arXiv subjects

Sara C. Beck

Publications and source records attributed to Sara C. Beck.

29 records · Page 2Linked to original sources

arXiv2006

Brackett Lines from the Super Star Cluster Nebulae in He 2-10

We present high spectral resolution (v~12-16 km/s) Brackett line spectroscopy of the blue compact dwarf galaxy Henize 2-10 made with NIRSPEC on the Keck Telescope. The spatial resolution is seeing limited, at 1". We detect two distinct kinematic features separated by approximately 3", with heliocentric velocities of ~860 and ~890 km/s. In addition to a narrow core, the line profiles also display a broad, low intensity feature on the blue side of the centroid, which we attribute to an outflow. This may be a sign of aging in the clusters. We compare to archival high resolution Very Large Array (VLA) data at 1.3 cm, and find that the centimeter wavelength emission is resolved into six sources. These radio sources are organized into two larger groups, which we associate with the two kinematic peaks in the Brackett spectrum. We estimate a Lyman continuum rate of at least 7 x 10^52 s^-1, with a corresponding stellar mass of 6 x 10^6 M_sun is required to ionize the nebulae. We also estimate the size of the nebulae from the radio continuum brightness and find that the observed sources probably contain many HII regions in smaller, unresolved clumps. Brackett line profiles have supersonic line widths, but, aside from the blue wing, are comparable to line widths observed in Galactic ultracompact HII regions, which are excited by a single star, or a few stars.

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arXiv2006

High Resolution Radio Maps of Four Nearby Spiral Galaxies

We report subarcsecond-resolution VLA imaging of the centers of four nearby spiral galaxies: IC 342, Maffei II, NGC 2903, and NGC 6946. In each galaxy, 7 - 12 compact radio continuum sources were identified within the central 15" x 15". Slightly over half of the compact sources appear to be HII regions with flat or positive spectral indices (alpha >~ -0.1). The HII regions with rising spectra are optically thick at centimeter wavelengths, and thus dense (n_i ~ 10^4 cm^(-3)) and young. The largest of these HII regions require the excitation of 500 - 800 O stars, within regions of only few parsecs extent. These clusters approach the sizes expected for globular clusters.

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arXiv2006

Subarcsecond-Resolution Radio Maps of Nearby Spiral Galaxies

We report subarcsecond-resolution VLA imaging of four nearby spiral galaxies: IC 342, Maffei II, NGC 2903, and NGC 6946. In each galaxy, compact radio continuum sources are identified in the central ~ 15" x 15" region. These compact sources are responsible for 20 - 30 % of the total emission from the central kpc of the host galaxies at 2 cm, but only ~ 5 - 10 % at 6 cm. More than half of the compact sources appear to be HII regions. The HII regions with rising spectra must be fairly dense (n_i ~ 10^4 cm^(-3)) and are presumably very young. The largest of these HII regions require the excitation of 500 - 800 O stars, within regions of only few parsecs extent. These clusters approach the sizes expected for globular clusters. Thermal free-free emission from compact sources contributes more significantly at 2 cm, while diffuse synchrotron emission dominates at 6 cm. The radio HII regions are found near the centers of giant molecular clouds in projection, and do not have obvious visual counterparts.

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arXiv2004

MERLIN and VLA Observations of VII Zw 19: Distant Cousin of M82

We have observed the starburst dwarf galaxy VII Zw 19 at subarcsecond resolution with the MERLIN and MERLIN+EVN networks at 18 cm, with MERLIN at 6 cm, and with the VLA in A array at 6 and 2 cm. The galaxy is detected at all VLA wavelengths and is resolved. It is also resolved at 18 cm by MERLIN, but is not detected by the EVN nor by MERLIN at 6 cm. VII Zw 19 has a complex structure of nonthermal radio emission at 18 cm extended over ~1200-1800 pc (4-6''). That the EVN did not detect this emission indicates that there are no obvious point sources that could be radio supernovae. The 2 cm emission, predominantly thermal free-free emission, has a markedly different spatial distribution from the nonthermal emission. The radio colors show that the galaxy contains numerous supernova remnants, as well as many young HII regions, many of which are optically thick at 6 cm. Two extended regions of 2 cm emission have little 18 cm flux and are probably emission from young star clusters. VII Zw 19 resembles M82 in its radio and infrared spectrum, but the starburst region in VII Zw 19 is twice the size of the one in M82 and twice as luminous.

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arXiv2004

Probing the Early Stages of Low-Mass Star Formation in LDN 1689N: Dust and Water in IRAS 16293-2422A, B, and E

We present deep images of dust continuum emission at 450, 800, and 850 micron of the dark cloud LDN 1689N which harbors the low-mass young stellar objects (YSOs) IRAS 16293-2422A and B (I16293A and I16293B) and the cold prestellar object I16293E. Toward the positions of I16293A and E we also obtained spectra of CO-isotopomers and deep submillimeter observations of chemically related molecules with high critical densities. To I16293A we report the detection of the HDO 1_01 - 0_00 and H2O 1_10 - 1_01 ground-state transitions as broad self-reversed emission profiles with narrow absorption, and a tentative detection of H2D+ 1_10 - 1_11. To I16293E we detect weak emission of subthermally excited HDO 1_01 - 0_00. Based on this set of submillimeter continuum and line data we model the envelopes around I16293A and E. The density and velocity structure of I16293A is fit by an inside-out collapse model, yielding a sound speed of a=0.7 km/s, an age of t=(0.6--2.5)e4 yr, and a mass of 6.1 Msun. The density in the envelope of I16293E is fit by a radial power law with index -1.0+/-0.2, a mass of 4.4 Msun, and a constant temperature of 16K. These respective models are used to study the chemistry of the envelopes of these pre- and protostellar objects. The [HDO]/[H2O] abundance ratio in the warm inner envelope of I16293A of a few times 1e-4 is comparable to that measured in comets. This supports the idea that the [HDO]/[H2O] ratio is determined in the cold prestellar core phase and conserved throughout the formation process of low-mass stars and planets.

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arXiv2004

The Birth of a Super Star Cluster: NGC 5253

We present images of the 7mm free-free emission from the radio "supernebula" in NGC 5253 made with the Very Large Array and the Pie Town link. The images reveal structure in the nebula, which has a <~ 1 pc (~50 mas radius) core requiring the excitation of 1200 O7 stars. The nebula is elongated, with an arc of emission curving to the northeast and to the south. The total ionizing flux within the central 1.2" (~20 pc) is 7 x 10^52 s^-1, corresponding to 7000 O7 stars. We propose that the radio source is coincident with a small, very red near-infrared cluster and apparently linked to a larger, optical source some 10 pc away on the sky. We speculate on the causes of this structure and what it might tell us about the birth of the embedded young super star cluster.

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arXiv2003

Watching the Birth of Super Star Clusters

Subarcsecond infrared and radio observations yield important information about the formation of super star clusters from their surrounding gas. We discuss the general properties of ionized and molecular gas near young, forming SSCs, as illustrated by the prototypical young forming super star cluster nebula in the dwarf galaxy, NGC 5253. This super star cluster appears to have a gravitationally bound nebula. The lack of molecular gas suggests a very high star formation efficiency, consistent with the formation of a large, bound star cluster.

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arXiv2002

Molecular Gas and the Young Starburst in NGC 5253, Revisited

We report the detection of CO(2-1) and 3.1 mm and 1.3 mm continuum emission towards the extremely young starburst in NGC 5253, with data taken from the Owens Valley Millimeter Array. Faint CO emission originates in five molecular clouds distributed along the prominent dust lane seen in visual images. With the gas, the morphology of NGC 5253 looks much like a dwarf elliptical version of the ``dust-lane ellipticals'' or ``polar-ring'' class of galaxies. The molecular gas resides in GMCs well away from the radio-IR super-star cluster/supernebula seen in the radio and infrared. The millimeter continuum data confirm that the 2 cm flux from the supernebula is optically thick; the Lyman continuum rate derived from the 1.3 mm continuum is N$_{Lyc} \sim 6x10^{52} s^{-1}$ for the central ~20''. CO may underestimate the true molecular column density, as expected for a low metallicity system, although there are regions along the dust lane that appear to have near-Galactic conversion factors. We estimate a total molecular gas mass of $M_{H_{2}}\lsim 10^{7} M_{\odot}$. The molecular gas in the dust lane is falling into the galaxy, supporting an accretion hypothesis. The dust lane gas cannot therefore be causally associated with the current burst of star formation. A relatively small amount, $M_{H_{2}}\lsim 5x10^{5} M_{\odot}$, of molecular gas is associated with the current starburst. We estimate a star formation efficiency of at least 25 % and more likely ~75 %, consistent with the formation of a bound cluster. Despite the extreme youth of the starburst, the specific trigger of the starburst remains elusive, although the infall of gas in the dust lane suggests that there is more star formation to come in NGC 5253.

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arXiv2001

Molecular Gas and Star Formation in NGC 3077

We present high resolution (~2.5'') CO(1-0) and CO(2-1) images of the central kiloparsec of NGC 3077 made with the Owens Valley Millimeter Array. CO emission is distributed in three major complexes which resolve into at least seven GMCs. Two complexes are associated with the central starburst. A third, more distant complex is not associated with strong star formation. The GMCs are ~70 pc in size and contain ~10^{6} M_{\odot} of molecular gas. The Galactic conversion factor appears applicable to NGC 3077, consistent with its solar metallicity. Galactic rotation in NGC 3077 is detected for the first time in the molecular gas. The molecular gas counterrotates with respect to the large scale HI tidal bridge. The molecular clouds closest to the starburst have super-virial linewidths, possibly related to the turbulence generated by the starburst. The 2.6 mm radio continuum flux indicates that thermal Bremsstrahlung dominates the emission from the starburst region below 5 cm, and that the N_{Lyc} \simeq 3.7\times 10^{52} s^{-1} or ~3000 O7 stars, corresponds to a star formation rate of 0.4 M_{\odot} yr^{-1}. At this rate the amount of molecular gas can sustain star formation for only ~10 Myrs; thus NGC 3077 is a true starburst galaxy. The derived age of the starburst is consistent with the inferred ages of the superbubbles, which suggests that the burst is much younger than the age of the M 81-M 82-NGC 3077 interaction. We suggest that it is caused by gas that was pulled out of NGC 3077 during the interaction with M 81, raining back down onto the galaxy.

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arXiv2001

Infrared Emission from the Radio Supernebula in NGC 5253: A Proto-Globular Cluster?

Hidden from optical view in the starburst region of the dwarf galaxy NGC 5253 lies an intense radio source with an unusual spectrum which could be interpreted variously as nebular gas ionized by a young stellar cluster or nonthermal emission from a radio supernova or an AGN. We have obtained 11.7 and 18.7 micron images of this region at the Keck Telescope and find that it is an extremely strong mid-infrared emitter. The infrared to radio flux ratio rules out a supernova and is consistent with an HII region excited by a dense cluster of young stars. This "super nebula" provides at least 15% of the total bolometric luminosity of the galaxy. Its excitation requires 10^5-10^6 stars, giving it the total mass and size (1-2 pc diameter) of a globular cluster. However, its high obscuration, small size, and high gas density all argue that it is very young, no more than a few hundred thousand years old. This may be the youngest globular cluster yet observed.

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arXiv2000

Warm Molecular Gas in Dwarf Starburst Galaxies: CO(3-2) Observations

Eight dwarf starburst galaxies have been observed with the Caltech Submillimeter Observatory (CSO) telescope in the CO J= 3 - 2 transition. The galaxies observed are He 2-10, NGC 5253, NGC 1569, NGC 3077, Haro 2, Haro 3, II Zw 40 and Mrk 86; all but the last two are detected. The central regions of He 2-10 and NGC 5253 were mapped and a CO(2-1) spectrum of NGC 5253 was obtained. The error weighted mean CO(3-2)/CO(1-0) ratio of the detected galaxies is 0.60$\pm$0.06, which is virtually identical to what is found for starbursts in the nuclei of nearby spirals, and suggests that the molecular gas is optically thick, warm (T$_{k}>$20 K), and moderately dense ($n_{H_{2}}\sim 10^{3-4} cm^{-3}$). The CO(3-2)/CO(1-0) ratio peaks at or close to the starburst in all cases. CO emission does not appear to be optically thin in these dwarfs, despite the low metallicity and intense radiation fields, which is probably because in order for CO to exist in detectable amounts it must be self-shielding and hence optically thick. Physical properties of the molecular clouds in these dwarf starbursts appear to be essentially the same as nearby spiral nuclei, with the possible exception that CO is more confined to the cloud cores.

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