Balmer Absorption Lines Research Articles

The stellar populations of low-luminosity active galactic nuclei - III. Spatially resolved spectral properties

In a recently completed survey of the stellar population properties of low-ionization nuclear emission-line regions (LINERs) and LINER/H II transition objects (TOs), we have identified a numerous class of galactic nuclei which stand out because of their conspicuous 10 8-9 yr populations, traced by high-order Balmer absorption lines and other stellar indices. These objects are called 'young-TOs', because they all have TO-like emission-line ratios. In this paper we extend this previous work, which concentrated on the nuclear properties, by investigating the radial variations of spectral properties in low-luminosity active galactic nuclei (LLAGNs). Our analysis is based on high signal-to-noise ratio (S/N) long-slit spectra in the 3500-5500 A interval for a sample of 47 galaxies. The data probe distances of typically up to 850 pc from the nucleus with a resolution of ∼100 pc (∼1 arcsec) and S/N ∼30. Stellar population gradients are mapped by the radial profiles of absorption-line equivalent widths and continuum colours along the slit. These variations are further analysed by means of a decomposition of each spectrum in terms of template galaxies representative of very young (≤10 7 yr), intermediate age (10 8-9 yr) and old (10 10 yr) stellar populations. This study reveals that young-TOs also differ from old-TOs and old-LINERs in terms of the spatial distributions of their stellar populations and dust. Specifically, our main findings are as follows. (i) Significant stellar population gradients are found almost exclusively in young-TOs. (ii) The intermediate age population of young-TOs, although heavily concentrated in the nucleus, reaches distances of up to a few hundred pc from the nucleus. Nevertheless, the half width at half-maximum of its brightness profile is more typically 100 pc or less. (iii) Objects with predominantly old stellar populations present spatially homogeneous spectra, be they LINERs or TOs. (iv) Young-TOs have much more dust in their central regions than other LLAGNs. (v) The B-band luminosities of the central ≤1 Gyr population in young-TOs are within an order of magnitude of M B = -15, implying masses of the order of ∼10 7 -10 8 M ○. . This population was 10-100 times more luminous in its formation epoch, at which time young massive stars would have completely outshone any active nucleus, unless the AGN too was brighter in the past.

Dynamical state and star formation properties of the merging galaxy cluster Abell 3921

We present the analysis and results of a new VRI photometric and spectroscopic survey of the central ~ region of the galaxy cluster A3921 (). We detect the presence of two dominant clumps of galaxies with a mass ratio of ~5: a main cluster centred on the Brightest Cluster Galaxy (BCG) (A3921-A), and an NW sub-cluster (A3921-B) hosting the second brightest cluster galaxy. The distorted morphology of the two sub-clusters suggests that they are interacting, while the velocity distribution of 104 confirmed cluster members does not reveal strong signatures of merging. By applying a two-body dynamical formalism to the two sub-clusters of A3921, and by comparing our optical results to the X-ray analysis of A3921 based on XMM observations (Belsole et al. [CITE]), we conclude that A3921-B is probably tangentially traversing the main cluster along the SW/NE direction. The two sub-clusters are observed in the central phase of their merging process (±0.3 Gyr), with a collision axis nearly perpendicular to the line of sight. Based on the spectral features of the galaxies belonging to A3921 we estimate the star formation properties of the confirmed cluster members. Substantial fractions of both emission-line (~13%) and post-star-forming objects (so called k+a's, ~16%) are detected, comparable to those measured at intermediate redshifts. Our analysis reveals a lack of bright post-star-forming objects in A3921 with respect to higher redshift clusters, while the fraction of k+a's increases towards fainter magnitudes (). Similar results were obtained in the Coma cluster by Poggianti et al. ([CITE]), but at still fainter magnitudes, suggesting that the maximum mass of actively star-forming galaxies increases with redshift (“downsizing effect”). The spatial and velocity distributions of k+a galaxies do not show significant differences to those of the passive population, and to the whole cluster. Most of these objects show relatively red colours and moderate Balmer absorption lines, which suggest that star formation has ceased ∼ Gyr ago. Their presence is therefore difficult to relate to the on-going merging event. We find that star-forming galaxies share neither the kinematics nor the projected distribution of the passive cluster members. Moreover, most emission-line galaxies are concentrated in A3921-B and in the region between the two sub-clusters. We therefore suggest that the ongoing merger may have triggered a star-formation episode in at least a fraction of the observed emission-line galaxies.

Gemini Multi-Object Spectrograph Integral Field Spectroscopy of a Merging System with Enhanced Balmer Absorption

In this paper we present the three-dimensional dynamics of the galaxy SDSS J101345.39+011613.66, selected for its unusually strong Balmer absorption lines [W0(Hδ) = 7.5 A]. Using the Gemini Multi-Object Spectrograph-South integral field unit (IFU) in nod+shuffle mode, we have mapped the continuum and optical absorption lines of this z = 0.1055 field galaxy. This galaxy has a disturbed morphology, with a halo of diffuse material distributed asymmetrically toward the north. Using the [O II] emission line (W0[O ] = 4.1 A), we find that the gas and hot OB stars are offset from the older stars in the system. The gas also has a spatially extended and elongated morphology with a velocity gradient of 100 ± 20 km s-1 across 6 kpc in projection. Using the strong Hγ and Hδ absorption lines, we find that the A stars are widely distributed across the system and are not centrally concentrated, arguing that the A star population has formed in molecular clouds outside the nucleus. By cross-correlating the spectra from the data cube with an A star template, we find evidence that the A star population has a 40 km s-1 shear in the same direction as the gas. The disturbed morphology, strong color gradients, and strong Hδ and Hγ absorption lines in SDSS J101345.39 argue that this is a recent tidal interaction/merger between a passive elliptical and a star-forming galaxy. Although based on a single object, these results show that we can spatially resolve and constrain the dynamics of this short-lived (yet important) phase of galaxy formation in which the evolutionary process takes galaxies from star-forming to their quiescent end products.

Are E+A galaxies dusty-starbursts?: VLA 20 cm radio continuum observation

E+A galaxies are characterized as a galaxy with strong Balmer absorption lines but without any [OII] or Halpha emission lines. Strong Balmer absorption lines indicate that E+A galaxies have experienced a starburst within the last <1-1.5 Gyr. However, the lack of emission lines indicates that E+A galaxies do not have any on-going star formation. Therefore, E+A galaxies are interpreted as a post-starburst galaxy. However, it has been a mystery why E+A galaxies underwent a starburst then stopped abruptly. One possible explanation to E+A galaxies is the dusty starburst scenario, where E+A galaxies have on-going star formation, but optical emission lines are invisible due to the heavy obscuration by dust. To test this dusty starburst scenario, we have observed 36 E+A galaxies carefully selected from the SDSS DR1 in 20cm radio continuum using the VLA. Since the radio emission is not affected by the dust extinction, the SFR in dusty galaxies can be revealed by the 20 cm continuum. All of our 36 target E+A galaxies are selected to have H\delta EW greater than 6A and no detection of [OII] or Halpha emission lines within 1 sigma. These selection criteria are much stronger than previous E+A selections in our attempt to select galaxies in the pure post-starburst phase without any remaining star formation. Except for the two galaxies with a nearby radio source, none of our 34 E+A galaxies are detected in 20 cm continuum to the limits reported in Table 1. The obtained upper limits on the radio estimated SFR suggest that E+A galaxies do not possess strong starburst (> 100 Msun yr^-1) hidden by dust extinction for the whole sample while 15 (z<0.08) E+As have lower SFR upper limits of ~15 Msun yr^-1. (Abridged)

The 2dF Galaxy Redshift Survey: the local E+A galaxy population

We select a sample of low-redshift (z ~ 0.1) E+A galaxies from the 2dF Galaxy Redshift Survey (2dFGRS). The spectra of these objects are defined by strong hydrogen Balmer absorption lines (H-delta, H-gamma, H-beta) combined with a lack of [OII] 3727A emission, together implying a recently-truncated burst of star formation. The E+A spectrum is thus a signpost to galaxies in the process of evolution. We quantify the local environments, clustering properties and luminosity function of the E+A galaxies. We find that the environments are consistent with the ensemble of 2dFGRS galaxies: low-redshift E+A systems are located predominantly in the field, existing as isolated objects or in poor groups. However, the luminosity distribution of galaxies selected using three Balmer absorption lines H-delta-gamma-beta appears more typical of ellipticals. Indeed, morphologically these galaxies are preferentially spheroidal (E/S0) systems. In a small but significant number we find evidence for recent major mergers, such as tidal tails. We infer that major mergers are one important formation mechanism for E+A galaxies, as suggested by previous studies. At low redshift the merger probability is high in the field and low in clusters, thus these recently-formed spheroidal systems do not follow the usual morphology-density relation for ellipticals. Regarding the selection of E+A galaxies: we find that basing the Balmer-line criterion solely on H-delta absorption leads to a significant sub-population of disk systems with detectable H-alpha emission. In these objects the [OII] emission is presumably either obscured by dust or present with a low signal-to-noise ratio, whilst the (H-gamma, H-beta) absorption features are subject to emission-filling.

E+A Galaxies and the Formation of Early‐Type Galaxies atz ∼ 0

E+A galaxies, whose spectra have deep Balmer absorption lines but no significant [O II] emission, are the best candidates for an evolutionary link between star-forming, gas-rich galaxies and quiescent, gas-poor galaxies. However, their current morphologies are not well known. We present Hubble Space Telescope WFPC2 observations of the five bluest E+A galaxies (z ~ 0.1) in the sample of Zabludoff and coworkers to study whether their detailed morphologies are consistent with late- to early-type evolution and to determine what drives that evolution. The morphologies of four galaxies are disturbed, indicating that a galaxy-galaxy merger is at least one mechanism that leads to the E+A phase. Two-dimensional image fitting shows that the E+As are generally bulge-dominated systems, even though at least two E+As may have underlying disks. In the fundamental plane, E+As stand apart from the E/S0 galaxies mainly because of their high effective surface brightness. Fading of the young stellar population and the corresponding increase in their effective radii will cause these galaxies to migrate toward the locus of E/S0 galaxies. E+As have profiles qualitatively like those of normal power-law early-type galaxies but have higher surface brightnesses. This result provides the first direct evidence supporting the hypothesis that power-law elliptical galaxies form via gas-rich mergers. In total, at least four E+As are morphologically consistent with early-type galaxies. We detect compact sources, possibly young star clusters, associated with the galaxies. These sources are much brighter (MR ~ -13) than Galactic globular clusters, have luminosities consistent with the brightest clusters in nearby starburst galaxies, and have blue colors consistent with the ages estimated from the E+A galaxy spectra (several 108 yr). Further study of such young star cluster candidates might provide the elusive chronometer needed to break the age/burst-strength degeneracy for these postmerger galaxies.

Higher-order Balmer line indices inα/Fe-enhanced stellar population models

We have computed the higher-order Balmer absorption line indices Hγ and Hδ for stellar population models with variable element ratios. The response of these indices to abundance ratio variations is taken from detailed line formation and model atmosphere calculations. We find that Hγ and Hδ, unlike Hβ, are very sensitive to α/Fe ratio changes at super-solar metallicities. Both line indices increase significantly with increasing α/Fe ratio. This effect cannot be neglected when these line indices are used to derive the ages of metal-rich, unresolved stellar populations such as early-type galaxies. We re-analyze the elliptical galaxy sample of Kuntschner, and show that consistent age estimates from Hβ and Hγ are obtained only if the effect of α/Fe enhancement on Hγ is taken into account in the models. This result rectifies a problem currently present in the literature, namely that Hγ and Hδ have up to now led to significantly younger ages for early-type galaxies than Hβ. Our work particularly impacts on the interpretation of intermediate- to high-redshift data, for which only the higher-order Balmer lines are accessible observationally.

Keck Spectroscopy of Two Young Globular Clusters in the Merger Remnant NGC 3921

Low-resolution, ultraviolet-to-visual spectra of two candidate globular clusters in the merger remnant NGC 3921 (czLG = 6021 km s-1) are presented. These two clusters of apparent magnitude V ≈ 22.2 (MV ≈ -12.5) lie at projected distances of about 5 kpc (0.9 Reff) from the center and move with halo-type radial velocities relative to the local galaxy background. Their spectra show strong Balmer absorption lines [EW(Hβ–Hδ) = 11–13 A] indicative of main-sequence turnoffs dominated by A-type stars. Comparisons with model-cluster spectra computed by Bruzual & Charlot and others yield cluster ages in the range of 200–530 Myr, and metallicities about solar to within a factor of three. Given their small half-light radii (Reff 5 pc) and ages corresponding to ~102 core-crossing times, these clusters are gravitationally bound and hence indeed young globulars. Assuming that they had Chabrier-type initial mass functions, their estimated current masses are (2.3 ± 0.1) × 106 ⊙ and (1.5 ± 0.1) × 106 ⊙, respectively, or roughly half the mass of ω Cen. Since NGC 3921 itself shows many signs of being a 0.7 ± 0.3 Gyr old protoelliptical, these two young globulars of roughly solar metallicity and their many counterparts observed with the Hubble Space Telescope provide supporting evidence that, in the process of forming elliptical-like remnants, major mergers of gas-rich disks can also increase the number of metal-rich globular clusters.

The Stellar Populations of Low‐Luminosity Active Galactic Nuclei. I. Ground‐based Observations

(Abridge): We present a spectroscopic study of the stellar populations of Low Luminosity AGN (LLAGN). Our main goal is to determine whether the stars who live in the innermost (100 pc-scale) regions of these galaxies are in some way related to the emission line properties, which would imply a link between the stellar population and the ionization mechanism. High signal to noise, ground based long-slit spectra in the 3500--5500 A interval were collected for 60 galaxies.Our main findings are: (1) Few LLAGN have a detectable young (< 10 Myr) starburst component, indicating that very massive stars do not contribute significantly to the optical continuum. In particular, no features due to Wolf-Rayet stars were convincingly detected. (2) High Order Balmer absorption lines of HI (HOBLs), on the other hand, are detected in 40% of LLAGN. These features, which are strongest in 100 Myr--1 Gyr intermediate age stellar populations, are accompanied by diluted metal absorption lines and bluer colors than other objects in the sample. (3) These intermediate age populations are very common (50 %) in LLAGN with relatively weak [OI] emission, but rare (10 %) in LLAGN with stronger [OI]. This is intriguing since LLAGN with weak [OI] have been previously hypothesized to be ``transition objects'' in which both an AGN and young stars contribute to the emission-line excitation. Massive stars, if present, are completely outshone by intermediate age and old stars in the optical. This happens in at least a couple of objects where independent UV spectroscopy detects young starbursts not seen in the optical. (4) Objects with predominantly old stars span the whole range of [OI]/Halpha values, but (5) sources with significant young and/or intermediate age populations are nearly all (90%) weak [OI] emitters.

Spectroscopic signatures of galaxy evolution in Cl0024+16 at $z\sim 0.4$

We report the first results from a panoramic spectroscopic survey of galaxies in the rich cluster Cl0024+1654 (, an overall trend consistent with previous work. Using diagnostic [O II] emission and Balmer absorption lines, we locate a population of intrinsically faint galaxies at 1–2.4 Mpc radius undergoing a period of star formation. The luminosity-dependent radial trends are suggestive of the gradual quenching of star formation for infalling galaxies. We discuss physical mechanisms that may be responsible for this environmental evolution.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Spectral dating of high-redshift stellar populations

Age derivation techniques for unresolved stellar populations at high redshifts are explored using the near-ultraviolet (NUV) spectrum of LBDS 53W091 (z = 1.55) and LBDS 53W069 (z = 1.43). The photometry and morphology of these galaxies - which are weak radio sources -suggest they are early-type systems, a feature that makes them ideal testbeds for the analysis of their ages and metallicities with the use of population-synthesis models. In the analysis that is based on simple stellar population models, we find a significant degeneracy between the derived ages and metallicities both in optical+near-infrared (NIR) photometric and NUV spectroscopic analyses. This degeneracy is not so strong for LBDS 53W069. However, even in this case the stellar age cannot be constrained better than to a range roughly encompassing one-third of the age of the Universe at z = 1.43 (90 per cent confidence level). We have explored several independent population-synthesis models and consistently found similar results. Broadband photometry straddling the rest-frame 4000-A break is also subject to a strong age-metallicity degeneracy. The use of realistic chemical enrichment assumptions significantly helps in disentangling the degeneracy. Based on this method, we derive the average stellar age for both galaxies around ∼ 3.6-3.8 Gyr with better constraints on the youngest possible ages (∼3 Gyr at the 90 per cent confidence level). The comparison with simple stellar population models suggest subsolar metallicities (log Z/Z ○. = -0.2). A composite model using chemical enrichment gives slightly higher metallicities in both galaxies (log Z/Z ○. = - 0.1). Given that the stellar component in galaxies forms over times which are larger than a typical chemical enrichment time-scale, we conclude that composite stellar populations must be used in all photospectroscopic analyses of galaxies. From the observational point of view, the most efficient (and feasible) way to set limits on unresolved stellar populations comprises a combination of Balmer absorption lines along with either low signal-to-noise ratio (SNR) rest-frame NUV spectroscopy or accurate optical and NIR photometry.

Formation and Evolution of Red Hδ‐strong Galaxies in Distant Clusters: Two Different Evolutionary Paths to S0 Galaxies?

We numerically investigate the spectrophotometric evolution of galaxies with a variety of different star formation histories and degrees of dust extinction in order to elucidate the origin of the "red Hδ-strong" (RHDS) galaxies—characterized by strong [EW(Hδ) > 3 Å] Balmer line absorption but with colors as red as the dormant E/S0 systems—first observed in distant clusters of galaxies. Specifically, we investigate the evolution in the [(B-R), EW(Hδ)]-plane of two different galaxy models: a "truncation" model in which a galaxy with a declining star formation rate (typical of a normal spiral) has its star formation abruptly truncated, and a "starburst" model in which a galaxy undergoes a secondary burst of star formation that is abruptly halted. This leads to the following three main results: (1) Both models pass through the zone inhabited by the RHDS galaxies in this plane, but the "truncation" models cannot explain the most extreme cases, which have EW(Hδ) > 6 Å. (2) The reddest RHDS galaxies can only be explained by "truncation" or "starburst" models that have very heavy dust extinction (Av > 0.5 mag). (3) Irrespective of whether dust effects are included, the "truncation" models are incapable of reproducing the very blue (B-R < 1.6) analogs of the RHDS galaxies. By comparing these results with recent Hubble Space Telescope-based morphological and ground-based spectroscopic studies of distant cluster galaxies, we propose that there are two possible different evolutionary paths for the transformation of cluster spirals into S0 galaxies.

Hδ-Strong Galaxies in the Sloan Digital Sky Survey: I. The Catalog

Abstract We present here a new and homogeneous sample of 3340 galaxies selected from the Sloan Digital Sky Survey (SDSS) based solely on the observed strength of their $\mathrm{H}\delta$ hydrogen Balmer absorption line. The presence of a strong $\mathrm{H}\delta$ line within the spectrum of a galaxy indicates that the galaxy has undergone a significant change in its star-formation history within the last Gigayear. Therefore, such galaxies have received considerable attention in recent years, because they provide an opportunity to study galaxy evolution in action. These galaxies are commonly known as “post-starburst”, “E$+$A”, “k$+$a”, and $\mathrm{H}\delta$-strong galaxies. Their study has been severely hampered by the lack of a large, statistical sample of such galaxies. In this paper, we rectify this problem by selecting a sample of galaxies which possess an absorption $\mathrm{H}\delta$ equivalent width of $\mathrm{EW}(\mathrm{H}\delta_{\mathrm{max}})-\Delta \mathrm{EW}(\mathrm{H}\delta_{\mathrm{max}}) \gt 4 \,{Å}$$\mathrm{EW}(\mathrm{H}\delta_{\mathrm{max}})-\Delta \mathrm{EW}(\mathrm{H}\delta_{\mathrm{max}}) \gt 4 \,$Å from 106682 galaxies in the SDSS. The measured abundance of our $\mathrm{H}\delta$-strong (HDS) galaxies is $2.6 \pm 0.1 \%$ of all galaxies within a volume-limited sample of $0.05 &amp;lt; z &amp;lt; 0.1$ and $M_{r^{*}} &amp;lt;-20.5$, which is consistent with previous studies of such galaxies described in the literature. We find that only 25 of our HDS galaxies in this volume-limited sample ($3.5 \pm 0.7\%$) show no, or little, evidence for [O II] and $\mathrm{H}\alpha$ emission lines, thus indicating that true E$+$A galaxies (as originally defined by Dressler and Gunn) are extremely rare objects at low redshift, i.e., only $0.09 \pm 0.02\%$ of all galaxies in this volume-limited sample are true E$+$A galaxies. In contrast, $89 \pm 5\%$ of our HDS galaxies in the volume-limited sample have significant detections of the [O II] and $\mathrm{H}\alpha$ emission lines. Of these, only 131 galaxies are robustly classified as Active Galactic Nuclei (AGNs) and, therefore, a majority of these emission-line HDS galaxies are star-forming galaxies. We find that $52 \pm 12\%$ ($27/52$) of the galaxies in our volume-limited HDS sample that possess no detectable [O II] emission, do however possess detectable $\mathrm{H}\alpha$ emission lines. These may be dusty star-forming galaxies. We provide the community with this new catalog of $\mathrm{H}\delta$-strong galaxies to aid in our understanding of these galaxies, via detailed follow-up observations, as well as providing a low-redshift sample for comparison with higher redshift studies of HDS galaxies.

Nuclear Starburst Activity in the Seyfert 2 Galaxy NGC 2273

We present spectrophotometric results of the Seyfert 2 galaxy NGC 2273. The presence of high-order Balmer absorption lines (H8, H9, H10) and weak equivalent widths of CaII K λ3933, CN λ4200, G-band λ4300 and MgIb λ5173 clearly indicate recent star-forming activity in the nuclear region. Using a simple stellar population synthesis model, we find that for the best fit, the contributions of a power-law featureless continuum, an intermediate-age (∼ 108 yr) and an old (> 109 yr) stellar population to the total light at the reference normalization wavelength are 10.0%, 33.4% and 56.6%, respectively. The existence of recent starburst activity is also consistent with its high far-infrared luminosity (log LFIR/L☉ = 9.9), its infrared color indexes [α(25, 60) = −1.81 and α(60, 100)= −0.79, typical values for Seyfert galaxies with circumnuclear starburst], and its q-value (2.23, ratio of infrared to radio flux, very similar to that of normal spirals and starburst galaxies). Byrd et al. have suggested that NGC 2273 might have interacted with NGC 2273B in less than 109 yr ago, so the starburst activity in this galaxy could have been triggered by tidal interaction, as indicated in recent numerical simulations.

The Fundamental Plane atz = 1.27: First Calibration of the Mass Scale of Red Galaxies at Redshiftsz &gt; 1

We present results on the Fundamental Plane (FP) of early-type galaxies in the cluster RDCS J0848+4453 at z=1.27. Internal velocity dispersions of three K-selected early-type galaxies are determined from deep Keck spectra. Structural parameters are determined from HST NICMOS images. The galaxies show substantial offsets from the FP of the nearby Coma cluster, as expected from passive evolution of their stellar populations. The offsets from the FP can be expressed as offsets in M/L ratio. The M/L ratios of the two most massive galaxies are consistent with an extrapolation of results obtained at z=0.02-0.83. The evolution of early-type galaxies with masses >10^11 M_sun is well described by ln M/L(B) = (-1.06 +- 0.09) z, corresponding to passive evolution of -1.50 +- 0.13 mag at z=1.3. Ignoring selection effects, the best fitting stellar formation redshift is z*=2.6, corresponding to a luminosity weighted age at the epoch of observation of ~2 Gyr. The M/L ratios of these two galaxies are also in excellent agreement with predictions from models that include progenitor bias. The third galaxy is a factor ~10 less massive than the other two, shows strong Balmer absorption lines in its spectrum, and is offset from the Coma Fundamental Plane by 2.9 mag in rest-frame B. Despite their large range in M/L ratios, all three galaxies fall in the ``Extremely Red Object'' (ERO) class with I-H>3 and R-K>5, and our results show that it is hazardous to use simple models for converting luminosity to mass for these objects. Measurements of M/L ratios at high redshift can be considered first steps to empirically disentangle luminosity and mass evolution at the high mass end of the galaxy population, lifting an important degeneracy in the interpretation of evolution of the luminosity function. [SHORTENED]

6-m telescope spectroscopic observations of the bubble complex in NGC 6946

We describe the results of a long-slit spectroscopic study of an unusual star complex in the nearby spiral galaxy NGC 6946 using the SAO 6-m telescope and the Keck 10-m telescope. The complex resembles a circular bubble 600 pc in diameter with a young super star cluster (SSC) near the center. The kinematics of ionized gas is studied through Ha emission with several slit positions. Position-velocity diagrams show two distinct features with high speed motions. One is an irregularly shaped region to the east of the SSC, 270 pc in size, in which most of the Ha emission is blue shifted by 120 km/s, and another is a 350 pc shell centered on the SSC with positive and negative velocity shifts of 60 km/s. Balmer and HeI absorption lines in the SSC give an age of 12-13 Myr, which is consistent with the photometric age but significantly older than the kinematic ages of the high speed regions. The energetics of the SSC and its interaction with the environment are considered. The expansion energies exceed 10^52 ergs, but the power outputs from winds and supernova in the SSC are large enough to account for this. The intensities of Balmer, [NII], and [SII] emission lines within and around the complex indicate that shock excitation makes a significant contribution to the emission from the most energetic region.

Optical Spectroscopy of Metal‐rich HiiRegions and Circumnuclear Hot Spots in M83 and NGC 3351

We present optical spectroscopy of a sample of metal-rich extragalactic HII regions in the spiral galaxies M83, NGC 3351 and NGC 6384, including a number of circumnuclear HII regions (hot spots). Different age estimators (equivalent width of the Hbeta emission line W(Hb), Balmer line absorption profiles, UV spectra) provide consistently young ages (4-6 Myr) for the hot spots. We have detected the W-R bump feature at 4650 A in five of the objects, and in fewer cases possibly the WC features at 5696 A and 5808 A. Six additional objects showing W-R features are drawn from our previous work on extragalactic HII regions. From the measured luminosity of the W-R blue bump we estimate a small number of WN stars (from 1-2 to about 30). By assuming instantaneous bursts of star formation the ages derived from W(Hb) (3-6 Myr) and the strength of the W-R bump are in agreement with the predictions of evolutionary stellar population models, provided we account for stochastic effects on the IMF. From a comparison with published photoionization models based on synthetic cluster spectral energy distributions we find some evidence for an overestimation of the number of He ionizing photons in the model fluxes. In general, however, the massive star diagnostics are in agreement with the predictions of recent evolutionary models calculated with a Salpeter IMF and a high mass limit. We find no compelling evidence for a depletion of massive stars (M > 40-50 solar masses) in the IMF of metal-rich clusters, contrary to our previous conclusions based on older evolutionary models.

Discovery of a cataclysmic variable with a sub-stellar companion

We find that the ROSAT source 1RXS J105010.3-140431 is a cataclysmic variable with orbital period of 88.6 minutes and a spectrum closely resembling WZ Sge. In particular, emission lines are flanked by Stark-broadened absorption wings probably originating in the photosphere of a compact object. The Balmer absorption lines can be modeled by the spectrum of a DA white dwarf with 13 000 $< T_{eff} <$ 24 000 K. The strong absorption lines allowed us to obtain direct radial velocities of the white dwarf using the cross-correlation technique. We find an extremely low white dwarf radial velocity half amplitude, $K_{wd}$ = 4 $\pm$ 1 km s$^{-1}$. This is consistent with the upper limit obtained from the H$\alpha$ emission line wing K < 20 km s$^{-1}$. The corresponding mass function is incompatible with a main sequence secondary, but is compatible with a post orbital period minimum cataclysmic variable with a brown dwarf-like secondary. The formal solution gives a secondary mass of 10-20 jovian masses. Doppler maps for the emission lines and the hypothesis of black-body emission indicate a steady state (T $\sim r^{-3/4}$) accretion disk mainly emitting in H$\alpha$ and an optically thicker hotspot with a strong contribution to the higher order Balmer lines and \ion{He}{I} 5875. As in other long cycle length dwarf novae, evidence for inner disk removal is found from the analysis of the emission lines.

UITBOC 1574: A Very Distant Helium‐poor Subdwarf O Star11Based on observations obtained with the Apache Point Observatory 3.5 m telescope, which is owned and operated by the Astrophysical Research Consortium.

ABSTRACTWe have obtained the optical spectrum (3750–7500 Å) and ultraviolet fluxes at 1521 and 2260 Å for the quasar candidate UITBOC 1574. The optical spectrum shows strong Balmer absorption lines through at least n = 9 and the He ii line at 4686 Å. We compared the optical spectrum with non‐LTE stellar atmosphere models and find Teff = 46,900 ± 3,000 K, log g = 5.6 ± 0.3, and helium abundance log (He/H) = -2.0 ± 0.4. We classify the object as a hot subdwarf O star (sdO). With its high effective temperature and low helium abundance, UITBOC 1574 may be considered as belonging to the hot end of the subdwarf B population. The location of this object on the theoretical Teff‐g diagram suggests that it is most likely in a post–extreme horizontal branch evolutionary stage. However, we cannot exclude the possibility that UITBOC 1574 may be a low‐mass helium white dwarf precursor. We estimate a spectroscopic distance of 5.6 ± 1.7 kpc with a height of 2.7 ± 0.8 kpc above the Galactic plane if it is a helium‐poor sdO. The heliocentric radial velocity of the star is 89 ± 18 km s−1.

The nuclear starburst activity in the Seyfert 2 galaxy NGC 7679

We present our recent spectrophotometric results of the infrared luminous Seyfert galaxy NGC 7679. We find compelling evidence of the presence of a recent nuclear starburst, revealed by the observations of a) the spectral features of high order Balmer absorption lines, b) the weak equivalent widths for CaIIK $\lambda$3933, CN $\lambda$4200, G-band $\lambda$4300 and MgIb $\lambda$5173, and c) the suggested UV stellar wind resonance lines (N V $\lambda$1240, Si IV $\lambda$1400 and C IV $\lambda$1550) in the IUE spectrum. Using the simple stellar population synthesis model, we find that in the nuclear 2 $\times$ 2 region, the contributions from the old, intermediate and young components are 21.7%, 42.9% and 35.4%, respectively. These nuclear starburst activities might be triggered by the close encounter with NGC 7682, as suggested by recent numerical simulations.