8 research outputs found
Keck Cosmic Web Imager Observations of He II Emission in I Zw 18
With a metallicity of 12 + Log(O/H) ≍ 7.1-7.2, I Zw 18 is a canonical low-metallicity blue compact dwarf (BCD) galaxy. A growing number of BCDs, including I Zw 18, have been found to host strong, narrow-lined, nebular He II (λ4686) emission with enhanced intensities compared to Hβ (e.g., He II(λ4686)/Hβ > 1%). We present new observations of I Zw 18 using the Keck Cosmic Web Imager. These observations reveal two nebular He II emission regions (or He III regions) northwest and southeast of the He III region in the galaxy's main body investigated in previous studies. All regions exhibit He II(λ4686)/Hβ greater than 2%. The two newly resolved He III regions lie along an axis that intercepts the position of I Zw 18's ultraluminous X-ray (ULX) source. We explore whether the ULX could power the two He III regions via shock activity and/or beamed X-ray emission. We find no evidence of shocks from the gas kinematics. If the ULX powers the two regions, the X-ray emission would need to be beamed. Another potential explanation is that a class of early-type nitrogen-rich Wolf-Rayet stars with low winds could power the two He III regions, in which case the alignment with the ULX would be coincidental
On the Kinematics of Cold, Metal-enriched Galactic Fountain Flows in Nearby Star-forming Galaxies
We use medium-resolution Keck/Echellette Spectrograph and Imager spectroscopy
of bright quasars to study cool gas traced by CaII 3934,3969 and NaI 5891,5897
absorption in the interstellar/circumgalactic media of 21 foreground
star-forming galaxies at redshifts 0.03 < z < 0.20 with stellar masses 7.4 <
log M_*/M_sun < 10.6. The quasar-galaxy pairs were drawn from a unique sample
of Sloan Digital Sky Survey quasar spectra with intervening nebular emission,
and thus have exceptionally close impact parameters (R_perp < 13 kpc). The
strength of this line emission implies that the galaxies' star formation rates
(SFRs) span a broad range, with several lying well above the star-forming
sequence. We use Voigt profile modeling to derive column densities and
component velocities for each absorber, finding that column densities N(CaII) >
10^12.5 cm^-2 (N(NaI) > 10^12.0 cm^-2) occur with an incidence f_C(CaII) =
0.63^+0.10_-0.11 (f_C(NaI) = 0.57^+0.10_-0.11). We find no evidence for a
dependence of f_C or the rest-frame equivalent widths W_r(CaII K) or W_r(NaI
5891) on R_perp or M_*. Instead, W_r(CaII K) is correlated with local SFR at
>3sigma significance, suggesting that CaII traces star formation-driven
outflows. While most of the absorbers have velocities within +/-50 km/s of the
host redshift, their velocity widths (characterized by Delta v_90) are
universally 30-177 km/s larger than that implied by tilted-ring modeling of the
velocities of interstellar material. These kinematics must trace galactic
fountain flows and demonstrate that they persist at R_perp > 5 kpc. Finally, we
assess the relationship between dust reddening and W_r(CaII K) (W_r(NaI 5891)),
finding that 33% (24%) of the absorbers are inconsistent with the best-fit
Milky Way E(B-V)-W_r relations at >3sigma significance.Comment: 38 pages, 16 figures, 4 tables. Accepted to Ap
DUVET Survey: Mapping Outflows in the Metal-Poor Starburst Mrk 1486
We present a method to characterize star-formation driven outflows from
edge-on galaxies and apply this method to the metal-poor starburst galaxy, Mrk
1486. Our method uses the distribution of emission line flux (from H and
[OIII] 5007) to identify the location of the outflow and measure the extent
above the disk, the opening angle, and the transverse kinematics. We show that
this simple technique recovers a similar distribution of the outflow without
requiring complex modelling of line-splitting or multi-Gaussian components, and
is therefore applicable to lower spectral resolution data. In Mrk 1486 we
observe an asymmetric outflow in both the location of the peak flux and total
flux from each lobe. We estimate an opening angle of depending
on the method and assumptions adopted. Within the minor axis outflows, we
estimate a total mass outflow rate of M yr, which
corresponds to a mass loading factor of . We observe a non-negligible
amount of flux from ionized gas outflowing along the edge of the disk
(perpendicular to the biconical components), with a mass outflow rate
M yr. Our results are intended to demonstrate a method that
can be applied to high-throughput, low spectral resolution observations, such
as narrow band filters or low spectral resolution IFS that may be more able to
recover the faint emission from outflows.Comment: 12 Pages, 6 Figure
DUVET: Spatially Resolved Observations of Star Formation Regulation via Galactic Outflows in a Starbursting Disk Galaxy
We compare 500~pc scale, resolved observations of ionised and molecular gas
for the starbursting disk galaxy IRAS08339+6517, using measurements
from KCWI and NOEMA. We explore the relationship of the star formation driven
ionised gas outflows with colocated galaxy properties. We find a roughly linear
relationship between the outflow mass flux () and star
formation rate surface density (), , and a strong correlation between
and the gas depletion time, such that
. Moreover, we find these
outflows are so-called ``breakout" outflows, according to the relationship
between the gas fraction and disk kinematics. Assuming that ionised outflow
mass scales with total outflow mass, our observations suggest that the regions
of highest in IRAS08 are removing more gas via the outflow
than through the conversion of gas into stars. Our results are consistent with
a picture in which the outflow limits the ability for a region of a disk to
maintain short depletion times. Our results underline the need for resolved
observations of outflows in more galaxies.Comment: 16 pages, 7 figures, Submitted to Ap
PHANGS–JWST First Results: Variations in PAH Fraction as a Function of ISM Phase and Metallicity
We present maps tracing the fraction of dust in the form of polycyclic aromatic hydrocarbons (PAHs) in IC 5332, NGC 628, NGC 1365, and NGC 7496 from JWST/MIRI observations. We trace the PAH fraction by combining the F770W (7.7 μ m) and F1130W (11.3 μ m) filters to track ionized and neutral PAH emission, respectively, and comparing the PAH emission to F2100W, which traces small, hot dust grains. We find the average R _PAH = (F770W + F1130W)/F2100W values of 3.3, 4.7, 5.1, and 3.6 in IC 5332, NGC 628, NGC 1365, and NGC 7496, respectively. We find that H ii regions traced by MUSE H α show a systematically low PAH fraction. The PAH fraction remains relatively constant across other galactic environments, with slight variations. We use CO+H i +H α to trace the interstellar gas phase and find that the PAH fraction decreases above a value of in all four galaxies. Radial profiles also show a decreasing PAH fraction with increasing radius, correlated with lower metallicity, in line with previous results showing a strong metallicity dependence to the PAH fraction. Our results suggest that the process of PAH destruction in ionized gas operates similarly across the four targets