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Extended Tidal Structure in Two Lyα-Emitting Starburst Galaxies
Evan D. Skillman (University of Minnesota), John M. Cannon (MPIA, Heidelberg), Daniel Kunth (Institut d’Astrophysique, Paris), Claus Leitherer
(STScI), Miguel Mas-Hesse (Centro de Astrobiologia CSIC-INTA, Spain), Göran Östlin (Stockholm Observatory, Sweden), Artashes Petrosian (Byurakan
Astrophysical Observatory and Isaac Newton Institute of Chile, Aremian Branch, Armenia)
The Propagation of Lya Photons
These two systems were chosen for imaging because they exhibit
prominent Lyα emission (Leitherer et al. 1995, Giavalisco et al. 1996, Kunth
et al. 1998, Leitherer et al. 2002). Hydrogen Lya is one of the most
important diagnostic emission lines in astrophysics. It is predicted to be
luminous in star-forming galaxies (Charlot & Fall 1993), and could
potentially be used as an indicator of star formation activity in distant
galaxies where the line is shifted into the visual or near-infrared region.
However, this application is not straightforward, since the appearance of Lyα
emission from starburst galaxies does not correlate well with the strength of
the burst, the metallicity of the ionized gas or the dust content as measured
by extinction. Rather, the geometry of the neutral gas and the presence of
outflows appear to be important factors in determining the appearance of
Lya emission.
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It is well known that due to high resonant scattering by neutral
hydrogen in the ISM, Lya photons can be attenuated by even small
amounts of dust. It is then expected that only young, relatively dust-free
galaxies should be prodigious sites of Lya emission. Low-metallicity
starburst galaxies in the local universe may be considered nearby analogs
to such objects which are expected in greater numbers at higher redshifts.
Thus it was surprising that HST observations of the most metal-poor galaxy
known, I Zw 18, showed only damped Lya absorption and no emission
(Kunth et al. 1994). In stark contrast, the more metal-rich starburst galaxy
Haro 2 showed prominent Lya emission (Lequeux et al. 1995). Further
observations have revealed the importance of ISM structure in determining
the escape fraction of Lya photons (Giavalisco et al. 1996, Kunth et al.
1998).
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The studies of Kunth et al. (1998), Tenorio-Tagle (1999), and MasHesse (2003) have elucidated the characteristics which appear to govern
the propagation of Lya photons in star-forming galaxies. If static,
homogeneous neutral gas with column densities  1018 cm-2 shields the
ionized gas, no emission will be detected. The resonant scattering of the
Lya photons will lead to increased probability of destruction by any dust
which is present. On the other hand, there may be diffuse Lya emission
which is detectable on sightlines not coincident with the sources of UV
photons. Similarly, if the areal coverage of the neutral gas is not uniform but
clumpy, some Lya emission may be detectable on favorable sightlines.
Finally, if the velocity structure of the neutral gas is not static but rather
outflowing from the ionizing regions (outflow velocities  200 km sec-1), Lya
photons to the red of 1216 Å can escape and Lya emission may be
significant. This explains the strong Lya emission detected in some
starburst galaxies with complete spatial coverage by neutral gas which is
also comparatively rich in both metals and dust.
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Astrophysical Journal, 2004, 608, 768
Abstract
We present new VLA C-configuration HI imaging of the Lyaemitting starburst galaxies Tol 1924-416 and IRAS 08339+6517. The
effective resolution probes neutral gas structures larger than 4.7
kpc in Tol 1924-416, and larger than 8.1 kpc in IRAS 08339+6517.
Both systems are revealed to be tidally interacting: Tol 1924-416
with ESO 338-IG04B (6.6΄ = 72 kpc minimum separation), and IRAS
08339+6517 with 2MASX J08380769+6508579 (2.4΄ = 56 kpc
minimum separation). The HI emission is extended in these
systems, with tidal tails and debris between the target galaxies and
their companions. Since Lyα emission has been detected from
both of these primary systems, these observations suggest that
the geometry of the ISM is one of the factors affecting the escape
fraction of Lyα emission from starburst environments. This could
be a very important factor to take into account when calculating
the escape fraction of ionizing radiation from starbursts in the
early universe. Furthermore, these observations argue for the
importance of interactions in triggering massive star formation
events. …………………………………………………………………….
Conclusions
VLA HI imaging of the starburst galaxies Tol 1924-416 and IRAS
08339 + 6517 has been presented. These two systems are remarkably
similar in HI content, mass, and current evolutionary state. In each, we find
extended neutral gas between the target and nearby neighbors, suggesting
that interactions have played an important role in triggering the massive
starbursts in the primary galaxies. The close proximity of the companions
suggests that the interactions were recent, and the similar velocities of both
primary and secondary galaxies argues that these systems may end up
gravitationally bound.
.
……………………………………
Since both primary systems are intense Lya emitters, these data
support the interpretation that the ISM kinematics are an important
mechanism that controls the escape of Lya photons from starburst regions.
This has immediate implications for the use of the strength of Lya emission
in determining star formation rates, since the results will be dependent on
the geometry of the ISM and not on properties inherent to the starburst
being considered. Further HI observations of Lya-emitting galaxies (and,
conversely, of starburst systems with no apparent Lya emission) are
certainly warranted to further explore the role of the ISM in regulating the
escape of Lya photons from starburst environments.
….
Distance (Mpc)
Tol 1924-416
37.5
ESO 338-IG04B
37.5
IRAS 08339+6517
80
2MASX J08380769+6508579
80
MV
-19.57
---
-20.35
---
VSYS (km sec-1)
HI Mass (M)
System Mass (M)
Tidal Fraction (%)
2830
(1.4±0.2) x 109
5750
(9.3±1.2) x 108
(4.2±0.5) x 109
~ 40
In some systems, regardless of total mass, gravitational interactions
can initiate powerful starburst episodes. In Taylor (1997), it was found that
HII galaxies (i.e., systems undergoing significant, concentrated star
formation events) have more HI-rich companions detected at small
separations than a similar sample of low surface brightness galaxies. This
suggests that at least some bursts of star formation in low-mass systems
are tidally triggered. Examples abound of higher-mass systems where
interactions have initiated prolific star formation events (e.g., the
``Antennae'' galaxies) and have even produced new galactic systems in
their own right (“tidal dwarf galaxies”). With these data we demonstrate the
extended tidal structure of two starburst systems, providing strong evidence
for triggered star formation episodes in these galaxies.
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Figure 1. (a) DSS image of Tol 1924-416, overlaid with contours of the HI
zeroth-moment image. Contours correspond to column densities of (7.3, 29,
51, 73) x 10 20 cm -2 . Each galaxy is labeled; beam size is shown at
bottom left. (b) Intensity-weighted velocity field of Tol 1924-416. From this
figure it is apparent that HI is being removed from one or both systems. Also,
there remains a component of solid-body rotation within the optical extent of
both galaxies. Beam size is labeled at lower left.
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Figure 2. (a) DSS image of IRAS 08339 + 6517, overlaid with contours of the
HI zeroth-moment image. Contours correspond to column densities of (5.5, 15,
24, 33, 42, 51) x 1020 cm-2. Each galaxy is labeled; beam size is shown at
bottom right. (b) Intensity-weighted velocity field of IRAS 08339 + 6517. From
this figure it is apparent that HI is being removed from one or both systems.
The companion galaxy appears to retain a component of solid-body rotation in
neutral gas; clear signs of rotation are less prominent in IRAS 08339 + 6517,
however, suggesting that this interaction has completely disrupted the neutral
gas in this system. Beam size is labeled at lower right.
.
Figure 3. DSS image of Tol 1924-416, overlaid with contours of HI emission at
the 3, 4.5, 6, 7.5 and 9 s levels; this corresponds to column densities of (2.3,
3.4, 4.6, 5.7, 6.9) x 1020 cm-2, respectively. Each galaxy is labeled in the upper
left plane; beam size is shown at bottom left of each frame, and heliocentric
velocities are labeled in the upper right.
.
Starburst Triggering Mechanisms.
(1.1±0.2) x 109
(7.0±0.9) x 108
(5.6±0.7) x 109
~ 70
Figure 4. DSS image of IRAS 08339 + 6517, overlaid with contours of HI
emission at the 3, 4.5, 6, 7.5 and 9 s levels; this corresponds to column
densities of (6.9, 10.3, 13.8, 17.2, 20.7) x 1020 cm-2, respectively. Each galaxy
is labeled in the upper left plane; beam size is shown at bottom right of each
frame, and heliocentric velocities are labeled in the upper left.
.
Relevance for Early Universe Studies.
Currently it is thought that QSOs alone are not capable of reionizing
the early universe (Madau et al. 1999). Starbursts then become a likely
candidate for providing the necessary ionizing photons. However, studies of
low-redshift starbursts indicate that only a small fraction of the ionizing
radiation can escape from starbursts (Heckman et al. 2001). The present
observation that Lyα emission escapes more easily from interacting systems
implies that Lyman continuum photons may also escape more easily. Since
the frequency of interactions is thought to be much higher in the early
universe, this may imply that starbursts are more efficient at contributing to
the reionization of the universe than inferred from observations of low
redshift starburst systems .
.
Acknowledgements
The National Radio Astronomy Observatory is a facility of the National
Science Foundation operated under cooperative agreement by Associated
Universities, Inc. Support for this work was provided by NASA through grant
number GO-9470 from the Space Telescope Science Institute, which is
operated by AURA, Inc., under NASA contract NAS5-26555. J.M.C. was
supported by NASA Graduate Student Researchers Program (GSRP)
Fellowship NGT 5-50346. E.D.S. acknowledges partial support from NASA
LTSARP grant NAG5-9221 and the University of Minnesota.
..
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