Local Group evolved stellar populations in the near infrared

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.

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...

2008

A programme has been started, using the recently commissioned Nagoya-South African 1.4m Infrared Survey Facility (IRSF) at SAAO Sutherland, to study the stellar populations, evolution and structures of Local Group galaxies.

Monthly Notices of the Royal Astronomical Society, 2008

We present a study of the evolved stellar populations in the dwarf spheroidal galaxy Leo II, based on JHK s observations obtained with the near-infrared array WFCAM at the UKIRT telescope. Combining the new data with optical data, we derived photometric estimates of the distribution of global metallicity [M/H] of individual red giant stars from their V −K s colours. Our results are consistent with the metallicities of RGB stars obtained from Ca ii triplet spectroscopy, once the age effects are considered. The photometric metallicity distribution function has a peak at [M/H] = −1.74 (uncorrected) or [M/H] = −1.64 ± 0.06 (random) ±0.17 (systematic) after correction for the mean age of Leo II stars (9 Gyr). The distribution is similar to a Gaussian with σ [M/H] = 0.19 dex, corrected for instrumental errors. We used the new data to derive the properties of a nearly complete sample of asymptotic giant branch (AGB) stars in Leo II. Using a near-infrared two-colour diagram, we were able to obtain a clean separation from Milky Way foreground stars and discriminate between carbonand oxygen-rich AGB stars, which allowed to study their distribution in K s -band luminosity and colour. We simulate the JHK s data with the trilegal population synthesis code together with the most updated thermally pulsing AGB models, and using the star formation histories derived from independent work based on deep HST photometry. After scaling the mass of Leo II models to the observed number of upper RGB stars, we find that present models predict too many O-rich TP-AGB stars of higher luminosity due to a likely under-estimation of either their mass-loss rates at low metallicity, and/or their degree of obscuration by circumstellar dust. On the other hand, the TP-AGB models are able to reproduce the observed number and luminosities of carbon stars satisfactorily well, indicating that in this galaxy the least massive stars that became carbon stars should have masses as low as ∼ 1 M ⊙ .

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.

Monthly Notices of the Royal Astronomical Society, 2009

We employ IRTF SpeX NIR (0.8µm-2.4µm) spectra to investigate the stellar population (SP), active galactic nuclei (AGN) featureless continuum (F C) and hot dust properties in 9 Sy 1 and 15 Sy 2 galaxies. Both the STARLIGHT code and the hot dust as an additional base element were used for the first time in this spectral range. We found evidence of correlation among the equivalent widths (W λ ) Si I 1.59µm × Mg I 1.58µm, equally for both kinds of activity. Part of the W Na I 2.21µm and W CO 2.3µm strengths may be related to galaxy inclination. Our synthesis shows significant differences between Sy 1 and Sy 2 galaxies: the hot dust component is required to fit the K-band spectra of ∼90% of the Sy 1 galaxies, and only of ∼25% of the Sy 2; about 50% of the Sy 2 galaxies require a F C component contribution 20%, while this fraction increases to about 60% in the Sy 1; also, in about 50% of the Sy2, the combined FC and young components contribute with more than 20%, while this occurs in 90% of the Sy1, suggesting recent star formation in the central region. The central few hundred parsecs of our galaxy sample contain a substantial fraction of intermediate-age SPs with a mean metallicity near solar. Our SP synthesis confirms that the 1.1µm CN band can be used as a tracer of intermediate-age SPs. The simultaneous fitting of SP, F C and hot dust components increased in ∼ 150% the number of AGNs with hot dust detected and the mass estimated. The NIR emerges as an excellent window to study the stellar population of Sy 1 galaxies, as opposed to the usually heavily attenuated optical range. Our approach opens a new way to investigate and quantify the individual contribution of the three most important NIR continuum components observed in AGNs.

arXiv (Cornell University), 2023

We present the JWST Resolved Stellar Populations Early Release Science (ERS) science program. We obtained 27.5 hours of NIRCam and NIRISS imaging of three targets in the Local Group (Milky Way globular cluster M92, ultra-faint dwarf galaxy Draco II, star-forming dwarf galaxy WLM), which span factors of ∼ 10 5 in luminosity, ∼ 10 4 in distance, and ∼ 10 5 in surface brightness. We describe the survey strategy, scientific and technical goals, implementation details, present select NIRCam color-magnitude diagrams (CMDs), and validate the NIRCam exposure time calculator (ETC). Our CMDs are among the deepest in existence for each class of target. They touch the theoretical hydrogen burning limit in M92 (< 0.08 M ; SNR ∼ 5 at m F 090W ∼ 28.2; M F 090W ∼ +13.6), include the lowest-mass stars observed outside the Milky Way in Draco II (0.09 M ; SNR = 10 at m F 090W ∼ 29; M F 090W ∼ +12.1), and reach ∼ 1.5 magnitudes below the oldest main sequence turnoff in WLM (SNR = 10 at m F 090W ∼ 29.5; M F 090W ∼ +4.6). The PARSEC stellar models provide a good qualitative match to the NIRCam CMDs, though are ∼ 0.05 mag too blue compared to M92 F090W−F150W data. The NIRCam ETC (v2.0) matches the SNRs based on photon noise from DOLPHOT stellar photometry in uncrowded fields, but the ETC may not be accurate in more crowded fields, similar to what is known for HST. We release beta versions of DOLPHOT NIRCam and NIRISS modules to the community. Results from this ERS program will establish JWST as the premier instrument for resolved stellar populations studies for decades to come.

The Astrophysical Journal, 2012

Using high spatial resolution Hubble Space Telescope Wide Field Camera 3 and Advance Camera for Surverys imaging of resolved stellar populations, we constrain the contribution of thermally-pulsing asymptotic giant branch (TP-AGB) stars and red helium burning (RHeB) stars to the 1.6 µm nearinfrared (NIR) luminosities of 23 nearby galaxies, including dwarfs and spirals. The TP-AGB phase contributes as much as 17% of the integrated F 160W flux, even when the red giant branch is well populated. The RHeB population contribution can match or even exceed the TP-AGB contribution, providing as much as 21% (18% after a statistical correction for foreground) of the integrated F 160W light. We estimate that these two short lived phases may account for up to 70% of the rest-frame NIR flux at higher redshift. The NIR mass-to-light (M/L) ratio should therefore be expected to vary significantly due to fluctuations in the star formation rate over timescales from 25 Myr to several Gyr, an effect that may be responsible for some of the lingering scatter in NIR galaxy scaling relations such as the Tully-Fisher and metallicity-luminosity relations. We compare our observational results to predictions based on optically derived star formation histories and stellar population synthesis (SPS) models, including models based on the 2008 Padova isochrones (used in popular SPS programs) and the updated 2010 Padova isochrones, which shorten the lifetimes of low-mass (old) low-metallicity TP-AGB populations. The updated (2010) SPS models generally reproduce the expected numbers of TP-AGB stars in the sample; indeed, for 65% of the galaxies, the discrepancy between modeled and observed numbers is smaller than the measurement uncertainties. The weighted mean model/data number ratio for TP-AGB stars is 1.5 (1.4 with outliers removed) with a standard deviation of 0.5. The same SPS models, however, give a larger discrepancy in the F 160W flux contribution from the TP-AGB stars, over-predicting the flux by a weighted mean factor of 2.3 (2.2 with outliers removed) with a standard deviation of 0.8. This larger offset is driven by the prediction of modest numbers of high luminosity TP-AGB stars at young (< 300 Myrs) ages. The best-fit SPS models simultaneously tend to under-predict the numbers and fluxes of stars on the RHeB sequence, typically by a factor of 2.0 ± 0.6 for galaxies with significant numbers of RHeBs. Possible explanations for both the TP-AGB and RHeB model results include: (1) difficulties with measuring the SFHs of galaxies especially on the short timescales over which these stars evolve (several Myrs); (2) issues with the way the SPS codes populate the CMDs (e.g. how they handle pulsations or self extinction), and/or (3) lingering issues with the lifetimes of these stars in the stellar evolution codes. Coincidentally these two competing discrepancies -over-prediction of the TP-AGB and under-prediction of the RHeBs -result in a predicted NIR M/L ratio largely unchanged for a rapid star formation rate, after correcting for these effects. However, the NIR-to-optical flux ratio of galaxies could be significantly smaller than AGB-rich models would predict, an outcome that has been observed in some intermediate redshift post-starburst galaxies.

Arxiv preprint arXiv: …, 2009

Context. The Red MSX Source (RMS) survey is a multi-wavelength campaign of follow-up observations of a colour-selected sample of candidate massive young stellar objects (MYSOs) in the galactic plane. This survey is returning the largest well-selected sample of MYSOs to date, while identifying other dust contaminant sources with similar mid-infrared colours including a large number of new ultra-compact (UC) H ii regions. Aims. To measure the far-infrared (IR) flux, which lies near the peak of the spectral energy distribution (SED) of MYSOs and UCH ii regions, so that, together with distance information, the luminosity of these sources can be obtained. Methods. Less than 50 % of RMS sources are associated with IRAS point sources with detections at 60 µm and 100 µm, though the vast majority are visible in Spitzer MIPSGAL or IRAS Galaxy Atlas (IGA) images. However, standard aperture photometry is not appropriate for these data due to crowding of sources and strong spatially variable far-IR background emission in the galactic plane. A new technique using a 2-dimensional fit to the background in an annulus around each source is therefore used to obtain far-IR photometry for young RMS sources. Results. Far-IR fluxes are obtained for a total of 1113 RMS candidates identified as young sources. Of these 734 have flux measurements using IGA 60 µm and 100 µm images and 724 using MIPSGAL 70 µm images, with 345 having measurements in both data sets.

2010

We present a study of a large, statistically complete sample of star-forming dwarf galaxies using mid-infrared observations from the Spitzer Space Telescope. The relationships between metallicity, star formation rate (SFR) and mid-infrared color in these systems show that the galaxies span a wide range of properties. However, the galaxies do show a deficit of 8.0 µm polycyclic aromatic hydrocarbon emission as is apparent from the median 8.0 µm luminosity which is only 0.004 L * 8.0 while the median B-band luminosity is 0.05 L * B . Despite many of the galaxies being 8.0 µm deficient, there is about a factor of 4 more extremely red galaxies in the [3.6] − [8.0] color than for a sample of normal galaxies with similar optical colors. We show correlations between the [3.6] − [8.0] color and luminosity, metallicity, and to a lesser extent SFRs that were not evident in the original, smaller sample studied previously. The luminosity-metallicity relation has a flatter slope for dwarf galaxies as has been indicated by previous work. We also show a relationship between the 8.0 µm luminosity and the metallicity of the galaxy which is not expected given the competing effects (stellar mass, stellar population age, and the hardness of the radiation field) that influence the 8.0 µm emission. This larger sample plus a well-defined selection function also allows us to compute the 8.0 µm luminosity function and compare it with the one for the local galaxy population. Our results show that below 10 9 L ⊙ , nearly all the 8.0 µm luminosity density of the local universe arises from dwarf galaxies that exhibit strong Hα emission -i.e., 8.0 µm and Hα selection identify similar galaxy populations despite the deficit of 8.0 µm emission observed in these dwarfs.