arXiv subjects

Amy E. Reines

Publications and source records attributed to Amy E. Reines.

45 records · Page 3Linked to original sources

arXiv2014

Extended Structure and Fate of the Nucleus in Henize 2-10

We investigate the structure and nuclear region of the black hole (BH) hosting galaxy Henize2-10. Surface brightness (SB) profiles are analyzed using Magellan/Megacam $g$- and $r$-band images. Excluding the central starburst, we find a best-fit two component Sérsic profile with $n_{\rm in} \sim 0.6$, $r_{\text{eff,in}} \sim$ 260~pc, and $n_{\rm out}\sim 1.8$, $r_{\text{eff, out}}\sim$ 1 kpc. Integrating out to our outer most data point (100 arc sec $\sim$ 4.3 kpc), we calculate $M_g=-19.2$ and $M_r=-19.8$. The corresponding enclosed stellar mass is $M_{\star}\sim(10\pm3)\times10^9$ M$_\odot$, $\sim3\times$ larger than previous estimates. Apart from the central $\lesssim$500 pc, with blue colors and an irregular morphology, the galaxy appears to be an early-type system. The outer color is quite red, $(g-r)_0=0.75$, suggesting a dominant old population. We study the nuclear region of the galaxy using archival Gemini/NIFS $K$-band adaptive optics spectroscopy and {\it Hubble Space Telescope} imaging. We place an upper limit on the BH mass of $\sim10^7M_{\odot}$ from the NIFS data, consistent with that from the $M_{\rm BH}$-radio-X-ray fundamental plane. No coronal lines are seen, but a Br$γ$ source is located at the position of the BH with a luminosity consistent with the X-ray emission. The starburst at the center of Henize 2-10 has led to the formation of several super star clusters, which are within $\sim$100 pc of the BH. We examine the fate of the nucleus by estimating the dynamical masses and dynamical friction timescales of the clusters. The most massive clusters ($\sim 10^6 M_\odot$) have $τ_{\rm dyn} \lesssim 200$ Myr, and thus Henize 2-10 may represent a rare snapshot of nuclear star cluster formation around a pre-existing massive BH.

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arXiv2013

High Resolution Radio and Optical Observations of the Central Starburst in the Low-Metallicity Dwarf Galaxy II Zw 40

The extent to which star formation varies in galaxies with low masses, low metallicities, and high star formation rate surface densities is not well-constrained. To gain insight into star formation under these physical conditions, this paper estimates the ionizing photon fluxes, masses, and ages for young massive clusters in the central region of II Zw 40 -- the prototypical low-metallicity dwarf starburst galaxy -- from radio continuum and optical observations. Discrete, cluster-sized sources only account for half the total radio continuum emission; the remainder is diffuse. The young (<5 Myr) central burst has a star formation rate surface density that significantly exceeds that of the Milky Way. Three of the 13 sources have ionizing photon fluxes (and thus masses) greater than R136 in 30 Doradus. Although isolating the effects of galaxy mass and metallicity is difficult, the HII region luminosity function and the internal extinction in the center of II Zw 40 appear to be primarily driven by a merger-related starburst. The relatively flat HII region luminosity function may be the result of an increase in ISM pressure during the merger and the internal extinction is similar to that generated by the clumpy and porous dust in other starburst galaxies.

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arXiv2013

Dwarf Galaxies with Optical Signatures of Active Massive Black Holes

We present a sample of 151 dwarf galaxies (10^8.5 < M_stellar < 10^9.5 Msun) that exhibit optical spectroscopic signatures of accreting massive black holes (BHs), increasing the number of known active galaxies in this stellar mass range by more than an order of magnitude. Utilizing data from the Sloan Digital Sky Survey Data Release 8 and stellar masses from the NASA-Sloan Atlas, we have systematically searched for active BHs in ~25,000 emission-line galaxies with stellar masses comparable to the Magellanic Clouds and redshifts z<0.055. Using the narrow-line [OIII]/H-beta versus [NII]/H-alpha diagnostic diagram, we find photoionization signatures of BH accretion in 136 galaxies, a small fraction of which also exhibit broad H-alpha emission. For these broad-line AGN candidates, we estimate BH masses using standard virial techniques and find a range of 10^5 < M_BH < 10^6 Msun and a median of M_BH ~ 2 x 10^5 Msun. We also detect broad H-alpha in 15 galaxies that have narrow-line ratios consistent with star-forming galaxies. Follow-up observations are required to determine if these are true type 1 AGN or if the broad H-alpha is from stellar processes. The median absolute magnitude of the host galaxies in our active sample is Mg = -18.1 mag, which is ~1-2 magnitudes fainter than previous samples of AGN hosts with low-mass BHs. This work constrains the smallest galaxies that can form a massive BH, with implications for BH feedback in low-mass galaxies and the origin of the first supermassive BH seeds.

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arXiv2012

Parsec-Scale Radio Emission from the Low-Luminosity Active Galactic Nucleus in the Dwarf Starburst Galaxy Henize 2-10

A candidate accreting massive black hole (BH) with M_BH ~ 10^6 Msun has recently been identified at the center of the dwarf starburst galaxy Henize 2-10 (He 2-10). This discovery offers the first possibility of studying a growing BH in a nearby galaxy resembling those in the earlier universe, and opens up a new class of host galaxies to search for the smallest supermassive BHs. Here we present very long baseline interferometry observations of He 2-10 taken with the Long Baseline Array (LBA) at 1.4 GHz with an angular resolution of ~0.1" x 0.03". A single compact radio source is detected at the precise location of the putative low-luminosity active galactic nucleus. The physical size of the nuclear radio emission is < 3 pc x 1 pc, an order of magnitude smaller than previous constraints from the Very Large Array (VLA), and the brightness temperature of T_B > 3 x 10^5 K confirms a non-thermal origin. These LBA observations indicate that the nuclear radio emission originates from a single object and exclude the possibility of multiple supernova remnants as the origin of the nuclear radio emission previously detected with the VLA at lower resolution. A weaker, more extended, off-nuclear source is also detected with the LBA and a comparison with multi-wavelength ancillary data indicate that, unlike the nuclear source, the off-nuclear source is co-spatial with a super star cluster, lacks a detectable X-ray point-source counterpart, and is almost certainly due to a supernova remnant in the host star cluster.

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arXiv2011

An actively accreting massive black hole in the dwarf starburst galaxy Henize 2-10

Supermassive black holes are now thought to lie at the heart of every giant galaxy with a spheroidal component, including our own Milky Way. The birth and growth of the first 'seed' black holes in the earlier Universe, however, is observationally unconstrained and we are only beginning to piece together a scenario for their subsequent evolution. Here we report that the nearby dwarf starburst galaxy Henize 2-10 contains a compact radio source at the dynamical centre of the galaxy that is spatially coincident with a hard X-ray source. From these observations, we conclude that Henize 2-10 harbours an actively accreting central black hole with a mass of approximately one million solar masses. This nearby dwarf galaxy, simultaneoulsy hosting a massive black hole and an extreme burst of star formation, is analogous in many ways to galaxies in the infant Universe during the early stages of black-hole growth and galaxy mass assembly. Our results confirm that nearby star-forming dwarf galaxies can indeed form massive black holes, and that by implication so can their primordial counterparts. Moreover, the lack of a substantial spheroidal component in Henize 2-10 indicates that supermassive black-hole growth may precede the build-up of galaxy spheroids.

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arXiv2009

The Importance of Nebular Continuum and Line Emission in Observations of Young Massive Star Clusters

In this spectroscopic study of infant massive star clusters, we find that continuum emission from ionized gas rivals the stellar luminosity at optical wavelengths. In addition, we find that nebular line emission is significant in many commonly used broad-band HST filters including the F814W I-band, the F555W V-band and the F435W B-band. Two young massive clusters (YMCs) in NGC 4449 were targeted for spectroscopic observations after Reines et al. (2008a) discovered an F814W I-band excess in their photometric study of radio-detected clusters in the galaxy. The spectra were obtained with the Dual Imaging Spectrograph on the 3.5 m APO telescope. We supplement these data with HST and SDSS photometry. By comparing our data to the Starburst99 and GALEV models, we find that nebular continuum emission competes with the stellar light in our observations and that the relative contribution is largest in the U- and I-bands, where the Balmer and Paschen jumps are located. The spectra also exhibit strong line emission including the [SIII] 9069,9532 lines in the HST F814W I-band. We find that the combination of nebular continuum and line emission can account for the F814W I-band excess found by Reines et al. (2008a). In an effort to provide a benchmark for estimating the impact of ionized gas emission on photometric observations of YMCs, we compute the relative contributions of the stellar continuum, nebular continuum, and emission lines to the total flux of a 3 Myr-old cluster through various HST filter/instrument combinations, including filters in the WFC3. We urge caution when comparing observations of YMCs to evolutionary synthesis models since nebular emission can have a large impact on magnitudes and colors of young (< 5 Myr) clusters, significantly affecting inferred properties such as ages, masses and extinctions. (Abridged)

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arXiv2008

A New View of the Super Star Clusters in the Low-Metallicity Galaxy SBS 0335-052

We present a study of the individual super star clusters (SSCs) in the low-metallicity galaxy SBS 0335-052 using new near-infrared and archival optical Hubble Space Telescope observations. The physical properties of the SSCs are derived from fitting model spectral energy distributions (SEDs) to the optical photometry, as well as from the H_alpha and Pa_alpha nebular emission. Among the clusters, we find a significant age spread that is correlated with position in the galaxy, suggesting successive cluster formation occurred in SBS 0335-052 triggered by a large-scale disturbance traveling through the galaxy at a speed of ~35 km/s. The SSCs exhibit I-band (~0.8 um) and near-IR (~1.6-2.1 um) excesses with respect to model SEDs fit to the optical data. We hypothesize that the I-band excess is dominated by a photoluminescent process known as Extended Red Emission; however, this mechanism cannot account for the excesses observed at longer near-IR wavelengths. From the cluster SEDs and colors, we find that the primary origin of the near-IR excess observed in the youngest SSCs (<3 Myr) is hot dust emission, while evolved red supergiants dominate the near-IR light in the older (>7 Myr) clusters. We also find evidence for a porous and clumpy interstellar medium (ISM) surrounding the youngest, embedded SSCs: the ionized gas emission underpredicts the expected ionizing luminosities from the optical stellar continuum, suggesting ionizing photons are leaking out of the immediate vicinity of the clusters before ionizing hydrogen. The corrected, intrinsic ionizing luminosities of the two SSCs younger than ~3 Myr are each ~ 5x10^52 s^-1, which is equivalent to each cluster hosting ~5000 O7.5 V stars. The inferred masses of these SSCs are ~10^6 M_sun.

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arXiv2008

Emerging Massive Star Clusters Revealed: High Resolution Imaging of NGC 4449 from the Radio to the Ultraviolet

We present a multi-wavelength study of embedded massive clusters in the nearby (3.9 Mpc) starburst galaxy NGC 4449 in an effort to uncover the earliest phases of massive cluster evolution. By combining high resolution imaging from the radio to the ultraviolet, we reveal these clusters in the process of emerging from their gaseous and dusty birth cocoons. We use Very Large Array (VLA) observations at centimeter wavelengths to identify young clusters surrounded by ultra-dense HII regions, detectable via their production of thermal free-free radio continuum. Ultraviolet, optical and infrared observations are obtained from the Hubble and Spitzer Space Telescope archives for comparison. We detect 39 compact radio sources towards NGC 4449 at 3.6 cm using the highest resolution (1.3") and sensitivity (RMS ~ 12 uJy) VLA image of the galaxy to date. We reliably identify 13 thermal radio sources and their physical properties are derived using both nebular emission from the HII regions and SED fitting to the stellar continuum. These radio detected clusters have ages < 5 Myr and stellar masses of order 10^4 Msun. The measured extinctions are quite low: 12 of the 13 thermal radio sources have Av < 1.5, while the most obscured source has Av ~ 4.3. By combining results from the nebular and stellar emission, we find an I-band excess that is anti-correlated with cluster age and an apparent mass-age correlation. Additionally, we find evidence that local processes such as supernovae and stellar winds are likely playing an important role in triggering the current bursts of star formation within NGC 4449.

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arXiv2002

Optical SETI: A Spectroscopic Search for Laser Emission from Nearby Stars

We have searched for nonastrophysical emission lines in the optical spectra of 577 nearby F, G, K, and M main-sequence stars. Emission lines of astrophysical origin would also have been detected, such as from a time--variable chromosphere or infalling comets. We examined ~20 spectra per star obtained during four years with the Keck/HIRES spectrometer at a resolution of 5 km/s, with a detection threshold 3% of the continuum flux level. We searched each spectrum from 4000-5000 angstroms for emission lines having widths too narrow to be natural from the host star, as well as for lines broadened by astrophysical mechanisms. We would have detected lasers that emit a power, P>60 kW, for a typical beam width of ~0.01 arcsec (diffraction-limit from a 10-m aperture) if directed toward Earth from the star. No lines consisstent with laser emission were found.

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