As part of a major program to use isolated Local Group dwarf galaxies as near-field probes of cosmology, we have obtained deep images of the dwarf irregular galaxy Leo A with the Advanced Camera for ...Surveys aboard the Hubble Space Telescope. From these images we have constructed a color-magnitude diagram (CMD) reaching apparent absolute magnitudes of (M sub(475), M sub(814)) (29.0 +4.4, 27.9 +3.4), the deepest ever achieved for any irregular galaxy beyond the Magellanic Clouds. We derive the star formation rate (SFR) as a function of time over the entire history of the galaxy. We find that over 90% of all the star formation that ever occurred in Leo A happened more recently than 8 Gyr ago. The CMD shows only a very small amount of star formation in the first few billion years after the big bang; a possible burst at the oldest ages cannot be claimed with high confidence. The peak SFR occurred -1.5-4 Gyr ago, at a level 5-10 times the current value. Our modeling indicates that Leo A has experienced very little metallicity evolution; the mean inferred metallicity is consistent with measurements of the present-day gas-phase oxygen abundance. We cannot exclude a scenario in which all of the ancient star formation occurred prior to the end of the era of reionization, but it seems unlikely that the lack of star formation prior to -8 Gyr ago was due to early loss or exhaustion of the in situ gas reservoir.
ABSTRACT
Based on color–magnitude diagrams (CMDs) from the
Hubble Space Telescope
Hubble Tarantula Treasury Project (HTTP) survey, we present the star formation history of Hodge 301, the oldest star ...cluster in the Tarantula Nebula. The HTTP photometry extends faint enough to reach, for the first time, the cluster pre-main sequence (PMS) turn-on, where the PMS joins the main sequence. Using the location of this feature, along with synthetic CMDs generated with the latest PARSEC models, we find that Hodge 301 is older than previously thought, with an age between 26.5 and 31.5 Myr. From this age, we also estimate that between 38 and 61 Type II supernovae exploded in the region. The same age is derived from the main sequence turn-off, whereas the age derived from the post-main sequence stars is younger and between 20 and 25 Myr. Other relevant parameters are a total stellar mass of ≈8800 ± 800
M
⊙
and average reddening
E
(
B
−
V
) ≈ 0.22–0.24 mag, with a differential reddening
δE
(
B
−
V
) ≈ 0.04 mag.
The conditions in the Galactic Center are often compared with those in starburst systems, which contain higher supernova rates, stronger magnetic fields, more intense radiation fields, and larger ...amounts of dense molecular gas than in our own Galactic disk. Interactions between such an augmented interstellar medium and cosmic rays result in brighter radio and gamma -ray emission. Here, we test how well the comparisons between the Galactic Center and starburst galaxies hold by applying a model for cosmic-ray interactions to the Galactic Center to predict the resulting gamma -ray emission. The model only partially explains the observed gamma -ray and radio emission. The model for the gamma -ray spectrum agrees with the data at TeV energies but not at GeV energies. Additionally, as the fits of the model to the radio and gamma -ray spectra require significant differences in the optimal wind speed and magnetic field strength, we find that the single-zone model alone cannot account for the observed emission from the Galactic Center. Our model is improved by including a soft, additional cosmic-ray population. We assess such a cosmic-ray population and its potential sources and find that a cosmic-ray electron spectrum is energetically favored over a cosmic-ray proton spectrum.
We study the star cluster population of NGC 2997, a giant spiral galaxy located at 9.5 Mpc and targeted by the Snapshot Hubble U-band Cluster Survey (SHUCS). Combining our U-band imaging from SHUCS ...with archival BVI imaging from the Hubble Space Telescope, we select a high confidence sample of clusters in the circumnuclear ring and disk through a combination of automatic detection procedures and visual inspection. The cluster luminosity functions in all four filters can be approximated by power laws with indices of -1.7 to -2.3. Some deviations from pure power-law shape are observed, hinting at the presence of a high-mass truncation in the cluster mass function. However, upon inspection of the cluster mass function, we find it is consistent with a pure power law of index -2.2 + or - 0.2 despite a slight bend at ~2.5 x 10 super(4) M sub(middot in circle). No statistically significant truncation is observed. From the cluster age distributions, we find a low rate of disruption (zeta ~ -0.1) in both the disk and circumnuclear ring. Finally, we estimate the cluster formation efficiency (Gamma) over the last 100 Myr in each region, finding 7% + or - 2% for the disk, 12% + or - 4% for the circumnuclear ring, and 10% + or - 3% for the entire UBVI footprint. This study highlights the need for wide-field UBVI coverage of galaxies to study cluster populations in detail, though a small sample of clusters can provide significant insight into the characteristics of the population.
We present optical spectroscopy obtained with the Space Telescope Imaging Spectrograph of five young massive star clusters in the starburst galaxy M82. A detailed analysis is performed for one ...cluster 'M82-A1' and its immediate environment in the starburst core. From Hubble Space Telescope archive images, we find that it is elliptical with an effective radius of 3.0 ± 0.5-pc and is surrounded by a compact (r= 4.5 ± 0.5-pc) H-ii region. We determine the age and reddening of M82-A1 using synthetic spectra from population synthesis models by fitting both the continuum energy distribution and the depth of the Balmer jump. We find an age of 6.4 ± 0.5-Myr and a photometric mass estimate of M= 7-13 × 105-M⊙. We associate its formation with the most recent starburst event 4-6-Myr ago. We find that the oxygen abundance of the H-ii region surrounding M82-A1 is solar or slightly higher. The H-ii region has a high pressure P/k= 1-2 × 107-cm−3-K. The diffuse gas in region A has a slightly lower pressure, which together with the broad Hα emission-line width, suggests that both the thermal and turbulent pressures in the M82 starburst core are unusually high. We discuss how this environment has affected the evolution of the cluster wind for M82-A1. We find that the high pressure may have caused the pressure-driven bubble to stall. We also obtain spectroscopic ages for clusters B1-2 and B2-1 in the 'fossil' starburst region and for the intermediate age clusters F and L. These are consistent with earlier studies and demonstrate that star formation activity, sufficiently intense to produce super star clusters, has been going on in M82 during the past Gyr, perhaps in discrete and localized episodes.
Context. Understanding the nuclear growth and feedback processes in galaxies requires investigating their often obscured central regions. One way to do this is to use (sub)millimeter line emission ...from vibrationally excited HCN (HCN-vib), which is thought to trace warm and highly enshrouded galaxy nuclei. It has been suggested that the most intense HCN-vib emission from a galaxy is connected to a phase of nuclear growth that occurs before the nuclear feedback processes have been fully developed. Aims. We aim to investigate if there is a connection between the presence of strong HCN-vib emission and the development of feedback in (ultra)luminous infrared galaxies ((U)LIRGs). Methods. We collected literature and archival data to compare the luminosities of rotational lines of HCN-vib, normalized to the total infrared luminosity, to the median velocities of 119 μm OH absorption lines, potentially indicating outflows, in a total of 17 (U)LIRGs. Results. The most HCN-vib luminous systems all lack signatures of significant molecular outflows in the far-infrared OH absorption lines. However, at least some of the systems with bright HCN-vib emission have fast and collimated outflows that can be seen in spectral lines at longer wavelengths, including in millimeter emission lines of CO and HCN (in its vibrational ground state) and in radio absorption lines of OH. Conclusions. We conclude that the galaxy nuclei with the highest LHCN − vib/LIR do not drive wide-angle outflows that are detectable using the median velocities of far-infrared OH absorption lines. This is possibly because of an orientation effect in which sources oriented in such a way that their outflows are not along our line of sight also radiate a smaller proportion of their infrared luminosity in our direction. It could also be that massive wide-angle outflows destroy the deeply embedded regions responsible for bright HCN-vib emission, so that the two phenomena cannot coexist. This would strengthen the idea that vibrationally excited HCN traces a heavily obscured stage of evolution before nuclear feedback mechanisms are fully developed.
The Hubble Tarantula Treasury Project (HTTP) is an ongoing panchromatic imaging survey of stellar populations in the Tarantula Nebula in the Large Magellanic Cloud that reaches into the sub-solar ...mass regime (<0.5 Modot). HTTP utilizes the capability of the Hubble Space Telescope to operate the Advanced Camera for Surveys and the Wide Field Camera 3 in parallel to study this remarkable region in the near-ultraviolet, optical, and near-infrared spectral regions, including narrow-band H alpha images. The combination of all these bands provides a unique multi-band view. The resulting maps of the stellar content of the Tarantula Nebula within its main body provide the basis for investigations of star formation in an environment resembling the extreme conditions found in starburst galaxies and in the early universe. Access to detailed properties of individual stars allows us to begin to reconstruct the temporal and spatial evolution of the stellar skeleton of the Tarantula Nebula over space and time on a sub-parsec scale. In this first paper we describe the observing strategy, the photometric techniques, and the upcoming data products from this survey and present preliminary results obtained from the analysis of the initial set of near-infrared observations.
Abstract
We investigate the variation in the upper end of the stellar initial mass function (uIMF) in 375 young and compact star clusters in five nearby galaxies within ∼5 Mpc. All the young stellar ...clusters (YSCs) in the sample have ages ≲ 4 Myr and masses above 500
M
⊙
, according to standard stellar models. The YSC catalogs were produced from Hubble Space Telescope images obtained as part of the Legacy ExtraGalactic UV Survey (LEGUS) Hubble treasury program. They are used here to test whether the uIMF is universal or changes as a function of the cluster’s stellar mass. We perform this test by measuring the H
α
luminosity of the star clusters as a proxy for their ionizing photon rate, and charting its trend as a function of cluster mass. Large cluster numbers allow us to mitigate the stochastic sampling of the uIMF. The advantage of our approach relative to previous similar attempts is the use of cluster catalogs that have been selected independently of the presence of H
α
emission, thus removing a potential sample bias. We find that the uIMF, as traced by the H
α
emission, shows no dependence on cluster mass, suggesting that the maximum stellar mass that can be produced in star clusters is universal, in agreement with previous findings.
We report metallicities and radial velocities derived from spectra at the near-infrared calcium triplet for 373 red giants in a 200 arcmin2 area at the optical center of the LMC bar. These are the ...first spectroscopic abundance measurements of intermediate-age and old field stars in the high surface brightness heart of the LMC. The metallicity distribution is sharply peaked at the median value Fe/H = -0.40, with a small tail of stars extending down to Fe/H <= -2.1; 10% of the red giants are observed to have Fe/H <= -0.7. The relative lack of metal-poor stars indicates that the LMC has a 'G dwarf' problem, similar to the Milky Way. The abundance distribution can be closely approximated by two Gaussians containing 89% and 11% of the stars, respectively: the first component is centered at Fe/H = -0.37 with s = 0.15, and the second at Fe/H = -1.08 with s = 0.46. The dominant population has a metallicity distribution similar to that of the LMC's intermediate-age star clusters. The mean heliocentric radial velocity of the sample is 257 km s-1, corresponding to the same center-of-mass velocity as the disk (measured at larger radii). Because of the central location of our field, kinematic constraints are not strong, but there is no evidence that the bar deviates from the general motion of the LMC disk. The velocity dispersion of the whole sample is sv = 24.7 ± 0.4 km s-1. When cut by metallicity, the most metal-poor 5% of stars (Fe/H < -1.15) show sv = 40.8 ± 1.7 km s-1, more than twice the value for the most metal-rich 5%; this suggests that an old, thicker disk or halo population is present. The age-metallicity relation (AMR) is almost flat during the period from 5 to 10 Gyr ago, with an apparent scatter of ±0.15 dex about the mean metallicity for a given age. Comparing this to chemical evolution models from the literature, we find that a burst of star formation 3 Gyr ago does not reproduce the observed AMR more closely than a steadily declining star formation rate. The AMR suggests that the epoch of enhanced star formation, if any, must have commenced earlier, 6 Gyr ago-the exact time is model dependent. We compare the properties of the LMC and the Milky Way, and discuss our results in the context of models that attempt to use tidal interactions with the Milky Way and Small Magellanic Cloud to explain the star and cluster formation histories of the LMC.
We investigate the properties of feedback-driven shocks in eight nearby starburst galaxies using narrow-band imaging data from the Hubble Space Telescope. We identify the shock-ionized component via ...the line diagnostic diagram OIII (lambda5007)/H beta versus SII (lambdalambda6716, 6731) (or NII (lambda6583))/H alpha , applied to resolved regions 3-15 pc in size. We divide our sample into three sub-samples: sub-solar, solar, and super-solar, for consistent shock measurements. For the sub-solar sub-sample, we derive three scaling relations: (1) L sub(shock) is proportional to SFR super(0.62), (2) L sub(shock) is proportional to capital sigma sub(SFR, HL) super(0.92), and (3) L sub(shock)/L sub(tot) is proportional to (L sub(H)/L sub(odot, H)) super(-0.65), where L sub(shock) is the H alpha luminosity from shock-ionized gas, capital sigma sub(SFR, HL) the star formation rate (SFR) per unit half-light area, L sub(tot) the total H alpha luminosity, and L sub(H)/L sub(odot, H) jLq sub(H) the absolute H-band luminosity from the Two Micron All Sky Survey normalized to solar luminosity. The other two sub-samples do not have enough number statistics, but appear to follow the first scaling relation. The energy recovered indicates that the shocks from stellar feedback in our sample galaxies are fully radiative. If the scaling relations are applicable in general to stellar feedback, our results are similar to those by Hopkins et al. for galactic superwinds. This similarity should, however, be taken with caution at this point, as the underlying physics that enables the transition from radiative shocks to gas outflows in galaxies is still poorly understood.
PDF
