Transcript Spatially Resolved Imaging at 350

Spatially Resolved Imaging at
350 μm of Cold Dust in Nearby
Elliptical Galaxies
(arXiv: 0801.0591, accepted for publication in The
Astrophysical Journal (ApJ) and tentatively scheduled
for the ApJ April 10, 2008, v677 n1 issue )
Lerothodi L. Leeuw, Jacqueline Davidson, C. Darren
Dowell, and Henry E. Matthews
Outline
•
•
•
•
1. Introduction
2. Observations
3. Discussion
4. Summary
1. Introduction
• Origin of dust in Elliptical galaxies:
• 1. Internal origin: From circumstellar envelopes of red
giant stars.
Evidences: 10μm emission in excess of stellar emission,
with similar luminosity profile.
Problems: How can these dust survive in hot gas? If
dust-induced-cooling timescale of hot gas is shorter than
dust destruction timescale. If this cooling is dominated,
dust should have strong FIR emission.
• 2. External origin: Mergers or accretion events including
a gas-rich galaxy.
Evidences: (a) FIR luminosity and cold gas content
uncorrelated with optical luminosities. (b) Irregular dust
lane and filaments detected with HST.
• Goals of this work:
• (a) Determine the physical properties of
the dust.
• (b) To see if the dust correlates with other
spatially resolved features.
• (c) Determine the heating source.
• (d) Address the question whether the dust
is internal or external origin.
2. Observations
• The sample:
• Seven Galaxies: NGC 83, NGC 759, NGC 807, UGC
1503, NGC 3656, NGC 4476, NGC 5666, all with
spatially resolved CO emission. The sample have
different environments, and are in different stages, from
on-going major mergers to very late accretion or
quiescent systems.
• Instrument:
• SHARC II. Field of view: 148”×56”. FWHM beamsize:
8.5”. Wavelength: 350μm.
• The presented submm mapping is sensitive to
continuum emission from optically-thin dust, thus will
detect dust deep or in the far-side of the galaxies which
is not visible in optical images.
• NGC3656:
• Early-age majormerger/accretion
• HI occurs in
shells and tidal
tails.
• Multi-wavelength
spur: warped
disk of dust?
• NGC 5666:
• Intermediate-age
merger/accretion
• HI: in circular
orbits, in dynamic
equilibrium.
• Optical: “spiral”
dusty structure
with tidal tails and
H II regions.
• NGC 83 and NGC 759:
• Late Merger/accretion
• The surface brightness profiles follow r1/4 law, submm not resolved.
NGC 83
NGC 759
•
•
•
•
UGC1503 and NGC 807:
Quiescent or Very-late-accretion
Field galaxies
No signatures of merger or accretion
UGC 1503
NGC 807
• NGC 4476:
• Accretion Virgo cluster dwarf
• No HI detected, but CO and submm
emission is detected
• Spiral-like dust lane, 19 degree
from major axis. It seems that the
dust component is separate from
the stellar one, so the dust lane is
more likely externally accreted.
• SED and luminosity:
• Data:
From the SHARC II 350 μm observation and
public archives and literature.
• Origin of mid-IR to submm emission:
A cold to cool (5K<T<100K) thermal component
plus a mid-IR power-law component that
includes PAHs, warm (T>100K) dust, and stellar
components.
• Model:
Power law in mid-IR (8 to 40 μm) + Graybody in
far-IR (40 to 2000 μm)
Here Power law is unphysical, only aid in the
calculation of total luminosity.
• Fit result:
• The cold dust
temperature:
22K<T<31K
• For most of the
galaxies, MIR-tosubmm excess are
~20-30% of the
total luminosities,
with ~70% or more
emitted at FIR-tomm (thermal cold
dust).
• Early and
intermediate-age
galaxies NGC3656
and NGC5666
have higher MIR-to
submm excess,
suggesting more or
denser dust and
star formation.
3. Discussion
• 3.1 Star formation rates and other luminosity
sources:
• Estimate star formation rate from FIR or CO
For NGC3656
and NGC759,
SFRco>>SFRFIR.
Most of the
galaxies in this
sample are using
their gas
reservoirs at
“normal” rate to
form stars, but
these two have
more gas.
• Is dust from red giant possible?
• Scenario: Cold dust from mass losing red
giants, rather than from recent galaxy
merger or accretion events. FIR excess is
due to the heating from hot ISM.
• Estimate: To achieve the observed FIR
luminosity, the red giant mass loss rate
needs to be 15-340 Msolar/yr for this set of
galaxies, which is far larger than those for
giant elliptical galaxies (~1 Msolar/yr), but
the optical/near-IR ratio is comparable to
normal giant ellipticals.
• Conclusion: Most of the FIR luminosity
comes from star formation.
• 3.2 Dust masses and Gas-to-Dust Mass
Ratios:
• Dust mass can be estimated from the
temperature and IR luminosity.
4. Summary
• 350 μm emission is detected in seven elliptical
galaxies with known CO-gas disks.
• The dust’s most likely and dominant heating
source is star formation triggered by an
accretion or merger event.
• Dust masses for the sample range from ~9X105
Msolar to ~2X107 Msolar. The total cold-gas-mass
to dust-mass ratio is in a relatively narrow
range: 230<M(H2+HI)/Mdust<400, about 3 times
the value for the Milky Way.
Thank you!