Dale Clayton Jackson

On extending the mass-metallicity relation of galaxies by 2.5 decades in stellar mass

We report 4.5 � m luminosities for 27 nearby (DP5 Mpc) dwarf irregular galaxies measured with the Spitzer Infrared Array Camera. We have constructed the 4.5 � m luminosity-metallicity (L-Z) relation for 25 dwarf galaxies with secure distance and interstellar medium oxygen abundance measurements. The 4.5 � m L-Z relation is 12þ log (O/H) ¼ (5:78 � 0:21) þ (� 0:122 � 0:012)M½4:5� , where M[4.5] is the absolute magnitude at 4.5 � m. The dispersion in the near-infrared L-Z relation is smaller than the corresponding dispersion in the optical L-Z relation. The subsequently derived stellar mass-metallicity (M� -Z) relation is 12 þ log (O/H) ¼ (5:65 � 0:23) þ (0:298 � 0:030)log M� , and extends the SDSSM� - Zrelation to lower mass by about 2.5 dex. We find that the dispersion in the M� -Z relation is similar over 5 orders of magnitude in stellar mass, and that the relationship between stellar mass and interstellar medium metallicity is similarly tight from high-mass to low-mass systems. We find a larger scatter at low mass in the relation between effective yield and total baryonic mass. In fact, there are a few dwarf galaxies with large yields, which is difficult to explain if galactic winds are ubiquitous in dwarf galaxies. The low scatter in the L‐Z and M� -Z relationships are difficult to understand if galactic superwinds or blowout are responsible for the low metallicities at low mass or luminosity. Naively, one would expect an ever increasing scatter at lower masses, which is not observed. Subject headingg galaxies: dwarf — galaxies: evolution — galaxies: irregular — infrared: galaxies Online material: color figures

A Spitzer IRAC Census of the Asymptotic Giant Branch Populations in Local Group Dwarfs. I. WLM

We present Spitzer IRAC observations at 3.6 and 4.5 ?m along with optical data from the Local Group Galaxies Survey to investigate the evolved stellar population of the Local Group dwarf irregular galaxy WLM. These observations provide a nearly complete census of the AGB stars. We find that 39% of the infrared-detected AGB stars are not detected in the optical data, even though our 50% completeness limit is 3 mag fainter than the red giant branch tip. An additional 4% of the infrared-detected AGBs are misidentified in the optical, presumably due to reddening by circumstellar dust. We also compare our results with those of a narrowband optical carbon star survey of WLM and find the latter study sensitive to only 18% of the total AGB population. We detect objects with infrared fluxes consistent with them being mass-losing AGB stars and derive a present-day total mass-loss rate from the AGB stars of = (0.7-2.4) ? 10-3 M? yr-1. The distributions of mass-loss rates and bolometric luminosities of AGBs and red supergiants are very similar to those in the LMC and SMC, and the empirical maximum mass-loss rate observed in the LMC and SMC is in excellent agreement with our WLM data.

A Spitzer IRAC Census of the Asymptotic Giant Branch Populations in Local Group Dwarfs. II. IC 1613

We present Spitzer/IRAC observations at 3.6 and 4.5 microns along with optical data from the Local Group Galaxies Survey to investigate the evolved stellar population of the Local Group dwarf irregular galaxy WLM. These observations provide a nearly complete census of the asymptotic giant branch (AGB) stars. We find 39% of the infrared-detected AGB stars are not detected in the optical data, even though our 50% completeness limit is three magnitudes fainter than the red giant branch tip. An additional 4% of the infrared-detected AGBs are misidentified in the optical, presumably due to reddening by circumstellar dust. We also compare our results with those of a narrow-band optical carbon star survey of WLM, and find the latter study sensitive to only 18% of the total AGB population. We detect objects with infrared fluxes consistent with them being mass-losing AGB stars, and derive a present day total mass-loss rate from the AGB stars of 0.7-2.4 x 10^(-3) solar masses per year. The distribution of mass-loss rates and bolometric luminosities of AGBs and red supergiants are very similar to those in the LMC and SMC and the empirical maximum mass-loss rate observed in the LMC and SMC is in excellent agreement with our WLM data.

H I Observations of the Local Group Dwarf WLM

We present Australia Telescope Compact Array mosaic H i imaging of the Local Group dwarf irregular galaxy WLM. We find an integrated flux of 149 Jy km s � 1 and a total H i mass of (3:2 � 0:3);10 7 M� .T he major axis of the Hi is aligned with the stellar component of the galaxy. The overall H i distribution is relatively smooth at our resolution and has a double-peaked central core. One of these peaks is aligned with a region found to have extinction that is internal to WLM, and we take this as possible evidence of a large molecular gas complex in the southern half of the galaxy. The other H i peak is in close proximity to the brightest H ii regions. WLM’s overall velocity field is consistent with rigid-body rotation. A rotation curve is derived, and we find a total dynamical mass of (3:00 � 0:80);10 8 M� . We also performed a wide-field search, 38 0 in radius, for H i companions or evidence of recent interactions (e.g., tidal tails) and found no detections to an H i mass limit of M HI > 8:4;10 5 M� .