68 research outputs found
Metal Abundances of KISS Galaxies III. Nebular Abundances for Fourteen Galaxies and the Luminosity-Metallicity Relationship for HII Galaxies
We report results from the third in a series of nebular abundance studies of
emission-line galaxies from the KPNO International Spectroscopic Survey (KISS).
Galaxies with coarse metallicity estimates of 12 + log(O/H) less than 8.2 dex
were selected for observation. Spectra of 14 galaxies, which cover the full
optical region from [OII]3727,3729 to beyond [SII]6717,6731, are presented, and
abundance ratios of N, O, Ne, S, and Ar are computed. The auroral [OIII]4363
line is detected in all 14 galaxies. Oxygen abundances determined through the
direct electron temperature T_e method confirm that the sample is metal-poor
with 7.61 <= 12 + log(O/H) <= 8.32. By using these abundances in conjunction
with other T_e-based measurements from the literature, we demonstrate that HII
galaxies and more quiescent dwarf irregular galaxies follow similar
metallicity-luminosity (L-Z) relationships. The primary difference is a
zero-point shift between the correlations such that HII galaxies are brighter
by an average of 0.8 B magnitudes at a given metallicity. This offset can be
used as evidence to argue that low-luminosity HII galaxies typically undergo
factor of two luminosity enhancements, and starbursts that elevate the
luminosities of their host galaxies by 2 to 3 magnitudes are not as common. We
also demonstrate that the inclusion of interacting galaxies can increase the
scatter in the L-Z relation and may force the observed correlation towards
lower metallicities and/or larger luminosities. This must be taken into account
when attempting to infer metal abundance evolution by comparing local L-Z
relations with ones based on higher redshift samples since the fraction of
interacting galaxies should increase with look-back time.Comment: 36 pages, 5 figures. ApJ, in pres
Resolved Near-infrared Stellar Populations in Nearby Galaxies
We present near-infrared (NIR) color-magnitude diagrams (CMDs) for the resolved stellar populations within 26 fields of 23 nearby galaxies (âČ 4 Mpc), based on images in the F110W and F160W filters taken with the 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 Myr 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 in F160W 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 red giant branch (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 Carnegie Hubble Program: The Infrared Leavitt Law in IC 1613
We have observed the dwarf galaxy IC 1613, at multiple epochs in the mid-infrared using Spitzer and contemporaneously in the near-infrared using the new FourStar near-infrared camera on Magellan. We have constructed Cepheid periodâluminosity relations in the J, H, K_s, [3.6] and [4.5] bands and have used the run of their apparent distance moduli as a function of wavelength to derive the line-of-sight reddening and distance to IC 1613. Using a nine-band fit, we find E(B â V) = 0.05 ± 0.01 mag and an extinction-corrected distance modulus of ÎŒ_0 = 24.29 ± 0.03_(statistical) ± 0.03_(systematic) mag. By comparing our multi-band and [3.6] distance moduli to results from the tip of the red giant branch and red clump distance indicators, we find that metallicity has no measurable effect on Cepheid distances at 3.6âÎŒm in the metallicity range â1.0 †[Fe/H] †0.2, hence derivations of the Hubble constant at this wavelength require no correction for metallicity
Spectral Energy Distributions of Local Luminous And Ultraluminous Infrared Galaxies
Luminous and ultraluminous infrared galaxies ((U)LIRGs) are the most extreme
star forming galaxies in the universe. The local (U)LIRGs provide a unique
opportunity to study their multi-wavelength properties in detail for comparison
to their more numerous counterparts at high redshifts. We present common large
aperture photometry at radio through X-ray wavelengths, and spectral energy
distributions (SEDs) for a sample of 53 nearby LIRGs and 11 ULIRGs spanning log
(LIR/Lsun) = 11.14-12.57 from the flux-limited Great Observatories All-sky LIRG
Survey (GOALS). The SEDs for all objects are similar in that they show a broad,
thermal stellar peak and a dominant FIR thermal dust peak, where nuLnu(60um) /
nuLnu(V) increases from ~2-30 with increasing LIR. When normalized at
IRAS-60um, the largest range in the luminosity ratio,
R(lambda)=log[nuLnu(lambda)/nuLnu(60um)] observed over the full sample is seen
in the Hard X-rays (HX=2-10 keV). A small range is found in the Radio (1.4GHz),
where the mean ratio is largest. Total infrared luminosities, LIR(8-1000um),
dust temperatures, and dust masses were computed from fitting thermal dust
emission modified blackbodies to the mid-infrared (MIR) through submillimeter
SEDs. The new results reflect an overall ~0.02 dex lower luminosity than the
original IRAS values. Total stellar masses were computed by fitting stellar
population synthesis models to the observed near-infrared (NIR) through
ultraviolet (UV) SEDs. Mean stellar masses are found to be log(M/Msun) =
10.79+/-0.40. Star formation rates have been determined from the infrared
(SFR_IR~45Msun/yr) and from the monochromatic UV luminosities
(SFR_UV~1.3Msun/yr), respectively. Multiwavelength AGN indicators have be used
to select putative AGN: about 60% of the ULIRGs would have been classified as
an AGN by at least one of the selection criteria.Comment: 39 pages, including 12 figures and 11 tables; accepted for
publication in ApJ
The ACS Nearby Galaxy Survey Treasury IX. Constraining asymptotic giant branch evolution with old metal-poor galaxies
In an attempt to constrain evolutionary models of the asymptotic giant branch
(AGB) phase at the limit of low masses and low metallicities, we have examined
the luminosity functions and number ratio between AGB and red giant branch
(RGB) stars from a sample of resolved galaxies from the ACS Nearby Galaxy
Survey Treasury (ANGST). This database provides HST optical photometry together
with maps of completeness, photometric errors, and star formation histories for
dozens of galaxies within 4 Mpc. We select 12 galaxies characterized by
predominantly metal-poor populations as indicated by a very steep and blue RGB,
and which do not present any indication of recent star formation in their
color--magnitude diagrams. Thousands of AGB stars brighter than the tip of the
RGB (TRGB) are present in the sample (between 60 and 400 per galaxy), hence the
Poisson noise has little impact in our measurements of the AGB/RGB ratio. We
model the photometric data with a few sets of thermally pulsing AGB (TP-AGB)
evolutionary models with different prescriptions for the mass loss. This
technique allows us to set stringent constraints to the TP-AGB models of
low-mass metal-poor stars (with M<1.5 Msun, [Fe/H]<~-1.0). Indeed, those which
satisfactorily reproduce the observed AGB/RGB ratios have TP-AGB lifetimes
between 1.2 and 1.8 Myr, and finish their nuclear burning lives with masses
between 0.51 and 0.55 Msun. This is also in good agreement with recent
observations of white dwarf masses in the M4 old globular cluster. These
constraints can be added to those already derived from Magellanic Cloud star
clusters as important mileposts in the arduous process of calibrating AGB
evolutionary models.Comment: To appear in ApJ, a version with better resolution is in
http://stev.oapd.inaf.it/~lgirardi/rgbagb.pd
Metal Abundances of KISS Galaxies. I. Coarse Metal Abundances and the Metallicity-Luminosity Relation
We derive metal abundance estimates for a large sample of starbursting
emission-line galaxies (ELGs). Our sample is drawn from the KPNO International
Spectroscopic Survey (KISS) which has discovered over 2000 ELG candidates to
date. Follow-up optical spectra have been obtained for ~900 of these objects. A
three step process is used to obtain metal abundances for these galaxies. We
first calculate accurate nebular abundances for 12 galaxies whose spectra cover
the full optical region from [OII]3726,29 to beyond [SII]6717,31 and include
detection of [OIII]4363. Using secondary metallicity indicators R_23 and p_3,
we calculate metallicities for an additional 59 galaxies with spectra that
cover a similar wavelength range but lack [OIII]4363. The results are used to
calibrate relations between metallicity and two readily observed emission-line
ratios, which allow us to estimate coarse metallicities for 519 galaxies in
total. The uncertainty in these latter abundance estimates is 0.16 dex. From
the large, homogeneously observed sample of star-forming galaxies we identify
low metallicity candidates for future study and investigate the
metallicity-luminosity relation. We find a linear metallicity-luminosity
relation of the following form: 12 + log(O/H) = 4.059 - 0.240 M_B, with an RMS
scatter of 0.252. This result implies that the slope of the metallicity-
luminosity relation is steeper than when dwarf galaxies are considered alone,
and may be evidence that the relationship is not linear over the full
luminosity range of the sample.Comment: 15 pages, including 7 figures and 4 tables. To appear in May, 2002 A
The Panchromatic Hubble Andromeda Treasury. III. Measuring Ages and Masses of Partially Resolved Stellar Clusters
The apparent age and mass of a stellar cluster can be strongly affected by stochastic sampling of the stellar initial mass function (IMF), when inferred from the integrated color of low-mass clusters (âŸ10^4 M_â). We use simulated star clusters to show that these effects are minimized when the brightest, rapidly evolving stars in a cluster can be resolved, and the light of the fainter, more numerous unresolved stars can be analyzed separately. When comparing the light from the less luminous cluster members to models of unresolved light, more accurate age estimates can be obtained than when analyzing the integrated light from the entire cluster under the assumption that the IMF is fully populated. We show the success of this technique first using simulated clusters, and then with a stellar cluster in M31. This method represents one way of accounting for the discrete, stochastic sampling of the stellar IMF in less massive clusters and can be leveraged in studies of clusters throughout the Local Group and other nearby galaxies
Metal Abundances of KISS Galaxies. IV. Galaxian Luminosity-Metallicity Relations in the Optical and Near-IR
We explore the galaxian luminosity-metallicity (L-Z) relationship in both the
optical and the near-IR using a combination of optical photometric and
spectroscopic observations from the KPNO International Spectroscopic Survey
(KISS) and near-infrared photometry from the Two-micron All Sky Survey (2MASS).
We supplement the 2MASS data with our own NIR photometry for a small number of
lower-luminosity ELGs that are under-represented in the 2MASS database. Our
B-band L-Z relationship includes 765 star-forming KISS galaxies with coarse
abundance estimates from our follow-up spectra, while the correlation with KISS
and 2MASS yields a total of 420 galaxies in our J-band L-Z relationship. We
explore the effect that changing the correlation between the strong-line
abundance diagnostic R_23 and metallicity has on the derived L-Z relation. We
find that the slope of the L-Z relationship decreases as the wavelength of the
luminosity bandpass increases. We interpret this as being, at least in part, an
effect of internal absorption in the host galaxy. Furthermore, the dispersion
in the L-Z relation decreases for the NIR bands, suggesting that variations in
internal absorption contribute significantly to the observed scatter. We
propose that our NIR L-Z relations are more fundamental than the B-band
relation, since they are largely free of absorption effects and the NIR
luminosities are more directly related to the stellar mass of the galaxy than
are the optical luminosities.Comment: 22 pages, including 6 figures and 6 tables. Accepted for publication
in the Astrophysical Journal (20 May 2005 issue
Strong PAH Emission from z~2 ULIRGs
Using the Infrared Spectrograph on board the Spitzer Space Telescope, we
present low-resolution (64 < lambda / dlambda < 124), mid-infrared (20-38
micron) spectra of 23 high-redshift ULIRGs detected in the Bootes field of the
NOAO Deep Wide-Field Survey. All of the sources were selected to have 1) fnu(24
micron) > 0.5 mJy; 2) R-[24] > 14 Vega mag; and 3) a prominent rest-frame 1.6
micron stellar photospheric feature redshifted into Spitzer's 3-8 micron IRAC
bands. Of these, 20 show emission from polycyclic aromatic hydrocarbons (PAHs),
usually interpreted as signatures of star formation. The PAH features indicate
redshifts in the range 1.5 =1.96 and a dispersion
of 0.30. Based on local templates, these sources have extremely large infrared
luminosities, comparable to that of submillimeter galaxies. Our results confirm
previous indications that the rest-frame 1.6 micron stellar bump can be
efficiently used to select highly obscured starforming galaxies at z~2, and
that the fraction of starburst-dominated ULIRGs increases to faint 24 micron
flux densities. Using local templates, we find that the observed narrow
redshift distribution is due to the fact that the 24 micron detectability of
PAH-rich sources peaks sharply at z = 1.9. We can analogously explain the
broader redshift distribution of Spitzer-detected AGN-dominated ULIRGs based on
the shapes of their SEDs. Finally, we conclude that z~2 sources with a
detectable 1.6 micron stellar opacity feature lack sufficient AGN emission to
veil the 7.7 micron PAH band.Comment: accepted for publication in ApJ; references corrected in Section 3.2
and Figure
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