at relatively low redshift. Whether the two galaxies
studied here are representative of their own sub-class of ETGs is still
an open question that will be addressed by an on-going deep optical
survey of ATLAS3D galaxies.
AB - The mass assembly of galaxies leaves imprints in their outskirts, such
as shells and tidal tails. The frequency and properties of such fine
structures depend on the main acting mechanisms - secular evolution,
minor or major mergers - and on the age of the last substantial
accretion event. We use this to constrain the mass assembly history of
two apparently relaxed nearby early-type galaxies (ETGs) selected from
the ATLAS3D sample, NGC 680 and 5557. Our ultra-deep optical
images obtained with MegaCam on the Canada-France-Hawaii Telescope reach
29 mag arcsec-2 in the g band. They reveal very low surface
brightness (LSB) filamentary structures around these ellipticals. Among
them, a gigantic 160 kpc long, narrow, tail east of NGC 5557 hosts three
gas-rich star-forming objects, previously detected in H I with the
Westerbork Synthesis Radio Telescope and in UV with GALEX. NGC 680
exhibits two major diffuse plumes apparently connected to extended H I
tails, as well as a series of arcs and shells. Comparing the outer
stellar and gaseous morphology of the two ellipticals with that
predicted from models of colliding galaxies, we argue that the LSB
features are tidal debris and that each of these two ETGs was assembled
during a relatively recent, major wet merger, which most likely occurred
after the redshift z ≃ 0.5 epoch. Had these mergers been older, the
tidal features should have already fallen back or be destroyed by more
recent accretion events. However, the absence of molecular gas and of a
prominent young stellar population in the core region of the galaxies
indicates that the merger is at least 1-2 Gyr old: the memory of any
merger-triggered nuclear starburst has indeed been lost. The
star-forming objects found towards the collisional debris of NGC 5557
are then likely tidal dwarf galaxies. Such recycled galaxies here appear
to be long-lived and continue to form stars while any star formation
activity has stopped in their parent galaxy. The inner kinematics of NGC
680 is typical for fast rotators which make the bulk of nearby ETGs in
the ATLAS3D sample. On the other hand, NGC 5557 belongs to
the poorly populated class of massive, round, slow rotators that are
predicted by semi-analytic models and cosmological simulations to be the
end-product of a complex mass accretion history, involving ancient major
mergers and more recent minor mergers. Our observations suggest that
under specific circumstances a single binary merger may dominate the
formation history of such objects and thus that at least some massive
ETGs may form at relatively low redshift. Whether the two galaxies
studied here are representative of their own sub-class of ETGs is still
an open question that will be addressed by an on-going deep optical
survey of ATLAS3D galaxies.
KW - galaxies: elliptical and lenticular
KW - cD
KW - galaxies: formation
KW - galaxies: individual: NGC 5557
KW - galaxies: individual: NGC 680
KW - galaxies: interactions
U2 - 10.1111/j.1365-2966.2011.19137.x
DO - 10.1111/j.1365-2966.2011.19137.x
M3 - Article
VL - 417
SP - 863
EP - 881
JO - Monthly Notices of the Royal Astronomical Society
JF - Monthly Notices of the Royal Astronomical Society
SN - 0035-8711
IS - 2
ER -