Near-infrared Spectroscopy of Five Ultra-massive Galaxies at 1.7 <z< 2.7 (original) (raw)
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Astronomy and Astrophysics, 2009
Context. Near-infrared (hereafter NIR) data may provide complementary information to the traditional optical population synthesis analysis of unresolved stellar populations because the spectral energy distribution of the galaxies in the 1-2.5 µm range is dominated by different types of stars than at optical wavelengths. Furthermore, NIR data are subjected to less absorption and hence could constrain the stellar populations in dust-obscured galaxies. Aims. We want to develop observational constraints on the stellar populations of unresolved stellar systems in the NIR. Methods. To achieve this goal we need a benchmark sample of NIR spectra of "simple" early-type galaxies, to be used for testing and calibrating the outputs of population synthesis models. We obtained low-resolution (R∼1000) long-slit spectra between 1.5 and 2.4 µm for 14 nearby early-type galaxies using SofI at the ESO 3.5-m New Technology Telescope and higher resolution (R∼3000) long-slit spectra, centered at the MgI at ∼1.51 µm for a heterogeneous sample of 5 nearby galaxies observed with ISAAC at Antu, one of the 8.2-m ESO Very Large Telescope. Results. We defined spectral indices for CO, NaI, CaI and MgI features and measured the strengths of these features in the sample galaxies. We defined a new global NIR metallicity index, suitable for abundance measurements in low-resolution spectra. Finally, we present an average NIR spectrum of an early-type galaxy, built from a homogenized subset of our sample. Conclusions. The NIR spectra of the sample galaxies show great similarity and the strength of some features does correlate with the iron abundance [Fe/H] and optical metal features of the galaxies. The data suggest that the NIR metal features, in combination with a hydrogen absorption feature may be able to break the age-metallicity degeneracy just like the Mg and Fe features in the optical wavelength range.
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Resolved Near-Infrared Stellar Populations in Nearby Galaxies
The Astrophysical Journal Supplement Series, 2012
We present near-infrared (NIR) color-magnitude diagrams (CMDs) for the resolved stellar populations within 26 fields of 23 nearby galaxies ( 4 Mpc), based on images in the F 110W and F 160W filters taken with Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST). The CMDs are measured in regions spanning a wide range of star formation histories, including both old dormant and young star-forming populations. We match key NIR CMD features with their counterparts in more familiar optical CMDs, and identify the red core Helium burning (RHeB) sequence as a significant contributor to the NIR flux in stellar populations younger than a few 100 Myrs old. The strength of this feature suggests that the NIR mass-to-light ratio can vary significantly on short timescales in star forming systems. The NIR luminosity of star forming galaxies is therefore not necessarily proportional to the stellar mass. We note that these individual RHeB stars may also be misidentified as old stellar clusters in images of nearby galaxies. For older stellar populations, we discuss the CMD location of asymptotic giant branch (AGB) stars in the HST filter set, and explore the separation of AGB subpopulations using a combination of optical and NIR colors. We empirically calibrate the magnitude of the NIR tip of the red giant branch (TRGB) in F 160W as a function of color, allowing future observations in this widely adopted filter set to be used for distance measurements. We also analyze the properties of the NIR RGB as a function of metallicity, showing a clear trend between NIR RGB color and metallicity. However, based on the current study, it appears unlikely that the slope of the NIR RGB can be used as an effective metallicity indicator in extragalactic systems with comparable data. Finally, we highlight issues with scattered light in the WFC3, which becomes significant for exposures taken close to a bright earth limb.
The Astrophysical Journal, 2009
We analyze a sample of galaxies chosen to have F 24µm > 0.5mJy and satisfy a certain IRAC color criterion. IRS spectra yield redshifts, spectral types, and PAH luminosities, to which we add broadband photometry from optical through IRAC wavelengths, MIPS from 24-160 µm, 1.1 millimeter, and radio at 1.4 GHz. Stellar population modeling and IRS spectra together demonstrate that the double criteria used to select this sample have efficiently isolated massive star-forming galaxies at z ∼ 1.9. This is the first starburst-dominated ULIRG sample at high redshift with total infrared luminosity measured directly from FIR and millimeter photometry, and as such gives us the first accurate view of broadband SEDs for starburst galaxies at extremely high luminosity and at all wavelengths. Similar broadband data are assembled for three other galaxy samples -local starburst galaxies, local AGN/ULIRGS, and a second 24µm-luminous z ∼ 2 sample dominated by AGN. L P AH /L IR for the new z ∼ 2 starburst sample is the highest ever seen, some three times higher than in local starbursts, whereas in AGNs this ratio is depressed below the starburst trend, often severely. Several pieces of evidence imply that AGNs exist in this starburst dominated sample, except two of which even host very strong AGN, while they still have very strong PAH emission. The ACS images show most objects have very extended morphologies in the rest-frame UV band, thus extended distribution of PAH molecules. Such an extended distribution prevents further destruction PAH molecules by central AGNs. We conclude that objects in this sample are ULIRGs powered mainly by starburst; and the total infrared luminosity density contributed by this type of objects is 0.9 − 2.6 × 10 7 L ⊙ /Mpc 3 .
2011
We present near-infrared (NIR) color-magnitude diagrams (CMDs) for the resolved stellar popula-tions within 26 fields of 23 nearby galaxies (. 4 Mpc), based on images in the F110W and F160W filters taken with Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST). The CMDs are measured in regions spanning a wide range of star formation histories, including both old dormant and young star-forming populations. We match key NIR CMD features with their counterparts in more familiar optical CMDs, and identify the red core Helium burning (RHeB) sequence as a significant contributor to the NIR flux in stellar populations younger than a few 100 Myrs old. The strength of this feature suggests that the NIR mass-to-light ratio can vary significantly on short timescales in star forming systems. The NIR luminosity of star forming galaxies is therefore not necessarily proportional to the stellar mass. We note that these individual RHeB stars may also be misidentified as old stellar cluste...
Mid-Infrared Spectroscopy of Two Luminous Submillimeter Galaxies at z ~ 2.8
The Astrophysical Journal, 2005
Using the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope, we have obtained rest frame mid-infrared spectroscopy of two bright submillimeter galaxies. SMMJ02399-0136 at z=2.81 shows a superposition of PAH emission features and a mid-infrared continuum, indicating significant and roughly equal contributions to its bolometric luminosity from star formation and from a Compton-thick AGN. We derive a new redshift of z=2.80 for MMJ154127+6616 from the IRS spectrum and find this object is dominated by starburst PAH emission. The rest frame mid-to far-infrared spectral energy distributions are consistent with these submillimeter galaxies being scaled up versions of local ultraluminous infrared galaxies. The mid-infrared spectra support the scenario that submillimeter galaxies are sites of extreme star formation and represent a key phase in the formation of massive galaxies.