arXiv subjects

C. L. Martin

Publications and source records attributed to C. L. Martin.

27 records · Page 2Linked to original sources

arXiv2006

Extended Star Formation and Molecular Gas in the Tidal Arms near NGC3077

We report the detection of ongoing star formation in the prominent tidal arms near NGC 3077 (member of the M 81 triplet). In total, 36 faint compact HII regions were identified, covering an area of ~4x6 kpc^2. Most of the HII regions are found at HI column densities above 1x10^21 cm^-2 (on scales of 200 pc), well within the range of threshold columns measured in normal galaxies. The HII luminosity function resembles the ones derived for other low-mass dwarf galaxies in the same group; we derive a total star formation rate of 2.6x10^-3 M_sun/yr in the tidal feature. We also present new high-resolution imaging of the molecular gas distribution in the tidal arm using CO observations obtained with the OVRO interferometer. We recover about one sixth of the CO flux (or M_H2~2x10^6 M_sun, assuming a Galactic conversion factor) originally detected in the IRAM 30m single dish observations, indicating the presence of a diffuse molecular gas component in the tidal arm. The brightest CO peak in the interferometer map (comprising half of the detected CO flux) is coincident with one of the brightest HII regions in the feature. Assuming a constant star formation rate since the creation of the tidal feature (presumably ~3x10^8 years ago), a total mass of ~7x10^5 M_sun has been transformed from gas into stars. Over this period, the star formation in the tidal arm has resulted in an additional enrichment of Delta(Z)>0.002. The reservoir of atomic and molecular gas in the tidal arm is ~3x10^8 M_sun, allowing star formation to continue at its present rate for a Hubble time. Such wide-spread, low-level star formation would be difficult to image around more distant galaxies but may be detectable through intervening absorption in quasar spectra.

astro-ph↗

arXiv2006

Kinematics of Interstellar Gas in Nearby UV-Selected Galaxies Measured with HST/STIS Spectroscopy

We measure Doppler shifts of interstellar absorption lines in HST/STIS spectra of individual star clusters in nearby UV-selected galaxies. Values for systemic velocities, which are needed to quantify outflow speeds, are taken from the literature, and verified with stellar lines. We detect outflowing gas in eight of 17 galaxies via low-ionization lines (e.g., CII, SiII, AlII), which trace cold and/or warm gas. The starbursts in our sample are intermediate in luminosity (and mass) to dwarf galaxies and luminous infrared galaxies (LIRGs), and we confirm that their outflow speeds (ranging from -100 km/s to nearly -520 km/s with an accuracy of ~80 km/s) are intermediate to those previously measured in dwarf starbursts and LIRGs. We do not detect the outflow in high-ionization lines (such as CIV or SiIV); higher quality data will be needed to empirically establish how velocities vary with the ionization state of the outflow. We do verify that the low-ionization UV lines and optical NaI doublet give roughly consistent outflow velocities solidifying an important link between studies of galactic winds at low and high redshift. To obtain higher signal-to-noise, we create a local average composite spectrum, and compare it to the high-z Lyman Break composite spectrum. Surprisingly, the low-ionization lines show similar outflow velocities in the two samples. We attribute this to a combination of weighting towards higher luminosities in the local composite, as well as both samples being on average brighter than the ``turnover'' luminosity in the v-SFR relation.

astro-ph↗

arXiv2004

Far Ultraviolet Observations of Molecular Hydrogen in the Diffuse Interstellar Medium of Starburst Galaxies

(Abridged) We have used the Far Ultraviolet Spectroscopic Explorer (FUSE) to search for H_2 absorption in five starburst galaxies: NGC 1705, NGC 3310, NGC 4214, M83 (NGC 5236), and NGC 5253. We tentatively detect weak absorption by H_2 in M83 and NGC 5253, and set upper limits on the H_2 column density in the other galaxies. Conservative upper limits on the mass of molecular gas detected with FUSE are many orders of magnitude lower than the H_2 mass inferred from CO emission. This indicates that almost all of the H_2 is in the form of clouds with N(H_2)>10^20 cm^-2 that are opaque to far-UV light and therefore cannot be probed with far-UV absorption measurements. The far-UV continuum visible in the FUSE spectra passes between the dense clouds, which have a covering factor <1. The complex observational biases related to varying extinction across the extended UV emission in the FUSE apertures prevent an unambiguous characterization of the diffuse H_2 in these starbursts. However, the evidence is suggestive that there is less H_2 in the diffuse interstellar medium between the dense clouds compared to similarly reddened sight lines in the Milky Way. This holds with the expectation that the destructive UV radiation field is stronger in starbursts. However, previous UV observations of these starbursts have shown that there is reddening caused by the diffuse interstellar medium. This suggests that while diffuse H_2 may be destroyed in the starburst, dust still exists.

astro-ph↗

arXiv2003

The Star-Forming Dwarf Galaxy Populations of two z ~ 0.4 Clusters: MS1512.4+3647 and Abell 851

We present the results of a deep narrow-band [OII] 3727 Åemission-line search for faint ($g <$ 27), star-forming galaxies in the field of the $z=0.37$ MS1512.4+3647 cluster. We find no evidence for an over-density of emission-line sources relative to the field at $z \sim$ 0.4 (Hogg et al. 1998), and therefore conclude that the MS1512.4+3647 sample is dominated by field [OII] emission-line galaxies which lie along the $\sim$ 180 Mpc line of sight immediately in front and behind the cluster. This is surprising, given that the previously surveyed $z=0.41$ cluster Abell 851 has 3-4 times the field emission-line galaxy density (Martin et al. 2000). We find that the MS1512.4+3647 sample is deficient in galaxies with intermediate colors (1.0 $< g-i <$ 2.0) and implied star-formation exponential decay timescales $τ\sim$ 100 Myr - 1 Gyr that dominate the Abell 851 emission-line galaxy population. Instead, the majority of [OII] emission-line galaxies surrounding the MS1512.4+3647 cluster are blue ($g-i \leq 1.0$) and forming stars in bursts with $τ< $ 100 Myr. In both samples, galaxies with the shortest star-formation timescales are preferentially among the faintest star-forming objects. Their i luminosities are consistent with young stellar populations $\sim 10^8 - 10^9 \Msun$, although an additional factor of ten in stellar mass could be hiding in underlying old stellar populations. We discuss the implications for the star-formation histories of dwarf galaxies in the field and rich clusters.

astro-ph↗

arXiv2001

13CI in high-mass star-forming clouds

We report measurements of the 12C/13C abundance ratio in the three galactic regions G 333.0-0.4, NGC 6334 A and G 351.6-1.3 from observations of the 12CI 3P2-3P1 transition and the hyperfine components of the corresponding 13CI transition near 809 GHz. These transitions were observed simultaneously with the CO 7-6 line emission at 806 GHz with the AST/RO telescope located at the South Pole. From a simultaneous fit to the 12CI 3P2-3P1 transition and the HF components of the corresponding 13CI transition and an independent estimate of an upper limit to the optical depth of the 12CI emission we determine intrinsic 12CI/13CI column density ratios of 23+-1 for G 333.0-0.4, 56+-14 for NGC 6334 A and 69+-12 for G 351.6-1.3. As the regions observed are photon dominated, we argue that the apparent enhancement in the abundance of 13C towards G 333.0-0.4 may be due to strong isotope-selective photodissociation of 13CO, outweighing the effects of chemical isotopic fractionation as suggested by models of PDRs. Towards NGC 6334 A and G 351.6-1.3 these effects appear to be balanced, similar to the situation for the Orion Bar region observed by Keene et al. (1998).

astro-ph↗

arXiv2001

On the Escape of Ionizing Radiation from Starbursts

Far-ultraviolet spectra obtained with $FUSE$ show that the strong $CIIλ$1036 interstellar absorption-line is essentially black in five of the UV-brightest local starburst galaxies. Since the opacity of the neutral ISM below the Lyman-edge will be significantly larger than in the $CII$ line, these data provide strong constraints on the escape of ionizing radiation from these starbursts. Interpreted as a a uniform absorbing slab, the implied optical depth at the Lyman edge is huge ($τ_0 \geq 10^2$). Alternatively, the areal covering factor of opaque material is typically $\geq$ 94%. Thus, the fraction of ionizing stellar photons that escape the ISM of each galaxy is small: our conservative estimates typically yield $f_{esc} \leq 6%$. Inclusion of extinction due to dust will further decrease $f_{esc}$. An analogous analysis of the rest-UV spectrum of the star-forming galaxy $MS 1512-CB58$ at $z$ =2.7 leads to similar constraints on $f_{esc}$. These new results agree with the constraints provided by direct observations below the Lyman edge in a few other local starbursts. However, they differ from the recently reported properties of star-forming galaxies at $z \geq$ 3. We assess the idea that the strong galactic winds seen in many powerful starbursts clear channels through their neutral ISM. We show empirically that such outflows may be a necessary - but not sufficient - part of the process for creating a relatively porous ISM. We note that observations will soon document the cosmic evolution in the contribution of star-forming galaxies to the metagalactic ionizing background, with important implications for the evolution of the IGM.

astro-ph↗

arXiv2001

FUSE Observations of Outflowing OVI in the Dwarf Starburst Galaxy NGC1705

We report FUSE far-UV spectroscopy of the prototypical dwarf starburst galaxy NGC 1705. These data allow us for the first time to probe the coronal-phase gas (T = 10E5 to 10E6 K) that may dominate the radiative cooling of the supernova-heated ISM and thereby determine the dynamical evolution of starburst-driven outflows. We detect a broad (100 km/s) and blueshifted (by 80 km/s) OVI absorption-line arising in the previously-known galactic outflow. The properties of the OVI absorption are inconsistent with the standard superbubble model in which this gas arises in a conductive interface inside the outer shell. We show that the superbubble in NGC 1705 is blowing out of the galaxy ISM. During blow-out, coronal-phase gas can be created by hydrodynamical mixing as hot gas rushes out through fissures in the fragmenting shell of cool gas. As the coronal gas cools radiatively, it can naturally produce the observed OVI column density and outflow speed. The OVI data show that the cooling rate in the coronal-phase gas is less than about 10% of the supernova heating rate. Since the X-ray luminosity from hotter gas is even smaller, we conclude that radiative losses are insignificant. The outflow should be able to vent its metals and kinetic energy out of the galaxy. This process has potentially important implications for the evolution of dwarf galaxies and the IGM.

astro-ph↗

arXiv1999

The Hubble Deep Field South - STIS Imaging

We present the imaging observations made with the Space Telescope Imaging Spectrograph of the Hubble Deep Field - South. The field was imaged in 4 bandpasses: a clear CCD bandpass for 156 ksec, a long-pass filter for 22-25 ksec per pixel typical exposure, a near-UV bandpass for 23 ksec, and a far-UV bandpass for 52 ksec. The clear visible image is the deepest observation ever made in the UV-optical wavelength region, reaching a 10 sigma AB magnitude of 29.4 for an object of area 0.2 square arcseconds. The field contains QSO J2233-606, the target of the STIS spectroscopy, and extends 50"x50" for the visible images, and 25"x25" for the ultraviolet images. We present the images, catalog of objects, and galaxy counts obtained in the field.

astro-ph↗

arXiv1999

Study of a generalized Metropolis decision rule in auxiliary field quantum Monte Carlo

We consider a generalization of the standard Metropolis algorithm acceptance/rejection decision rule and numerically explore its properties using auxiliary field quantum Monte Carlo. The generalization involves a free parameter which, given a criterion for proposing attempted moves, can be used to tune the average acceptance rate in a particular way. Such tuning can also potentially change Monte Carlo autocorrelation times, and the combination of the changing acceptance rate and autocorrelation times raises the possibility of more efficient simulations. We explore these issues using primarily massively parallel quantum Monte Carlo runs of the ``test case'' two-dimensional Hubbard model, and discuss results and applications.

cond-mat.str-el↗