12 research outputs found
Rest-Frame Optical Spectra of Three Strongly Lensed Galaxies at z~2
We present Keck II NIRSPEC rest-frame optical spectra for three recently
discovered lensed galaxies: the Cosmic Horseshoe (z = 2.38), the Clone (z =
2.00), and SDSS J090122.37+181432.3 (z = 2.26). The boost in signal-to-noise
ratio (S/N) from gravitational lensing provides an unusually detailed view of
the physical conditions in these objects. A full complement of high S/N
rest-frame optical emission lines is measured, spanning from rest-frame 3600 to
6800AA, including robust detections of fainter lines such as H-gamma,
[SII]6717,6732, and in one instance [NeII]3869. SDSS J090122.37+181432.3 shows
evidence for AGN activity, and therefore we focus our analysis on star-forming
regions in the Cosmic Horseshoe and the Clone. For these two objects, we
estimate a wide range of physical properties, including star-formation rate
(SFR), metallicity, dynamical mass, and dust extinction. In all respects, the
lensed objects appear fairly typical of UV-selected star-forming galaxies at
z~2. The Clone occupies a position on the emission-line diagnostic diagram of
[OIII]/H-beta vs. [NII]/H-alpha that is offset from the locations of z~0
galaxies. Our new NIRSPEC measurements may provide quantitative insights into
why high-redshift objects display such properties. From the [SII] line ratio,
high electron densities (~1000 cm^(-3)) are inferred compared to local
galaxies, and [OIII]/[OII] line ratios indicate higher ionization parameters
compared to the local population. Building on previous similar results at z~2,
these measurements provide further evidence (at high S/N) that star-forming
regions are significantly different in high-redshift galaxies, compared to
their local counterparts (abridged).Comment: 16 pages, 8 figures. Accepted for publication in the Astrophysical
Journa
A Young Super Star Cluster in the Nuclear Region of NGC 253
We present observations of a massive star cluster near the nuclear region of
the nearby starburst galaxy NGC 253. The peak of near-infrared emission, which
is spatially separated by 4" from the kinematic center of the galaxy, is
coincident with a super star cluster whose properties we examine with
low-resolution (R ~ 1,200) infrared CTIO spectroscopy and optical/near-infrared
HST imaging. Extinction, measured from [FeII] lines, is estimated at Av = 17.7
+/- 2.6. The age of the cluster is estimated at 5.7 Myr, based on Bry
equivalent width for an instantaneous burst using Starburst99 modeling.
However, a complex star formation history is inferred from the presence of both
recombination emission and photospheric CO absorption. The ionizing photon flux
has a lower limit of 7.3 +/- 2.5 x 10^53 inverse seconds, corrected for
extinction. Assuming a Kroupa IMF, we estimate a cluster mass of 1.4 +/- 0.4 x
10^7 solar masses. We observe a strong Wolf-Rayet signature at 2.06 microns and
report a weak feature at 2.19 microns which may be due to a massive stellar
population, consistent with the derived mass and age of this cluster.Comment: 8 pages, 5 figures, accepted for publication in Ap