ABSTRACT
We here present the luminosity function (LF) of globular clusters (GCs) in five nearby spiral galaxies using the samples of GC candidates selected in Hubble Space Telescope mosaic images in ...F435W, F555W, and F814W filters. Our search, which surpasses the fractional area covered by all previous searches in these galaxies, has resulted in the detection of 158 GC candidates in M81, 1123 in M101, 226 in NGC 4258, 293 in M51, and 173 in NGC 628. The LFs constructed from this data set, after correcting for relatively small contamination from reddened young clusters, are lognormal in nature, which was hitherto established only for the Milky Way (MW) and Andromeda among spiral galaxies. The magnitude at the turn-over (TO) corresponds to MV0(TO) = −7.41 ± 0.14 in four of the galaxies with Hubble types Sc or earlier, in excellent agreement with MV(TO) = −7.40 ± 0.10 for the MW. The TO magnitude is equivalent to a mass of ∼3 × 105 M⊙ for an old, metal-poor population. MV0(TO) is fainter by ∼1.16 magnitude for the fifth galaxy, M 101, which is of Hubble type Scd. The TO dependence on Hubble type implies that the GCs in early-type spirals are classical GCs, which have a universal TO, whereas the GC population in late-type galaxies is dominated by old disc clusters, which are in general less massive. The radial density distribution of GCs in our sample galaxies follows the Sérsic function with exponential power-law indices, and effective radii of 4.0–9.5 kpc. GCs in the sample galaxies have a mean specific frequency of 1.10 ± 0.24, after correcting for magnitude and radial incompleteness factors.
ABSTRACT
We present a complete set of structural parameters for a sample of 99 intermediate-age super star cluster (SSCs) in the disc of M82, and carry out a survival analysis using the ...semi-analytical cluster evolution code emacss. The parameters are based on the profile-fitting analysis carried out in previous work, with the mass-related quantities derived using a mass-to-light ratio for a constant age of 100 Myr. The SSCs follow a power-law mass function with an index α = 1.5, and a lognormal size function with a typical half-light radius, Rh = 4.3 pc, which is both comparable with the values for clusters in the Magellanic Clouds, rather than in giant spirals. The majority of the SSCs follow a power-law mass−radius relation with an index of b = 0.29 ± 0.05. A dynamical analysis of M82 SSCs using emacss suggests that 23 per cent of the clusters are tidally limited, with the rest undergoing expansion at present. Forward evolution of these clusters suggests that the majority would dissolve in ∼2 Gyr. However, a group of four massive compact clusters, and another group of five SSCs at relatively large galactocentric distances, are found to survive for a Hubble time. The model-predicted mass, Rh, μV, and core radius of these surviving SSCs at 12 Gyr are comparable with the corresponding values for the sample of Galactic globular clusters.
ABSTRACT
We here present the results of an analysis of the optical spectroscopy of 42 globular cluster (GC) candidates in the nearby spiral galaxy M81 (3.61 Mpc). The spectra were obtained using the ...long-slit and multi-object spectroscopic modes of the OSIRIS instrument at the 10.4-m Gran Telescopio Canarias at a spectral resolution of ∼1000. We used the classical H β versus MgFe′ index diagram to separate genuine old GCs from clusters younger than 3 Gyr. Of the 30 spectra with continuum signal-to-noise ratio >10, we confirm 17 objects to be classical GCs (age >10 Gyr, −1.4 < Fe/H < −0.4), with the remaining 13 being intermediate-age clusters (1–7.5 Gyr). We combined age and metallicity data of other nearby spiral galaxies (≲18 Mpc) obtained using similar methodology like the one we have used here to understand the origin of GCs in spiral galaxies in the cosmological context. We find that the metal-poor (Fe/H < −1) GCs continued to form up to 6 Gyr after the first GCs were formed, with all younger systems (age <8 Gyr) being metal rich.
Abstract
We investigate the relation between gas and star formation in subgalactic regions, ∼360 pc to ∼1.5 kpc in size, within the nearby starburst dwarf NGC 4449, in order to separate the ...underlying relation from the effects of sampling at varying spatial scales. Dust and gas mass surface densities are derived by combining new observations at 1.1 mm, obtained with the AzTEC instrument on the Large Millimeter Telescope, with archival infrared images in the range 8–500
μ
m from the
Spitzer Space Telescope
and the
Herschel Space Observatory
. We extend the dynamic range of our millimeter (and dust) maps at the faint end, using a correlation between the far-infrared/millimeter colors
F
(70)/
F
(1100) (and
F
(160)/
F
(1100)) and the mid-infrared color
F
(8)/
F
(24) that we establish for the first time for this and other galaxies. Supplementing our data with maps of the extinction-corrected star formation rate (SFR) surface density, we measure both the SFR–molecular gas and the SFR–total gas relations in NGC 4449. We find that the SFR–molecular gas relation is described by a power law with an exponent that decreases from ∼1.5 to ∼1.2 for increasing region size, while the exponent of the SFR–total gas relation remains constant with a value of ∼1.5 independent of region size. We attribute the molecular law behavior to the increasingly better sampling of the molecular cloud mass function at larger region sizes; conversely, the total gas law behavior likely results from the balance between the atomic and molecular gas phases achieved in regions of active star formation. Our results indicate a nonlinear relation between SFR and gas surface density in NGC 4449, similar to what is observed for galaxy samples.
ABSTRACT
We present the structural parameters of 99 super star clusters (SSCs) in the disc of M82. Moffat-EFF, King, and Wilson models were fitted using a χ2-minimization method to ...background-subtracted surface brightness profiles in the F435W (B), F555W (V), and F814W (I) bands of the Advanced Camera for Surveys (ACS) of the Hubble Space Telescope. The majority of the SSC profiles are best fitted by the Moffat-EFF profile. The scale parameter rd and the shape parameter γ in the three filters are identical within the measurement errors. The analysed sample is big enough to allow the characterization of the distributions of core radii Rc and γ. The obtained distribution of Rc follows a lognormal form, with centre and $\sigma \mathrm{ log}\big {(}\frac{R_{\rm c}}{pc}\big {)}$ being 1.73 pc and 0.25, respectively. The γ distribution is also lognormal with centre and σlog(γ) being 2.88 and 0.08, respectively. M82 is well known for the absence of current star formation in its disc, with all disc SSCs older than 50 Myr and hardly any cluster older than ∼300 Myr. The derived distributions compare very well with the distributions for intermediate-age clusters in the Large Magellanic Cloud (LMC), which is also a low-mass late-type galaxy similar to M82. On the other hand, the distributions of Rc in both these galaxies are shifted towards larger values as compared to SSCs of similar age in the giant spiral galaxy M83. M82 and LMC also span a narrower range of γ values as compared to that in M83.
We study the sensitivity of the methods available for abundance determinations in H ii regions to potential observational problems. We compare the dispersions they introduce around the oxygen and ...nitrogen abundance gradients when applied to five different sets of spectra of H ii regions in the galaxy M81. Our sample contains 116 H ii regions with galactocentric distances of 3 to 33 kpc, including 48 regions observed by us with the OSIRIS long-slit spectrograph at the 10.4-m Gran Telescopio Canarias telescope. The direct method can be applied to 31 regions, where we can get estimates of the electron temperature. The different methods imply oxygen abundance gradients with slopes of -0.010 to -0.002 dex kpc..., and dispersions in the range 0.06-0.25 dex. The direct method produces the shallowest slope and the largest dispersion, illustrating the difficulty of obtaining good estimates of the electron temperature. Three of the strong-line methods, C, ONS, and N2, are remarkably robust, with dispersions of ~0.06 dex, and slopes in the range -0.008 to -006 dex kpc... The robustness of each method can be directly related to its sensitivity to the line intensity ratios that are more difficult to measure properly. Since the results of the N2 method depend strongly on the N/O abundance ratio and on the ionization parameter, we recommend the use of the C and ONS methods when no temperature estimates are available or when they have poor quality, although the behaviour of these methods when confronted with regions that have different properties and different values of N/O should be explored. (ProQuest: ... denotes formulae/symbols omitted.)
Local dwarf spheroidal galaxies (dSphs) are nearby dark-matter dominated systems, making them excellent targets for searching for gamma rays from particle dark matter interactions. If dark matter ...annihilates or decays directly into two gamma rays (or a gamma ray and a neutral particle), a monochromatic spectral line is created. At TeV energies, no other process is predicted to produce spectral lines, making this a very clean indirect dark matter search channel. With the development of event-by-event energy reconstruction, we can now search for spectral lines with the High Altitude Water Cherenkov (HAWC) Observatory. HAWC is a wide field of view survey instrument located in central Mexico that observes gamma rays from ∼ 200 GeV to ∼ 200 TeV . In this work we present results from a recent search for spectral lines from local, dark matter dominated, dwarf galaxies using 1038 days of HAWC data. We also present updated limits on several continuum channels that were reported in a previous publication. Our gamma-ray spectral line limits are the most constraining obtained so far from 20 TeV to 100 TeV.
Abstract
In this work, we present the results of the survey carried out on one of the deepest X-ray fields observed by the XMM-Newton satellite. The 1.75 Ms Ultra Narrow Deep Field (XMM175UNDF) ...survey is made by 13 observations taken over 2 yr with a total exposure time of 1.75 Ms (1.372 Ms after flare-filtered) in a field of 30′ × 30′ centered around the blazar 1ES 1553+113. We stacked the 13 observations reaching flux limits of 4.03 × 10
−16
, 1.3 × 10
−15
, and 9.8 × 10
−16
erg s
−1
cm
−2
in the soft (0.2–2 keV), hard (2–12 keV), and full (0.2–12 keV) bands, respectively. Using a conservative threshold of Maximum Likelihood significance of ML ≥ 6, corresponding to 3
σ
, we detected 301 point-sources for which we derived positions, fluxes in different bands, and hardness ratios. Thanks to an optical follow-up that was carried out using the 10.4m the Gran Telescopio Canarias on the same field in the
u
′
g
′
r
′
i
′
z
′ bands, combined with WISE/2MASS IR data, we identified 244 optical/IR counterpart candidates for our X-ray sources and estimated their X-ray luminosities, redshift distribution, X-ray/optical–X-ray/IR flux ratios, and absolute magnitudes. Finally, we divided this subsample into 40 non-active sources and 204 active galactic nuclei, of which 139 are classified as Seyfert galaxies and 41 as Quasars.
Abstract
The radio galaxy M87 is the central dominant galaxy of the Virgo Cluster. Very high-energy (VHE, ≳0.1 TeV) emission from M87 has been detected by imaging air Cherenkov telescopes. Recently, ...marginal evidence for VHE long-term emission has also been observed by the High Altitude Water Cherenkov Observatory, a gamma-ray and cosmic-ray detector array located in Puebla, Mexico. The mechanism that produces VHE emission in M87 remains unclear. This emission originates in its prominent jet, which has been spatially resolved from radio to X-rays. In this paper, we construct a spectral energy distribution from radio to gamma rays that is representative of the nonflaring activity of the source, and in order to explain the observed emission, we fit it with a lepto-hadronic emission model. We found that this model is able to explain nonflaring VHE emission of M87 as well as an orphan flare reported in 2005.
ABSTRACT
We here report the detection of extended He ii λ4686 nebular emission in the central region of NGC 1569 using the integral field spectrograph MEGARA at the 10.4 m Gran Telescopio Canarias. ...The observations cover a field of view (FoV) of 12.5 arcsec × 11.3 arcsec at a seeing-limited spatial resolution of ∼15 pc and at a spectral resolution of R = 6000 in the wavelength range 4330–5200 Å. The emission extends over a semicircular arc of ∼40 pc width and ∼150 pc diameter around the superstar cluster A (SSC-A). The AV derived using Balmer decrement varies from the Galactic value of 1.6 mag to a maximum of ∼4.5 mag, with a mean value of 2.65 ± 0.60 mag. We infer 124 ± 11 Wolf–Rayet (WR) stars in SSC-A using the He ii λ4686 broad feature and AV = 2.3 mag. The He+ ionizing photon rate from these WR stars is sufficient to explain the luminosity of the He ii nebula. The observationally determined total He+ and H0 ionizing photon rates, their ratio, and the observed number of WR stars in SSC-A are all consistent with the predictions of simple stellar population models at an age of 4.0 ± 0.5 Myr and a mass of (5.5 ± 0.5) × 105 M⊙. Our observations reinforce the absence of WR stars in SSC-B, the second most massive cluster in the FoV. None of the other locations in our FoV where He ii λ4686 emission has been reported from narrow-band imaging observations contain WR stars.
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