Dwarf Galaxies with Ionizing Radiation Feedback II: Spatially-resolved Star Formation Relation Page: 7 of 16
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Dwarf Galaxies with Radiation Feedback. II
log[ SFR from Estimated Ha Emission (M.,yr/kpe2 )]
S 10D 15 2DD 250
x (20 kpc)
log[ Approx. H, Surface Density (Mpc2 ) ]
S 100 SO 200
x (20 kpc)
FIG. 7. Estimated SFR surface density (left, in M yr-i kpc-2, from mock Ha emission) and H2 surface density (right, in M pc-2). Shown at 13.3 Myr
into the high-resolution evolution of the MC-RTF run in a 20 kpc box centered on the galactic center. The same mock observation technique as in Figure 5 is
employed with uniform 50 pc resolution. The central region of radius 2.5 kpc is cut out in these maps (inner dashed circle; see 2 and 5.2). The locations of 50
local maxima are marked with black circles. Note that star-forming clumps are highly clustered; compare with the projected density in the left panel of Figure 1.
2011). Here Z' is the metallicity normalized to the solar value,
ad is the mean extinction cross section by dust per hydrogen
nucleus, pH is the mean mass per hydrogen nucleus, and pgas
is in the unit of g cm-3.
We hereafter define pH2, est fH2 Pgas and EH2, est as the H2
(surface) density evaluated from a simulated galaxy. This is to
emphasize that the our H2 density is an approximate estimate,
not a real value which is explicitly followed in simulations. In
Appendix B we discuss whether the use of the Krumholz et al.
(2008, 2009a) equilibrium model is adequate in our experi-
ment to determine molecular gas content. We also note that
in observational studies, '2CO J 1 -> 0 transition line emis-
sion is typically used to trace molecular gas, and we are ne-
glecting any additional scatter in the SFR relation that might
be induced by spatially-variable CO-to-H2 conversion factors.
The observations of the spatially-resolved star formation law
to which we will compare below all target normal spiral galax-
ies of roughly solar metallicity, and both observations and the-
oretical investigations of the CO-to-H2 conversion factor in
such galaxies suggest that its variation is at the factor of ~ 2
level (e.g. Wolfire et al. 2010; Narayanan et al. 2011, 2012;
Feldmann et al. 2012a,b; Bolatto et al. 2013). Thus the error
we make by neglecting this aspect of the observations is likely
to be a fairly modest effect.
5.1.3. Correlation Between SFR and H2 Surface Densities
Because the location of ionized gas is self-consistently
computed in our high-resolution radiation hydrodynamics
simulation, we are able to investigate the correlation between
ionized and molecular gas in detail. With PSFR,est and pH2,est
estimated as above, projections of both densities can be read-
ily generated. One such example is shown in Figure 5 where
a face-on projection of SFR density, EsFR,est, and that of H2
density, EH2,est, are produced and compared side by side.
These maps are generated with uniform 15 pc resolution at
13.3 Myr into the high-resolution evolution of the MC-RTF
run, in a 3 kpc box centered on a gas clump that harbors mul-
tiple young SFMC particles. Also displayed are the locations
of 20 local maxima of each map marked with black circles.11
The resolution of these maps can be regarded as a mock ob-
servation equivalent of an aperture size (diameter) in photo-
metric observations of local galaxies. For Figure 5, the reso-
lution of 15 pc is chosen in order to demonstrate how the loca-
tions of peaks in one map differ from the ones in the other. In
these images, it is unmistakable that Ha peaks and H2 peaks
are closely related, but they do not always coincide perfectly.
This experiment verifies the proposition we discussed in the
beginning of this section. That is, the site of highly ionized
gas (typically identified with Ha peaks) may not necessar-
ily overlap with the site of on-going star formation (typically
identified with H2 peaks).
To portray this discrepancy in a clearer way, in Figure 6 we
place the local maxima of Figure 5 on the plane of SFR sur-
face density, EsFR, est, and H2 surface density, EH2, est. Both
axes represent estimated values from the simulated galaxy,
MC-RTF, at 13.3 Myr into the high-resolution evolution. Each
data point can be regarded as a 15 pc aperture average at the
local maxima in the map of either EsFR,est or EH2,est. SFR sur-
face density peaks are marked with blue circles and H2 peaks
with red circles. It is obvious that the EsFR, est peaks are not
necessarily the peaks in EH2,est map, and vice versa. Further,
some data points suggest that EsFR,est peaks could have very
low EH2,est (upper left in the panel), and vice versa (lower
right in the panel). In other words, the locations of highly ion-
ized gas do not precisely correlate with those of dense molec-
ular gas. The discrepancy can be encapsulated by a separation
between the averages of the two sets of peaks. A blue star
represents the average EsFR, est and EH2, est for the peaks of the
ESFR,est map, whereas a red filled circle represents the average
for the peaks of the EH2, est map. The separation between the
blue star and the red filled circle characterizes how well Ha
performs as star formation tracers. The separation becomes
1oUsing FixedResolutionBuffer in yt (Turk et al. 2011) that deposits
meshes of varying sizes into a pixelized, fixed-resolution array.
t Using maximum filter in scipy package at http://www.scipy.org/.
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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/m1/7/: accessed July 22, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.