Dwarf Galaxies with Ionizing Radiation Feedback II: Spatially-resolved Star Formation Relation Page: 4 of 16
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J. Kim et al.
- - C -
2 4 1 2I 2
-- . - -
t n y 13 I t . M
., M C - R T F
12 14 16 18 20 22 24 26 28 30
Time into High-resolution Evolution (Myr)
FIG. 2. Time evolution of the instantaneous galactic star formation rate in
the unit of MoyX1 from 13.3 Myr to 30.7 Myr of the high-resolution evolu-
tion. For the MC-TF (solid line, only with supernova feedback) and MC-RTF
runs (dashed line, with both supernova and stellar radiation feedback).
Egas, for the two runs with and without the stellar radia-
tion feedback, between 13.3 Myr and 30.7 Myr of the high-
resolution evolution. Each of the densities is measured on a
galactic disk of 9 kpc in radius excluding the central 2.5 kpc
in which SFMC particles do not radiate ionizing photons (see
2). We estimate the SFR surface density by using the stars
of age less than T 0.37 Myr. The movement of the data
points in time on this plane is relatively small during the an-
alyzed period which covers only 17.4 Myrs. The error bars
here show the extent of such movements; and the movement
in x-axis (variation in Egas) is so small that its error bar is not
visible in the plot. We recognize that the additional suppres-
sion by stellar radiation is not large enough to move the data
points down to the observed global Kennicutt-Schmidt rela-
tion (Kennicutt 1998). The global star formation surface den-
sity would have been lower if a larger energy input had been
employed for thermal supernovae explosion (see 2 and/or Pa-
per I; we intentionally choose a relatively small fiducial value
for thermal feedback in order to further contrast the effect of
radiation feedback). The suppression by feedback could have
been even more effective if the SFMC particles in the inner
disk had also radiated ionizing photons (< 2.5 kpc from the
4.2. Interstellar Medium Structure
We now focus on how ionizing radiation feedback regulates
star formation. Photoionizing radiation from massive stars is
one of the important factors that drives changes in the environ-
ment of star-forming clumps, and self-regulates star formation
(Whitworth 1979; Matzner 2002). In order to characterize the
nature of the impact of ionizing stellar radiation feedback, we
compare the galactic ISM structures of the two runs, MC-TF
and MC-RTF. The bottom rows of Figure 4 displays the two
dimensional probability distribution functions (PDFs) of pro-
ton number density and gas temperature, colored by gas mass
in each bin. The measurement is made in the sphere of 10 kpc
radius centered on a galactic center (but excluding the inner
2.5 kpc sphere) at 13.3 Myrs into the high-resolution evolu-
tion for each of the performed simulations.
The ionizing radiation from young SFMC particles heats
the surrounding dense gas up to 105 K and pushes the PDF
in an upward direction. In the MC-TF run dense gas cells
0.5 1.0 1.5
log[ Gas Surface Density (I/Ipc2 ) ]
FIG. 3. Time-averaged location of global SFR surface density, EsFR,
and global gas surface density, Egas, between 13.3 Myr and 30.7 Myr of
the high-resolution evolution, for the MC-TF (red circle) and MC-RTF runs
(blue square). Also plotted are the observed global Kennicutt-Schmidt re-
lations: the best fits to the disk-averaged galactic observations by Kennicutt
(1998, long dashed line with a shaded region representing their errors) and
by Bouch6 et al. (2007, dot dashed line that includes high-z galaxies).
with number density above 103 cm-3, the threshold density
for the SFMC particle formation (see 2), are almost com-
pletely turned into SFMC particles. However, in the MC-RTF
run, zone "A" still hosts very dense gas cells above 103 cm-3,
which are beyond the threshold density. Photoheating by stel-
lar radiation retains such hot dense gas which otherwise would
have been unstable against Jeans fragmentation and deposited
into SFMC particles. At the same time, zone "B" depicts rel-
atively less dense gas cells heated up to 105 K, but mostly
clustered around ~ 2 x 104 K just above the bump in the cool-
ing curve by the [011I] forbidden line. The top rows of the
same figure show one dimensional PDFs of gas number den-
sity at the same epoch, binned by cold (< 2 x 103 K), warm
(2 x 103 -2 x 105 K), and hot gas (> 2 x 105 K). In the plot
for the MC-RTF run, the increased masses of the warm gas
above 103 cm-3 (corresponding to zone "A') and the hot gas
at around ~ 10-2 cm-3 are prominent.
5. RESULTS II: SPATIALLY-RESOLVED STAR FORMATION
In this section we move to the spatially-resolved star forma-
tion relation on a simulated galactic disk. We start by describ-
ing our methodology which for allows us to make spatially-
resolved mock observations of galactic star formation tracers,
such as Ha emission.
5.1. Simulated Observation of Star Formation Relation
The traditional star formation tracer of Ha emission
(Balmer line of n 3 -> 2, 1.88eV) maps out the ionized
gas around young massive stars. The site of highly ionized
gas, however, may not overlap with the site of dense collaps-
ing gas in which on-going star formation takes place, usually
identified by peaks of molecular hydrogen, H2. This discrep-
ancy may become even more problematic when the observed
galaxy is resolved with higher spatial accuracy which distin-
guishes such regions from one another. Therefore in an ob-
servation with high spatial resolution, one may question the
validity of the traditional star formation tracers, in particu-
lar, Ha emission (e.g. Onodera et al. 2010; Schruba et al.
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Kim, Ji-hoon; Krumholz, Mark R.; Wise, John H.; Turk, Matthew J.; Goldbaum, Nathan J. & Abel, Tom. Dwarf Galaxies with Ionizing Radiation Feedback II: Spatially-resolved Star Formation Relation, article, October 21, 2013; United States. (https://digital.library.unt.edu/ark:/67531/metadc867631/m1/4/: accessed June 25, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.